Interest Calculator
Calculate simple or compound interest, compare compounding frequencies, and review the projected balance over time.
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This calculator provides estimates. Actual financial products may use different day-count conventions, fees, or compounding rules.
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Growth schedule
| Period | Opening balance | Interest | Closing balance |
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How interest is calculated
Simple interest formula
Interest = Principal × Rate × Time, where the rate and time are expressed in matching units. Simple interest is calculated on the original principal only.
Compound interest formula
A = P(1 + r/n)nt, where P is principal, r is the annual rate as a decimal, n is the number of compounding periods per year, and t is time in years. For a rate quoted monthly, the monthly rate is converted to an annual nominal rate for the formula.
How to use this calculator
- Enter the principal and interest rate.
- Choose whether the rate is quoted per year or per month.
- Enter the duration and choose years, months, or days.
- Select simple or compound interest; for compound interest, choose a compounding frequency.
- Select a display currency and click Calculate interest.
Frequently asked questions
What is the difference between simple and compound interest?
Simple interest is based only on the initial principal. Compound interest adds accrued interest to the balance, so future interest can be earned on earlier interest.
Can I enter an interest rate per month?
Yes. Choose “Per month” when the quoted rate is monthly. The rate period is used consistently with the duration for simple interest and converted to a nominal annual rate for compound calculations.
How are days handled?
For this estimate, a year is treated as 365 days and a month as one-twelfth of a year. Lenders and financial institutions may use different conventions.
Does this calculator include taxes or fees?
No. Results show mathematical interest estimates only and do not include tax, fees, inflation, or product-specific rules.