Evidence hierarchy
Grade A evidence is a controlling regulator, statute, official public dataset or authoritative disclosure. Grade B is qualified professional guidance or multiple credible technical sources. Unverified summaries and machine-generated assertions are never published as fact.
Applicability
A citation establishes the definition or current rule checked. It does not imply affiliation, endorsement or that the same rule applies to every user, product, state or transaction.
How a source earns a place here
A source is cited when it is the authority that defines a term, sets a rule, or publishes an official figure: a regulator, a statistical agency, or the body that administers the programme in question. Commentary, vendor material and secondary summaries are not cited as evidence, however convenient, because they can restate an authority incorrectly.
What a citation does and does not establish
A citation establishes what a definition or rule is. What it cannot establish is that a calculated result is correct for your circumstances. Nor does it imply that the cited organisation endorses this site or reviewed anything on it. Free Calculators Online is independent of every organisation listed here.
Keeping the ledger current
Source links are re-checked as part of the release process, and a link that has moved or broken is treated as a correction , not a cosmetic issue, because a citation that cannot be followed is not evidence. Where an authority republishes a figure annually, the page that depends on it names the year it applies to.
cfpb-creditConsumer Financial Protection Bureau: consumer credit
Official U.S. guidance on credit cards, borrowing costs and disclosures.
View primary page ↗cfpb-mortgageConsumer Financial Protection Bureau: mortgages
Official mortgage shopping, Loan Estimate, interest-rate and APR guidance.
View primary page ↗cfpb-autoConsumer Financial Protection Bureau: auto loans
Official guidance for comparing vehicle financing and total loan cost.
View primary page ↗sec-investorSEC Investor.gov: financial calculators
Primary investor-education material covering compounding, savings goals and risk.
View primary page ↗irs-2026IRS: tax year 2026 inflation adjustments
Official 2026 ordinary-income brackets and standard deductions used by the simplified estimator.
View primary page ↗irs-retirementIRS: inflation-adjusted tax items by year
Official index of retirement-plan and IRA contribution-limit releases.
View primary page ↗irs-self-employedIRS: self-employed individuals tax center
Official overview of self-employment tax responsibilities.
View primary page ↗irs-capital-gainsIRS Topic No. 409: capital gains and losses
Official overview of adjusted basis, gains, losses and holding periods.
View primary page ↗irs-rentalIRS Publication 527: residential rental property
Official context for rental income and expenses; ratios remain management estimates.
View primary page ↗irs-529IRS Topic No. 313: qualified tuition programs
Official federal tax overview for 529 and qualified tuition programs.
View primary page ↗state-tax-portalsIRS: state government tax websites
Official federal directory for reaching current state revenue and taxation authorities; enter the rules from the applicable state.
View primary page ↗hud-fhaHUD: FHA home loans
Official federal overview of FHA-insured home loans and HUD-approved housing counseling.
View primary page ↗va-funding-feeU.S. Department of Veterans Affairs: funding fee and closing costs
Official VA guidance for current funding-fee applicability, exemptions and closing costs.
View primary page ↗usda-home-loanUSDA Rural Development: Single Family Housing Guaranteed Loan Program
Official USDA program eligibility, lender and guarantee-fee context.
View primary page ↗treasury-ratesU.S. Treasury: daily interest-rate data
Official Treasury yield-curve, bill-rate and real-yield data and methodology.
View primary page ↗ecb-frankfurterEuropean Central Bank reference rates (via Frankfurter)
Official ECB euro foreign-exchange reference rates, published each ECB business day; frankfurter.dev republishes them as a free, no-key API.
View primary page ↗ssa-retirementSocial Security Administration: retirement benefits
Official claiming-age, retirement-estimate and household benefit guidance.
View primary page ↗irs-social-securityIRS Publication 915: Social Security and equivalent railroad retirement benefits
Official federal worksheet and guidance for taxable Social Security benefits.
View primary page ↗irs-annuityIRS Topic No. 410: pensions and annuities
Official federal tax overview for pension and annuity distributions.
View primary page ↗medicare-costsMedicare.gov: Medicare costs
Official premiums, deductibles and cost-sharing reference for current Medicare coverage.
View primary page ↗fdic-ratesFDIC: national rates and rate caps
Official deposit-rate context; users enter their own rate and terms.
