Simple Interest Calculator
Enter any three of principal, rate, time, and interest — the fourth is solved instantly with the full I = Prt working shown.
With interest compounded yearly at the same terms you would have 157.63 in interest — 7.63 more than simple interest. For monthly or daily compounding, try the compound interest calculator.
Estimates for information only — actual products may use different day-count or compounding rules.
How it works
- Choose which variable to solve for: interest, principal, annual rate, or time.
- Enter the three known values, picking years, months, or days for the time period.
- Read the solved value, the substituted I = Prt steps, and the comparison against compound interest at the same terms.
Frequently asked questions
- How is simple interest calculated?
- Simple interest uses the formula I = P × r × t, where P is the principal, r is the annual rate as a decimal, and t is the time in years. Interest is charged only on the original principal, so it grows linearly — it never earns interest on itself. This calculator shows each substitution so you can follow the arithmetic.
- What is the difference between simple and compound interest?
- Simple interest applies the rate to the principal alone, while compound interest applies it to the principal plus all previously earned interest. Over short terms the two are close, but the gap widens with time and rate. The results panel here shows exactly how much more (or less) annual compounding would produce at your terms.
- Can I solve for the rate or the time instead of the interest?
- Yes. The solve-for selector rearranges the formula for you: P = I ÷ (r × t), r = I ÷ (P × t), or t = I ÷ (P × r). Enter the three values you know and the calculator returns the missing one along with the rearranged formula and its substitution.
- How are months and days converted to years?
- The time you enter is converted to years before the formula runs: months are divided by 12 and days by 365, which is the ordinary (banker’s unofficial) convention for annual rates. Some loan contracts use a 360-day year instead, so results on day-counted products may differ slightly from a lender’s statement.
- Is my financial data uploaded anywhere?
- No. Every calculation runs in JavaScript inside your browser tab, and the page makes no network requests with your numbers. You can disconnect from the internet after loading the page and it will keep working, which makes it safe to use with real account figures.
About this tool
This simple interest calculator computes interest and total repayment from a principal, an annual rate, and a time period — or works backwards to find whichever of the four variables you do not know. The core formula is I = P × r × t: principal times the annual rate (as a decimal) times the time in years. Because the interest never compounds, the result grows in a straight line, which is why simple interest is common for short-term personal loans, car loans in some markets, late-payment penalties, and bonds that pay coupons out rather than reinvesting them.
Everything runs client-side in your browser. As you type, the calculator converts your time period to years (months ÷ 12, days ÷ 365), substitutes your numbers into the formula, and prints each step of the working — the symbolic formula, the substituted line, and the final value. Nothing is sent to a server, so it is safe to use with real loan or savings figures, and results update live with no submit button.
The solve-for-any-variable mode makes the tool useful beyond the textbook case. Lenders often quote a total payoff figure without stating the rate; enter the principal, interest, and term to recover the effective annual rate. Savers can ask how long a deposit needs to sit at a given rate to earn a target amount, and students can check homework in any of the four rearrangements without doing the algebra by hand.
A practical tip: always compare the simple-interest result against compounding before committing to a product. The calculator shows what annual compounding would yield at identical terms — over one year the two match, but at 8% over 10 years compound interest earns roughly 36% more. If a savings product compounds, the linked compound interest calculator models it properly with monthly, quarterly, or daily compounding.