Free Calculators Online learning center
Calculator guides
46 plain-language guides to the formulas, risks and tradeoffs behind the calculators — no product pitch, no recommended provider. Each works a real example and names the mistakes that produce a plausible-but-wrong answer.
How the library is spread
Guides by topic
The most-covered topics across the 46 guides.
- Retirement5 guides
- Investing4 guides
- Mortgage3 guides
- Household3 guides
- Energy3 guides
- Business finance3 guides
- Loans2 guides
- Taxes2 guides
- Business2 guides
- Fixed income2 guides
- Startup2 guides
- Math2 guides
- Time2 guides
- Debt1 guides
- Auto finance1 guides
- Real estate1 guides
- Freelance1 guides
- Education1 guides
- Physics1 guides
- Health1 guides
- Electronics1 guides
- Chemistry1 guides
- Home1 guides
- Statistics1 guides
How loan payments work
Understand amortization, APR, term length and the difference between a smaller payment and a lower-cost loan.
Read guide →MortgageThe full monthly cost of a mortgage
Build a housing estimate that includes principal, interest, taxes, insurance, association dues and upfront cash.
Read guide →InvestingCompound interest and investment projections
Use future-value estimates responsibly by separating contributions, growth, time and uncertainty.
Read guide →RetirementA practical retirement planning framework
Connect spending, fixed income, portfolio withdrawals, saving rate and uncertainty in one repeatable process.
Read guide →DebtHow to build a credit card payoff plan
Turn balances, APRs and monthly cash flow into a payoff date while avoiding common promotional-rate traps.
Read guide →TaxesHow the 2026 federal tax estimate works
Understand progressive brackets, standard deductions and the limits of a simplified federal ordinary-income estimate.
Read guide →Auto financeHow to compare auto financing and leases
Compare payment, amount financed, total interest, depreciation, residual value and ownership horizon.
Read guide →Real estateRental property metrics that work together
Understand NOI, cap rate, cash-on-cash return, DSCR, GRM and break-even occupancy without mixing their definitions.
Read guide →BusinessEssential business finance ratios
Use margin, markup, break-even, working capital, current ratio, burn and runway without confusing their purposes.
Read guide →LoansHow to read an amortization schedule
Follow a level-payment loan from opening balance to final payoff and understand why early payments contain more interest.
Read guide →MortgageA complete rent-versus-buy decision framework
Compare unrecoverable costs, equity, transaction costs, flexibility and holding period without treating rent as wasted money.
Read guide →MortgageARM and interest-only mortgage risk
Understand reset rates, caps, payment shock and the difference between temporary payment relief and permanent affordability.
Read guide →Fixed incomeBond price, yield and duration explained
Connect coupon cash flows, market price, yield to maturity, duration and DV01 in one fixed-income framework.
Read guide →Fixed incomeT-bills, CDs and maturity ladders
Compare short-term yield conventions and design a ladder around liquidity dates , not chasing one headline rate.
Read guide →InvestingIRR, time-weighted and annualized return
Choose the return measure that matches the question and avoid misleading comparisons across cash-flow patterns.
Read guide →InvestingInvestment fees and risk metrics
Read expense drag, Sharpe, Sortino and maximum drawdown as complementary measurements , not a single scorecard.
Read guide →RetirementRMD and QCD planning
Use prior year-end balances, official life-expectancy factors and charitable distribution rules without turning a planning estimate into tax advice.
Read guide →RetirementRoth conversion and bracket planning
Model conversion tax, bracket room, IRA basis and future tax-rate assumptions before moving retirement assets.
Read guide →RetirementSocial Security claiming and pension choices
Compare break-even age, survivor value, inflation protection and present value without reducing lifetime income to one crossover date.
Read guide →RetirementFIRE, Coast FIRE and retirement target variants
Translate lifestyle spending into Lean, traditional, Fat and Coast FIRE targets while keeping withdrawal-rate uncertainty visible.
Read guide →FreelanceFreelancer and contractor pricing
Replace salary, benefits, taxes, overhead and nonbillable time before setting an hourly or project rate.
Read guide →TaxesQuarterly estimated tax and safe harbor
Separate projected liability from payment-timing protection and update the estimate as self-employment income changes.