View primary page ↗sba-financeU.S. SBA: manage your finances
Official small-business guidance on statements and cash flow.
View primary page ↗studentaidFederal Student Aid: loan simulator
Official federal resource for plan-specific student-loan repayment results.
View primary page ↗bls-inflationU.S. Bureau of Labor Statistics: CPI
Official source for historical U.S. consumer-price inflation.
View primary page ↗cdc-bmiCDC: About Adult BMI
Official U.S. guidance on body mass index, its calculation and its limitations.
View primary page ↗dietary-guidelinesDietary Guidelines for Americans (HHS/USDA)
Official federal guidance on calorie needs, macronutrient ranges and healthy eating patterns.
View primary page ↗cdc-physical-activityCDC: Physical Activity Basics
Official U.S. guidance on physical activity, intensity and target heart rate.
View primary page ↗acog-pregnancyACOG: Weight Gain During Pregnancy
Official clinical guidance on recommended pregnancy weight gain by pre-pregnancy BMI.
View primary page ↗niddk-kidneyNIDDK: Estimating Glomerular Filtration Rate
Official NIH guidance on eGFR equations and their clinical interpretation.
View primary page ↗nhtsa-alcoholNHTSA: Drunk Driving
Official U.S. guidance on alcohol impairment; blood alcohol content estimates are not a substitute for a breath or blood test.
View primary page ↗nist-dlmfNIST Digital Library of Mathematical Functions
Official NIST reference for mathematical formulas, definitions and identities.
View primary page ↗nist-stat-handbookNIST/SEMATECH e-Handbook of Statistical Methods
Official NIST reference for statistical methods, formulas and the assumptions behind them.
View primary page ↗nist-timeNIST: Official U.S. Time
Official U.S. reference for civil timekeeping and calendar conventions.
View primary page ↗nist-unitsNIST Handbook 44: Units and Systems of Measurement
Official U.S. reference for units of measurement and conversion factors.
View primary page ↗nist-physical-constantsNIST Physical Measurement Laboratory
Official U.S. reference for physical constants and measurement standards.
View primary page ↗nws-weatherNational Weather Service: Heat Index and Wind Chill
Official NWS formulas and safety guidance for wind chill, heat index and related weather indices.
View primary page ↗iapmo-upcIAPMO: Uniform Plumbing Code
Official reference for fixture unit values, drain slope and pipe-sizing tables used by plumbing trades.
View primary page ↗ashrae-hvacASHRAE Handbook — Fundamentals
Official engineering reference for duct design, airflow and load-estimating methods used by HVAC trades.
View primary page ↗nfpa-necNFPA 70: National Electrical Code
Official U.S. electrical code governing conduit fill, conductor ampacity and box-fill calculations.
View primary page ↗iccsafe-ibcICC: International Building Code
Official model building code reference for presumptive soil-bearing values and general construction requirements.
View primary page ↗awc-spanAmerican Wood Council: Span Options Calculator
Official industry tool for allowable joist and rafter spans by species, grade and spacing, per the NDS Supplement.
View primary page ↗asme-b1ASME B1.1: Unified Inch Screw Threads
Official U.S. standard defining thread geometry used in tap-drill and thread-engagement calculations.
View primary page ↗craft-yarn-councilCraft Yarn Council: Standard Yarn Weight System
Industry-standard reference for yarn weight categories and gauge used across knitting and crochet patterns.
View primary page ↗rfc4632IETF RFC 4632: Classless Inter-domain Routing (CIDR)
Official Internet standard defining CIDR notation and the network/broadcast/usable-host arithmetic for an IPv4 subnet.
View primary page ↗nist-800-63bNIST SP 800-63B: Digital Identity Guidelines, Authentication and Lifecycle Management
Official U.S. federal guidance on password and authenticator strength; does not itself publish a crack-time table, which this calculator's own attack-speed scenarios are illustrative estimates for.
View primary page ↗nist-binary-prefixesNIST: Prefixes for Binary Multiples
Official U.S. reference distinguishing decimal (SI, powers of 1000) from binary (IEC, powers of 1024) prefixes for digital storage.
View primary page ↗eia-electricityU.S. Energy Information Administration: electricity data
Official U.S. source for average residential electricity prices and consumption by state.
View primary page ↗fueleconomy-govFuelEconomy.gov: U.S. Department of Energy and EPA
Official fuel-economy and electric-vehicle efficiency ratings, and the EPA's method for comparing electricity and gasoline costs.
