Interest
Calculate simple interest and final value, or solve the same formula for principal, annual rate, or time.
Simple interest
$1,500.00
Final amount
$11,500.00
Principal
$10,000.00
Annual rate
5%
Time
3 yr
Result comparison
This is a mathematical estimate. Fees, taxes, day-count conventions and changing rates are not included.
Calculation method
I is interest, P is principal, r is the annual rate written as a decimal, t is time in years, and A is the final amount. A 5% rate is entered as 5 and converted to 0.05 for the formula.
Detailed guide
A simple interest calculator applies the annual rate only to the original principal. Interest earned in one period does not become part of the balance used for the next period. The result therefore grows in a straight line rather than accelerating over time.
Use the first mode to calculate interest and final value. The other modes rearrange the same equation to find the starting principal, annual rate, or time required to produce a known interest amount. Keeping these questions on one page prevents a partial calculator from forcing users to solve the formula manually.
I is interest, P is principal, r is the annual rate written as a decimal, t is time in years, and A is the final amount. A 5% rate is entered as 5 and converted to 0.05 for the formula.
To solve principal, divide interest by rate and time. To solve rate, divide interest by principal and time. To solve time, divide interest by principal and rate.
Decision guide
High-ranking pages contrast simple and compound interest, show the formula, and identify products that may use daily or periodic simple interest. The key assumption is that prior interest is not added to principal for later interest calculations.
Simple interest equals principal multiplied by the periodic rate and number of periods. Keep the units aligned: an annual rate needs time in years, while a daily calculation needs a daily convention. Convert percentages to decimals before multiplying.
For an installment loan, payments reduce principal over time, so interest may be calculated on the outstanding daily balance rather than the original amount for the entire term. This calculator models the inputs shown; it is not automatically an amortization schedule.
If earned interest remains in an account and begins earning interest, use a compound-interest model. Compare both methods over longer terms to see why the difference grows. Fees and taxes remain outside either basic formula unless added separately.
Save a baseline, then change only one input at a time: amount, rate, term, or recurring payment. Record the supporting outputs as well as the headline result—contributions, interest, total cost, remaining balance, or target date. This isolates the assumption that actually changes the decision instead of hiding it inside several simultaneous edits.
Do not tune every assumption until the calculator produces a preferred answer. Start with a cautious case, follow with a central case, and treat the favorable case as an upside scenario. A calculator result is not a lender quote, market forecast, or tax determination; actual dates, rounding, fees, contract rules, and required payments must be checked in primary documents.
Keep enough precision in the inputs and round only the output used for a decision. If the estimate supports a transaction, loan, tax calculation, or long-term plan, repeat it with figures from the contract or official statement. Any difference helps identify the assumption that still needs verification.
Some notes, private loans and simplified examples quote interest on the original amount. Confirm the contract method before relying on this result.
Compound interest adds earned interest to the balance. If interest is capitalized daily, monthly or annually, use the compound interest calculator instead.
This page uses years, including decimal years. Exact daily interest may use a 360-day or 365-day convention that produces a different result.
Multiply principal by the annual rate as a decimal and by time in years. Add the interest to principal to get the final amount.
Yes. Select annual interest rate, then enter principal, time and the known interest amount.
No. Monthly compounding changes the balance after each period and belongs in a compound-interest calculation.
Updated: August 1, 2026
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