Read guide →BusinessWorking capital and the cash conversion cycle
Connect inventory days, receivable days, payable days, liquidity ratios and cash forecasting.
Read guide →StartupSaaS revenue, retention and unit economics
Define MRR, ARR, NRR, GRR, churn, LTV and CAC consistently before using them for planning or valuation.
Read guide →StartupStartup valuation and dilution
Connect pre-money value, post-money value, new investor ownership and existing-holder dilution before a financing round.
Read guide →HouseholdHealth plan, HSA and FSA cost comparison
Compare premiums, expected care, deductibles, coinsurance, maximum exposure and tax-advantaged accounts.
Read guide →HouseholdEmergency funds, sinking funds and financial runway
Separate known future expenses from true emergencies and use depletion modeling to set action deadlines.
Read guide →HouseholdHow to estimate life insurance need
Convert survivor income, debts, education, final expenses, assets and existing coverage into a transparent range.
Read guide →EducationCollege cost, savings and student debt planning
Combine net price, education inflation, 529 growth, aid uncertainty and student-loan terms.
Read guide →EnergyHow to size a backup generator
Build an essential-load list, separate running watts from starting surge and choose a generator, battery or both without buying too big or too small.
Read guide →MathGeometry for DIY and design: area, volume and angles
Break odd shapes into simple ones, choose the right formula for the shape in front of you and avoid the mistakes that cost material and money.
Read guide →PhysicsPhysics formulas for everyday questions
Use the standard formulas for speed, force, energy, work, power and projectiles to answer practical questions, and know where the textbook model stops working.
Read guide →HealthWhat body and nutrition calculators can tell you
Use BMI, healthy weight ranges, calorie deficits, macronutrient ranges and heart rate estimates as screening tools, and know when to ask a clinician.
Read guide →MathFractions, decimals and percentages explained
Move between the three ways of writing a part of a whole, add and simplify fractions, and choose the right percent measure for a change or a difference.
Read guide →ElectronicsElectronics basics: Ohm's law, resistors, dividers and LEDs
Use Ohm's law, resistor color codes, voltage dividers, RC circuits and LED resistors to size parts for a simple circuit, and know the limits of the formulas.
Read guide →ChemistryLab calculations: dilutions, pH, half-life and yield
Work through common bench and classroom calculations, from diluting a stock solution to converting a trace concentration, and avoid slips that spoil results.
Read guide →Business financeOperations metrics: OEE, takt time and safety stock
Measure how well a line runs, set its pace from demand, buffer inventory against uncertainty and size a queue from two numbers you can observe.
Read guide →Business financeMarketing metrics: conversion rate, CTR, CPC, CPA and ROAS
Read an ad campaign or a website through a few ratios, know what each one measures and avoid optimizing the wrong number.
Read guide →EnergyHow to work out what an appliance costs to run
Turn a wattage label, the hours of use and your electricity rate into a monthly and yearly cost, then decide which changes are worth making.
Read guide →TimeTime cards, decimal hours and overtime explained
Convert clock times into paid hours, handle breaks and shifts that cross midnight, and check gross pay before a paycheck goes out.
Read guide →HomeHow to estimate materials for a home project
Measure a room correctly, subtract openings, add the right waste allowance and round up to what suppliers sell, for paint, drywall, flooring and fencing.
Read guide →Business financeDepreciation methods compared
See how straight-line, double-declining balance, sum-of-the-years'-digits and units-of-production depreciation spread the same cost differently.
Read guide →StatisticsA/B tests, p-values and Bayes' theorem
Learn what a p-value does and does not say, how to run a fair test, and why a positive result from a good test can still be wrong.
Read guide →EnergyComparing fuel economy: mpg, L/100 km and electric cars
Convert between fuel economy units, measure your real mileage and compare gasoline and electric cars on cost per mile.
Read guide →InvestingHow to read stock valuation ratios
Understand P/E, PEG, dividend payout, earnings per share, market cap, enterprise value, book value and the Graham number, and what each one leaves out.
Read guide →TimeSleep cycles and how to time a bedtime
Understand what a sleep cycle is, how to count backward from a wake-up time and why the result is a scheduling aid and not a prescription.
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