View primary page ↗irs-pub-946IRS Publication 946: How To Depreciate Property
Official federal guide to depreciation methods, useful lives and salvage rules for business property.
View primary page ↗niddk-bwpNIDDK Body Weight Planner
Official NIH tool built on a dynamic model of weight change, which shows why the fixed 3,500-calorie rule overstates long-run loss.
View primary page ↗medlineplus-menstruationMedlinePlus: Menstruation
Official NIH health information on the menstrual cycle, its normal range of length and its variation.
View primary page ↗investor-peInvestor.gov: Price-to-Earnings Ratio
Official SEC investor-education definition of the P/E ratio and how it is read.
View primary page ↗doe-going-solarU.S. Department of Energy: Homeowner's Guide to Going Solar
Official guidance on sizing a solar system, estimating output and weighing costs and incentives.
View primary page ↗medlineplus-cholesterolMedlinePlus: Cholesterol
Official NIH health information on LDL, HDL, triglycerides and how cholesterol levels are read.
View primary page ↗nhlbi-blood-pressureNHLBI: High Blood Pressure
Official NIH guidance on blood pressure readings, categories and what they mean.
View primary page ↗tanaka-max-hrTanaka, Monahan and Seals (2001): Age-predicted maximal heart rate revisited
The peer-reviewed analysis behind the 208 minus 0.7 times age formula.
View primary page ↗bea-gdpU.S. Bureau of Economic Analysis: What to Know About GDP
Official U.S. explanation of gross domestic product and the expenditure approach used to measure it.
View primary page ↗investor-debt-equityInvestor.gov: Debt-to-Equity Ratio
Official SEC investor-education definition of the debt-to-equity ratio and how it is read.
View primary page ↗investor-optionsInvestor.gov: Options
Official SEC investor-education definition of options, calls, puts, premiums and the risks of trading them.
View primary page ↗nhlbi-sleepNHLBI: Stages of Sleep
Official NIH description of sleep stages and the roughly 80 to 100 minute cycles they repeat in, four to six times a night.
View primary page ↗epa-watersenseEPA WaterSense: Outdoor Water Use
Official EPA guidance on outdoor water use, including collecting and reusing water around the home.
View primary page ↗canada-interest-actInterest Act (Canada), section 6
The federal law that governs how interest on a mortgage on real property must be stated, including yearly or half-yearly calculation, which is why fixed-rate Canadian mortgages compound half-yearly.
View primary page ↗acog-due-dateACOG Committee Opinion: Methods for Estimating the Due Date
Official clinical guidance on estimating a due date from last menstrual period, and why an ultrasound-based estimate can supersede it.
View primary page ↗Which calculators cite which source
| Source | What it establishes | Calculators citing it |
|---|---|---|
| SEC Investor.gov: financial calculators | Primary investor-education material covering compounding, savings goals and risk. | 117 |
| U.S. SBA: manage your finances | Official small-business guidance on statements and cash flow. | 68 |
| NIST Digital Library of Mathematical Functions | Official NIST reference for mathematical formulas, definitions and identities. | 61 |
| NIST Handbook 44: Units and Systems of Measurement | Official U.S. reference for units of measurement and conversion factors. | 44 |
| IRS: inflation-adjusted tax items by year | Official index of retirement-plan and IRA contribution-limit releases. | 43 |
| Consumer Financial Protection Bureau: mortgages | Official mortgage shopping, Loan Estimate, interest-rate and APR guidance. | 37 |
| NIST Physical Measurement Laboratory | Official U.S. reference for physical constants and measurement standards. | 30 |
| Consumer Financial Protection Bureau: consumer credit | Official U.S. guidance on credit cards, borrowing costs and disclosures. | 29 |
| IRS Publication 527: residential rental property | Official context for rental income and expenses; ratios remain management estimates. | 19 |
| Consumer Financial Protection Bureau: auto loans | Official guidance for comparing vehicle financing and total loan cost. | 18 |
| IRS: self-employed individuals tax center | Official overview of self-employment tax responsibilities. | 16 |
| Federal Student Aid: loan simulator | Official federal resource for plan-specific student-loan repayment results. | 16 |
| IRS: tax year 2026 inflation adjustments | Official 2026 ordinary-income brackets and standard deductions used by the simplified estimator. | 15 |
| NIST/SEMATECH e-Handbook of Statistical Methods | Official NIST reference for statistical methods, formulas and the assumptions behind them. | 13 |
