Simple Interest Calculator
Free simple interest calculator: interest and total on a principal at a yearly rate for days, months or years, with a 365- or 360-day year.
What it does
Simple interest is calculated only on the original principal, never on interest already earned: interest = principal × yearly rate × time in years. It is used in some car and personal loans, short-term notes and many school exercises. For periods in days you can use the actual 365-day year or the 360-day "banker's year" that many contracts use.
How to use it
- Type the principal and the yearly interest rate.
- Type the time and choose years, months or days (and the days in a year).
- See the interest, the total and the interest per day.
The arithmetic runs in your browser. Nothing you type is sent anywhere and nothing is stored, so you can use these with real numbers.
Frequently asked questions
Simple or compound interest?
With simple interest the interest never earns interest, so it grows in a straight line. With compound interest it is added to the balance and grows faster. Over one year the difference is small; over many years it is large.
How do I convert a monthly rate?
With simple interest a monthly rate times 12 gives the yearly rate: 1% a month is 12% a year. With compound interest that is not true: 1% a month compounds to about 12.68% a year.
Why 360 days?
Many banks and contracts count a year as 360 days (the banker's year) for daily interest, which gives slightly more interest than 365 days. The contract says which one applies.
How do I find the rate or the time?
Rearrange the formula: rate = interest ÷ (principal × time), and time = interest ÷ (principal × rate). For example, 900 of interest on 5,000 over 3 years is 900 ÷ 15,000 = 6% a year.
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