Simple Interest
I = PRT — forwards, backwards, any variable you need.
Fill the three known values; the calculator solves the fourth.
Estimates only — not financial advice. Most real loans compound; see the compound interest calculator for those.
How to use simple interest math
- Choose what to solve for. Interest, total, principal, rate, or time.
- Fill the three known values. Rate is annual percent; time is in years.
- Press Calculate. The missing variable appears with the formula used.
- Compare with compounding in the compound interest calculator to feel the difference.
Frequently asked questions
What is the simple interest formula?
I = P × R × T: principal times annual rate (as a decimal) times years. Total owed is A = P + I.
When is interest simple vs compound?
Simple interest accrues only on the original principal — common in short-term loans and some auto financing. Compound interest accrues on principal plus past interest — that's most savings and mortgages.
How do I solve for the rate?
R = I / (P × T), then ×100 for percent. If $1,500 of interest came from $10,000 over 3 years, R = 1500/(10000×3) = 5%.
How do I solve for time?
T = I / (P × R). Time comes out in years when R is the annual rate.
Why is my real loan's interest higher than this?
Because it compounds — interest earns interest. Simple interest is the floor; compounding only adds.
Is this financial advice?
No. Planning estimates only.
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Simple Interest
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