About the Simple Interest Calculator
Simple interest is interest worked out only on the original amount, never on interest already earned. This calculator finds the interest and the total amount from the principal, the yearly rate and the time, which you can enter in years, months or days. Switch the Find setting and it solves for the rate, the time or the principal instead, and it writes out the formula with your numbers filled in. Days are counted on a 365-day year. Simple interest does not compound, and real loans and deposits often do, so use the Compound Interest Calculator for those. The figures are estimates for information only, not financial advice.
Updated September 2026
How to use the Simple Interest Calculator
- 1Choose what to find: Interest & total, Rate, Time or Principal.
- 2Fill in the other boxes. Time can be years, months or days.
- 3Read the answer and the formula with your numbers in it.
- 4For Interest & total, check the year-by-year table.
- 5Press Copy answer to keep the result.
The simple interest formula
The formula has three inputs. The principal P is the starting amount. The rate R is the yearly percentage. The time T is measured in years. Interest is I = P × R × T / 100, and the total is P + I. Because the interest is always taken on P alone, doubling the time doubles the interest.
The year-by-year table lists the cumulative interest at the end of each whole year, so you can see the straight-line growth. Fractions of a year are used in the answer, but the table shows whole years only and stops at 50 rows.
Months and days
Interest for part of a year is found by converting the time to years. Six months is 0.5 years and 90 days is 90 divided by 365, about 0.2466 years. On 10,000 at 5 percent, 90 days earns about 123.29. Banks may use a 360-day year or count days differently, so their figure can differ slightly from this estimate.
Simple vs compound interest
To see both side by side, try your numbers here and then in the Compound Interest Calculator.
- Simple: the same interest is added each year
- Compound: each year's interest earns interest of its own
- Over short periods the two are close
- Over many years, compound interest pulls far ahead
Where simple interest is used
You meet simple interest in short-term personal loans, some savings bonds, late-payment charges and lessons on percentages. For regular monthly investing, the SIP Calculator is more suitable. If you need a quick percentage of an amount, the Percentage Calculator is faster. Results here are estimates and do not include tax, fees or penalties.
Features
- Interest and total amount
- Solve for rate, time or principal
- Time in years, months or days
- Formula shown with your numbers
- Year-by-year table of interest and total
- Currency symbol picker
- Runs in your browser
Frequently asked questions
What is the simple interest formula?
Interest equals principal times rate times time, divided by 100, when the rate is a yearly percentage and the time is in years. For 10,000 at 5 percent for 3 years that is 10,000 × 5 × 3 / 100 = 1,500, so the total is 11,500.
How do I calculate simple interest for months or days?
Turn the time into years first: months divided by 12, or days divided by 365. Eighteen months is 1.5 years, so 10,000 at 5 percent earns 750. Choose Months or Days in the Time unit and the page converts it for you.
What is the difference between simple and compound interest?
Simple interest is calculated on the original principal only, so it grows by the same amount every year. Compound interest is calculated on the principal plus earlier interest, so it grows faster over time. The Compound Interest Calculator shows the compound case.
How do I find the interest rate from the interest amount?
Rearrange the formula: rate equals interest times 100, divided by principal times time. Choose Rate in the Find setting and enter the principal, the interest earned and the time. For 1,500 interest on 10,000 over 3 years the rate is 5 percent.
How do I find the time period?
Time equals interest times 100 divided by principal times rate. Choose Time in the Find setting. The answer is given in years, and also in months and days.
Does simple interest apply to home loans?
Usually not. Most home loans and credit cards charge interest on a reducing balance or compound it, so the Loan Calculator is the better tool. Simple interest is common for short personal loans, some bonds and school problems.
Can I find the principal if I know the interest?
Yes. Choose Principal, then enter the interest earned, the yearly rate and the time. The principal equals interest times 100 divided by rate times time.