Simple interest, solved any way round
Simple interest is charged only on the original amount, never on earlier interest. It's used for many short-term loans, promissory notes and personal loans between family and friends, and it's a staple of math classes. The formula is I = P × r × t.
This calculator solves it in every direction: choose Interest, Rate, Time or Principal and fill in the other values. Time can be entered in years, months or days, and the total amount (principal plus interest) is shown as well.
How to calculate simple interest
- 1
Choose what to find
Interest, rate, time or principal.
- 2
Enter the known values
Principal, yearly rate and time.
- 3
Read the result
Along with the total amount.
Key features
Four-way solver
Find I, P, r or t.
Flexible time units
Years, months or days.
Clear breakdown
Principal, interest and total.
Any currency
Dollars, euros, pounds and more.
Common uses
- Homework: Check simple interest answers step by step.
- Family loans: Agree fair interest on money lent to friends or relatives.
- Short-term notes: Estimate interest where nothing compounds.
Tips and good to know
- For periods in days, the calculator divides by 365, the convention most US banks use for consumer products.
- If interest is added to the balance over time, use the Compound Interest Calculator instead.
Frequently asked questions
What is the simple interest formula?
I = P × r × t, where P is the principal, r the yearly rate as a decimal and t the time in years. $10,000 at 5% for 3 years earns $1,500.
How do I find the interest rate?
Choose Rate and enter the principal, time and interest. The tool uses r = I ÷ (P × t).
How do I calculate interest for a number of months?
Pick Months as the time unit; the calculator divides by 12 for you.