Compound Interest Calculator
This free compound interest calculator shows what a lump sum turns into when interest itself starts earning interest — the effect Einstein allegedly called the eighth wonder of the world.
How to use
- Enter the starting principal.
- Enter the annual interest rate and the number of years.
- Pick how often interest compounds per year and add an optional per-period contribution.
- Read the final value, total invested and interest earned — instantly, on your device.
Frequently asked questions
What formula does the calculator use?
The standard compound interest formula: FV = P(1 + r/n)^(nt) + PMT·[((1 + r/n)^(nt) − 1)/(r/n)], where P is the principal, r the annual rate, n the compounding frequency, t the years and PMT the contribution added at the end of each compounding period. Example: 10,000 at 5% compounded monthly for 10 years gives about 16,470.09.
How much difference does the compounding frequency make?
More frequent compounding credits interest sooner, so each subsequent period earns interest on it. 10,000 at 5% for 10 years ends at about 16,288.95 compounded annually, 16,470.09 monthly and 16,486.65 daily. The gap grows with both the rate and the number of years.
When are contributions added?
At the end of every compounding period — a deposit made with the monthly option is added after that month’s interest is credited, so it does not earn interest for the period it arrives in. This matches how most recurring deposits actually settle.
Does the result include taxes, inflation or fees?
No. The calculator models pure interest arithmetic. Taxes on interest, inflation eroding purchasing power, account fees and investment risk are all excluded, so treat the final value as a nominal upper reference before those real-world deductions.
What happens at a 0% rate?
The formula degenerates to a simple linear sum: the final value is the principal plus every contribution, with no growth. That is the mathematical limit of the formula as the rate approaches zero, so a 0% case is computed exactly rather than failing.
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About this tool
This free compound interest calculator shows what a lump sum turns into when interest itself starts earning interest — the effect Einstein allegedly called the eighth wonder of the world. Enter four numbers: the starting principal, the annual rate, the number of years, and how often interest compounds each year (1, 2, 4, 12 or 365 times). The final value appears instantly together with the total you put in and exactly how much of the result is interest. You can also add an optional regular contribution deposited at the end of every compounding period, which turns the tool into a savings-plan projector: 10,000 at 5% for 10 years with monthly compounding grows to about 16,470.09 on its own, and to about 31,998.32 if you add 100 every month. The calculation uses the standard formula FV = P(1 + r/n)^(nt) + PMT·[((1 + r/n)^(nt) − 1)/(r/n)], and a 0% rate cleanly degenerates to a simple linear sum. Results update live as you type and everything runs locally in your browser. Note the scope: the result is pure interest arithmetic — it does not model taxes, inflation, fees or market risk.