Transparent method
The formula
Worked example
Example inputs
How to interpret the result
Simple interest is calculated only against the original principal for the entire term, so it never compounds. Unlike an amortized loan, where interest recalculates each period against a changing balance, simple interest produces the identical dollar interest charge for every period regardless of how much has already been paid.
Before you rely on it
What to check
Confirm whether the actual product is genuinely simple interest or an amortized loan being described loosely as one, since the two produce different total interest over an identical stated rate and term.
The common error
Where people go wrong with simple interest calculator
Applying amortized-loan intuition, where balance and interest both shrink together over time, to a simple-interest product. Simple interest charges the identical amount each period against the original principal, not a shrinking balance.
Sensitivity evidence
How annual rate changes the interest
| Annual rate | Interest | Ending balance |
|---|---|---|
| 4.0% | $1,200 | $11,200 |
| 5.0% | $1,500 | $11,500 |
| 6.0%worked example | $1,800 | $11,800 |
| 7.0% | $2,100 | $12,100 |
| 8.0% | $2,400 | $12,400 |
Every input, tested
Which input moves the interest most
| Input | Tested from | To | Interest at each end | Swing |
|---|---|---|---|---|
| Time | 2.0 years | 4.0 years | $1,200 to $2,400 | $1,200 (67%) |
| Annual rate | 5.0% | 7.0% | $1,500 to $2,100 | $600 (33%) |
| Principal | $9,000 | $11,000 | $1,620 to $1,980 | $360 (20%) |
Two variables at once
Interest by annual rate and principal
| Annual rate \ Principal | $8,000 | $10,000 | $12,000 |
|---|---|---|---|
| 4.0% | $960 | $1,200 | $1,440 |
| 5.0% | $1,200 | $1,500 | $1,800 |
| 6.0% | $1,440 | $1,800 | $2,160 |
| 7.0% | $1,680 | $2,100 | $2,520 |
| 8.0% | $1,920 | $2,400 | $2,880 |
Step by step
The worked example, input by input
| Input | Value used |
|---|---|
| Principal | $10,000 |
| Annual rate | 6.0% |
| Time | 3.0 years |
| Interest | $1,800 |
| Ending balance | $11,800 |
Where the inputs come from
Principal is the amount borrowed or invested at the start. The rate is the annual percentage, entered as a number such as 6 for 6%. Time is measured in years, so eighteen months is 1.5. A common slip is entering months in the years field, which makes the interest twelve times too large. If a note quotes a daily or monthly rate, convert it to an annual rate before using it.
When the result stops applying
Simple interest appears in short-term notes, some auto loans, certain bonds' coupons and many textbook problems. If the agreement says interest is added to the balance monthly or daily, use a compound interest tool instead.
What to compare it with
Put the same principal, rate and time into a compound interest calculator to see the difference. On a three-year, 6% example the gap is small, but on twenty years it becomes large, which is why savings and investments compound while some short-term notes are simple. If you are checking a contract, ask whether the rate is an annual rate and how the lender counts days, since a 360-day year makes interest slightly higher than a 365-day year. The gap is worth knowing on large notes.
People also ask
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is charged only on the original principal. Compound interest is charged on principal plus interest already added, which is why it grows faster.
Do auto loans use simple interest?
Many do, in the sense that interest accrues daily on the unpaid balance and is not added to principal. That is different from the flat-total method, where interest is computed once for the whole loan.
Can I use it for a term shorter than a year?
Yes. Enter the fraction of a year, for example 0.25 for three months or 0.5 for half a year.