| NIST: Official U.S. Time | Official U.S. reference for civil timekeeping and calendar conventions. | 13 |
| U.S. Bureau of Labor Statistics: CPI | Official source for historical U.S. consumer-price inflation. | 12 |
| FDIC: national rates and rate caps | Official deposit-rate context; users enter their own rate and terms. | 9 |
| EPA WaterSense: Outdoor Water Use | Official EPA guidance on outdoor water use, including collecting and reusing water around the home. | 9 |
| CDC: About Adult BMI | Official U.S. guidance on body mass index, its calculation and its limitations. | 8 |
| Dietary Guidelines for Americans (HHS/USDA) | Official federal guidance on calorie needs, macronutrient ranges and healthy eating patterns. | 8 |
| CDC: Physical Activity Basics | Official U.S. guidance on physical activity, intensity and target heart rate. | 6 |
| IAPMO: Uniform Plumbing Code | Official reference for fixture unit values, drain slope and pipe-sizing tables used by plumbing trades. | 6 |
| American Wood Council: Span Options Calculator | Official industry tool for allowable joist and rafter spans by species, grade and spacing, per the NDS Supplement. | 6 |
| Craft Yarn Council: Standard Yarn Weight System | Industry-standard reference for yarn weight categories and gauge used across knitting and crochet patterns. | 6 |
| IRS: state government tax websites | Official federal directory for reaching current state revenue and taxation authorities; enter the rules from the applicable state. | 5 |
| National Weather Service: Heat Index and Wind Chill | Official NWS formulas and safety guidance for wind chill, heat index and related weather indices. | 5 |
| ASHRAE Handbook — Fundamentals | Official engineering reference for duct design, airflow and load-estimating methods used by HVAC trades. | 5 |
| NFPA 70: National Electrical Code | Official U.S. electrical code governing conduit fill, conductor ampacity and box-fill calculations. | 5 |
| ICC: International Building Code | Official model building code reference for presumptive soil-bearing values and general construction requirements. | 5 |
| NIST: Prefixes for Binary Multiples | Official U.S. reference distinguishing decimal (SI, powers of 1000) from binary (IEC, powers of 1024) prefixes for digital storage. | 5 |
| ACOG Committee Opinion: Methods for Estimating the Due Date | Official clinical guidance on estimating a due date from last menstrual period, and why an ultrasound-based estimate can supersede it. | 4 |
| ASME B1.1: Unified Inch Screw Threads | Official U.S. standard defining thread geometry used in tap-drill and thread-engagement calculations. | 4 |
| IRS Publication 946: How To Depreciate Property | Official federal guide to depreciation methods, useful lives and salvage rules for business property. | 4 |
| U.S. Energy Information Administration: electricity data | Official U.S. source for average residential electricity prices and consumption by state. | 4 |
| HUD: FHA home loans | Official federal overview of FHA-insured home loans and HUD-approved housing counseling. | 3 |
| FuelEconomy.gov: U.S. Department of Energy and EPA | Official fuel-economy and electric-vehicle efficiency ratings, and the EPA's method for comparing electricity and gasoline costs. | 3 |
| IRS Topic No. 313: qualified tuition programs | Official federal tax overview for 529 and qualified tuition programs. | 2 |
| U.S. Department of Veterans Affairs: funding fee and closing costs | Official VA guidance for current funding-fee applicability, exemptions and closing costs. | 2 |
| USDA Rural Development: Single Family Housing Guaranteed Loan Program | Official USDA program eligibility, lender and guarantee-fee context. | 2 |
| IRS Topic No. 410: pensions and annuities | Official federal tax overview for pension and annuity distributions. | 2 |
| Social Security Administration: retirement benefits | Official claiming-age, retirement-estimate and household benefit guidance. | 2 |
| NIDDK: Estimating Glomerular Filtration Rate | Official NIH guidance on eGFR equations and their clinical interpretation. | 2 |
| MedlinePlus: Menstruation | Official NIH health information on the menstrual cycle, its normal range of length and its variation. | 2 |
| U.S. Department of Energy: Homeowner's Guide to Going Solar | Official guidance on sizing a solar system, estimating output and weighing costs and incentives. | 2 |
| Investor.gov: Debt-to-Equity Ratio | Official SEC investor-education definition of the debt-to-equity ratio and how it is read. | 2 |
