Simple interest calculator
Calculate simple interest on a principal amount over a set period. Shows interest earned, total amount and a clear formula breakdown.
- Free, no sign-up
- No data stored
- Reviewed 2026-01-15
Your details
Results
- Principal
- £5,000.00
- Total value
- £5,900.00
- Daily interest
- £0.8219
- Compound comparison
- £955.08
£55.08 more with compound interest
- Simple interest
- Compound interest
About the Simple Interest Calculator
Simple interest is calculated only on the original principal — it does not compound. Enter the principal, annual rate and duration to see total interest and the final amount.
This method is used for short-term personal loans, Treasury bills and some savings products where interest is not reinvested.
How to use the Simple Interest Calculator
- Enter the principal amount.
- Set the annual interest rate.
- Enter the duration and select the unit (days, months or years).
- View the interest earned and total amount.
Formula and method
I = P × r × t where P = principal, r = annual rate (decimal), t = time in years.
Frequently asked questions
When is simple interest used?
Short-term loans, car finance (flat rate), Treasury bills, and promissory notes commonly use simple interest.
How is it different from compound interest?
Simple interest is always calculated on the original principal. Compound interest calculates on principal plus accumulated interest, leading to faster growth.
Can I calculate for days?
Yes. Select "days" as the duration unit. The calculator divides by 365 (or choose a 360-day convention in some markets).