Use this simple interest calculator to find the interest on a loan or savings deposit when interest is charged only on the original amount. Enter the principal, the yearly rate and the time in years, months or days.
Simple interest formula
Interest = principal × rate × time
The rate is written as a decimal (5% = 0.05) and time is in years, so 18 months is 1.5 years and 90 days is 90 ÷ 365 years. The total you repay or receive is the principal plus the interest.
Worked example
You deposit $10,000 at 5% simple interest for 3 years. Interest = 10,000 × 0.05 × 3 = $1,500, so the total after 3 years is $11,500.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is calculated only on the original principal. Compound interest is also calculated on interest already earned, so it grows faster over time. Use the compound interest calculator to compare.
Where is simple interest used?
Many car loans, short-term personal loans, some bonds and certain savings products use simple interest. Credit cards and most savings accounts compound.
Why do some lenders use a 360-day year?
Some banks use a 360-day “banker’s year” for daily interest, which produces slightly more interest than a 365-day year. Check your loan agreement.