Compound Interest Calculator

See how your money can grow — enter your numbers and watch the future value, contributions and interest add up.

📖 How it works & FAQ

Starting amount ($) Annual interest rate (%) Years Compounds per year Monthly contribution ($)

Future value

Total contributed

Interest earned

What compound interest actually does

Compound interest is the interest you earn on your interest. Instead of only your original deposit earning a return, every bit of growth gets added back to the balance and starts earning too. Over a few years the effect is modest, but over decades it becomes the main driver of your total — which is why starting early matters so much.

This calculator uses the standard formula FV = P(1 + r/n)^nt for your starting amount, where P is the principal, r is the annual rate as a decimal, n is how many times a year interest compounds, and t is the number of years. If you add a monthly contribution, its future value is calculated as a monthly annuity and added on top, so you see the combined result.

Contributions vs. interest

The two smaller boxes split your future value into money you put in versus money the account earned for you. Early on, "total contributed" dominates. Given enough time and a decent rate, "interest earned" can overtake everything you deposited — that crossover is the whole point of investing for the long term.

How to use it

Enter your starting amount (your current balance or opening deposit).
Type the expected annual interest rate as a percent, e.g. 7 for 7%.
Set the number of years you'll leave it invested.
Pick how often interest compounds — monthly is common for savings.
Add an optional monthly contribution to model regular deposits.
Read the future value, then check the split of contributions vs. interest below it.

FAQ

Does compounding frequency really matter?: A little. More frequent compounding (daily vs. annual) raises the result slightly at the same rate, but the rate itself and the time horizon matter far more.
Are contributions added at the start or end of the month?: This tool assumes end-of-month deposits (an ordinary annuity). Depositing at the start of each month would grow slightly faster.
Does it account for taxes or inflation?: No. It shows nominal growth only. Real spending power will be lower after inflation and any taxes on gains.
Is my data uploaded anywhere?: No. Everything runs in your browser and nothing is sent to a server. Results are estimates for planning, not financial advice.