Simple Interest Calculator
Calculate interest, total amount, principal, rate, or term with simple interest, and compare it with compound interest.
Processed on your device. Nothing you type is sent anywhere.
How to use: Simple Interest Calculator
- Choose what you want to find: interest and total amount, principal, rate, or time.
- Enter the other values, then choose the rate unit (day, month, or year) and the time unit.
- See the result, the formula used, and how it compares with compound interest.
About this tool
With simple interest, interest for each period is calculated only on the original principal: J = C × i × t and M = C + J. For example, $1,000 at 2% per month for 6 months earns $120 in interest, for a total of $1,120. With compound interest, the same scenario would total $1,126.16.
The rate and time must use the same unit. The calculator converts automatically using a 360-day year (12 30-day months), a convention used in some financial calculations. Simple interest is used for late fees and overdue interest, commercial discounts, and practice problems.
Frequently asked questions
What’s the formula for simple interest?
J = C × i × t, where C is the principal, i is the rate (as a decimal), and t is the time in the same unit as the rate. The total amount is M = C × (1 + i × t).
What’s the difference between simple and compound interest?
With simple interest, interest is always calculated on the original amount. With compound interest, it’s calculated on the growing balance (interest on interest). Over more than one period, compound interest always earns more.
How do you calculate the simple interest rate?
Select “Interest rate” under “What do you want to find?” and enter the principal, total amount, and time. The formula is i = (M ÷ C − 1) ÷ t.
Is it really free? Is there a daily limit?
It's free with no daily limits. Processing happens on your device, so there's no per-use cost and no need to limit you.
Does it work on phones?
Yes. The site works in any modern browser on Android, iPhone, Windows, Mac, and Linux—no installation needed.
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