Calculate simple interest

Enter the principal, the yearly interest rate and the time in years, months or days to see the interest, the total amount and the interest per month.

Principal

Interest rate (% a year)

For a monthly rate, multiply by 12.

Time

Time in

YearsMonthsDays

Calculations run on this device. We don't see, store or log the amounts you enter.

How it works

Enter the principal: the amount lent, borrowed or deposited.
Enter the yearly interest rate and the time, and choose years, months or days.
Read the interest, the total amount and the interest per month, or tap Copy.

The formula

Simple interest = P × R × T ÷ 100, where P is the principal, R the yearly rate in percent and T the time in years. 10,000 at 8% a year for 2 years earns 10,000 × 8 × 2 ÷ 100 = 1,600, so the total is 11,600. For months, T = months ÷ 12; for days, T = days ÷ 365.

Simple vs compound interest

Simple interest is paid only on the original principal, so it grows by the same amount every year. Compound interest is also paid on interest already earned: 10,000 at 8% for 10 years earns 8,000 simple interest but 11,589 compounded yearly. Use simple interest for short loans between people, some deposits and gold or vehicle loans quoted as simple interest; for savings that compound, use the compound interest calculator.

Interest for a number of days

For short periods many banks count the exact days on a 365-day year: 100,000 at 7.5% for 90 days earns 100,000 × 7.5 × 90 ÷ 36,500 = 1,849.32. Some lenders use a 360-day year instead, which gives slightly more interest (1,875 here), so check how your agreement counts days.

Worked examples

Simple interest and total amount
Principal	Yearly rate	Time	Interest	Total amount
10,000	8%	2 years	1,600.00	11,600.00
50,000	12%	6 months	3,000.00	53,000.00
100,000	7.5%	90 days	1,849.32	101,849.32
25,000	10%	3 years	7,500.00	32,500.00
200,000	6.5%	18 months	19,500.00	219,500.00

Frequently asked questions

How do I calculate simple interest per month?

Divide the yearly interest by 12: 100,000 at 12% a year earns 12,000 a year, which is 1,000 a month. The calculator shows this as Interest per month.

My rate is per month, not per year. What do I enter?

Multiply the monthly rate by 12: 2% a month is 24% a year.

Can I use it for an Islamic financing profit rate?

Islamic banks quote a profit rate instead of interest. If the profit is a flat percentage of the amount per year, this calculator gives the same figure; the contract decides what you actually pay.

Sources

Interest: simple interest formula (Wikipedia)

Last reviewed: October 4, 2026