Two years of simple interest
A 5,000 principal at 6% simple annual interest for two years earns 600. Principal plus interest is 5,600. Interest is not added to principal to earn more interest.
Simple interest uses the original principal only: I = P × r × t. Choose the missing value. There are no periodic payments or interest-on-interest.
Calculated interest
USD 600.00
USD 5,000.00Principal
USD 600.00Interest
USD 5,600.00Principal + interest
USD 5,000.00 × 0.06000000 × 2.000000 years = USD 600.00.
Days are divided by your selected 360 or 365 denominator, with no calendar dates or leap-year adjustment. A zero rate or zero duration gives zero interest; solving an unknown with a zero divisor has no unique solution.
Your inputs and calculations stay in this browser. No live financial data is fetched; amounts use your selected USD or GBP labels.
How to use the Simple Interest Calculator
- 01
Choose the unknown value you want to calculate.
- 02
Enter the remaining known values; use interest alone rather than principal plus interest.
- 03
Select a day-count basis when entering days, then copy or download the result and its inputs.
Examples
Day-count conventions differ
A 1,000 principal at 12% for 30 days earns about 9.86 using 365 days per year, or 10.00 using 360 days per year.
The formula and its units
Simple interest is I = P × r × t, where r is the annual decimal rate and t is time in years. This tool rearranges that equation for whichever quantity is unknown. A rate entered as 6 is interpreted as 6%, or 0.06 in the multiplication.
When days become years
For a day input, time in years equals elapsed days divided by the chosen denominator. Entering 90 days with a 360-day denominator gives 0.25 years. This is a user-selected arithmetic convention, not a determination of a particular contract’s day-count method.
Zero values and unique answers
Known principal, rate, or time can be zero when calculating interest, which then becomes zero. When solving a different unknown, a zero multiplier can make the equation impossible or leave infinitely many answers. The tool reports that the answer is not unique rather than producing an infinite or misleading value.
What to know
- No installments, changing principal, fees, tax rules, calendar dates, or leap-day logic.
- Known annual rates are restricted to 0–100%; computed unknowns may exceed that input range.
Common questions
Can this model a loan with monthly payments?
No. The principal stays unchanged throughout the elapsed time. Use the loan or amortization calculator for a declining balance.
Does it compound annually?
No. Simple interest applies only to principal. Compound interest also applies to accumulated interest.
Why is solved time shown in years?
The equation uses an annual rate, so its natural time unit is years. A 365-day equivalent is displayed for convenience; it is not a calendar-date calculation.