| IRS Topic No. 409: capital gains and losses | Official overview of adjusted basis, gains, losses and holding periods. | 1 |
| U.S. Treasury: daily interest-rate data | Official Treasury yield-curve, bill-rate and real-yield data and methodology. | 1 |
| IRS Publication 915: Social Security and equivalent railroad retirement benefits | Official federal worksheet and guidance for taxable Social Security benefits. | 1 |
| Medicare.gov: Medicare costs | Official premiums, deductibles and cost-sharing reference for current Medicare coverage. | 1 |
| ACOG: Weight Gain During Pregnancy | Official clinical guidance on recommended pregnancy weight gain by pre-pregnancy BMI. | 1 |
| NHTSA: Drunk Driving | Official U.S. guidance on alcohol impairment; blood alcohol content estimates are not a substitute for a breath or blood test. | 1 |
| European Central Bank reference rates (via Frankfurter) | Official ECB euro foreign-exchange reference rates, published each ECB business day; frankfurter.dev republishes them as a free, no-key API. | 1 |
| IETF RFC 4632: Classless Inter-domain Routing (CIDR) | Official Internet standard defining CIDR notation and the network/broadcast/usable-host arithmetic for an IPv4 subnet. | 1 |
| NIST SP 800-63B: Digital Identity Guidelines, Authentication and Lifecycle Management | Official U.S. federal guidance on password and authenticator strength; does not itself publish a crack-time table, which this calculator's own attack-speed scenarios are illustrative estimates for. | 1 |
| NIDDK Body Weight Planner | Official NIH tool built on a dynamic model of weight change, which shows why the fixed 3,500-calorie rule overstates long-run loss. | 1 |
| Investor.gov: Price-to-Earnings Ratio | Official SEC investor-education definition of the P/E ratio and how it is read. | 1 |
| MedlinePlus: Cholesterol | Official NIH health information on LDL, HDL, triglycerides and how cholesterol levels are read. | 1 |
| NHLBI: High Blood Pressure | Official NIH guidance on blood pressure readings, categories and what they mean. | 1 |
| Tanaka, Monahan and Seals (2001): Age-predicted maximal heart rate revisited | The peer-reviewed analysis behind the 208 minus 0.7 times age formula. | 1 |
| U.S. Bureau of Economic Analysis: What to Know About GDP | Official U.S. explanation of gross domestic product and the expenditure approach used to measure it. | 1 |
| Investor.gov: Options | Official SEC investor-education definition of options, calls, puts, premiums and the risks of trading them. | 1 |
| NHLBI: Stages of Sleep | Official NIH description of sleep stages and the roughly 80 to 100 minute cycles they repeat in, four to six times a night. | 1 |
| Interest Act (Canada), section 6 | The federal law that governs how interest on a mortgage on real property must be stated, including yearly or half-yearly calculation, which is why fixed-rate Canadian mortgages compound half-yearly. | 1 |
Frequently asked questions
What earns a source a place in the ledger?
A source must be the actual authority that defines a term, sets a rule or publishes an official figure: a regulator, a statistical agency or the body that administers the programme in question.
What do Grade A and Grade B evidence mean?
Grade A is a controlling regulator, statute, official public dataset or authoritative disclosure. Grade B is qualified professional guidance or the convergence of several credible technical sources.
Are commentary, vendor material or secondary summaries ever cited?
No. They are never cited as evidence, however convenient, because a secondary source can restate a primary rule incorrectly without anyone noticing.
How many sources are in the ledger?
63 primary sources, cited across the 663 calculators. The table on this page shows how many calculators cite each one.
Which source is cited most?
SEC Investor.gov: financial calculators, cited by 117 calculators. The table on this page lists every source with the number of calculators that cite it.
Does a citation mean the cited body endorses this site?
No. A citation implies no affiliation or endorsement. Free Calculators Online is independent of every organisation listed.
Does a source prove that a calculated result is correct?
No. A citation establishes what a definition or rule currently is, and a citation never establishes that a calculated result is correct for your own circumstances.
What happens when a source link breaks?
Source links are re-checked as part of every release, and a link that has moved or broken is treated as a correction in its own right, because a citation nobody can follow is not evidence.