Compound Interest Calculator
Calculate compound interest with optional regular deposits. See the future balance, total interest, APY and a year-by-year growth table.
Compound interest means each period's interest is added to the balance, so the next period earns interest on a slightly larger amount. This calculator projects a savings or investment balance from a starting deposit, an interest rate, the compounding frequency and optional regular deposits. It also splits the final amount into your own deposits and interest, so you can compare savings accounts, CDs or a long-term saving plan.
How to use this tool
- Enter your starting deposit and the annual interest rate the account pays.
- Choose how often interest compounds — savings accounts usually compound daily or monthly.
- Add a regular deposit and how often you make it, or leave it at 0 for a lump sum.
- Pick whether deposits go in at the start or the end of each period, then read the balance and the year-by-year table.
Formula
A = P × (1 + r/m)^(m·t) + D × ((1 + i)^(k·t) − 1) / i
P is the starting deposit, r the annual rate as a decimal, m the number of compounding periods per year and t the number of years. D is the regular deposit made k times a year, and i = (1 + r/m)^(m/k) − 1 is the interest rate for one deposit period. For start-of-period deposits multiply the second part by (1 + i). This formula assumes whole deposit periods; partial periods count only deposits already made and grow them to the end date. At 0%, add the deposits without interest. APY = (1 + r/m)^m − 1.
Example
Put $10,000 in an account paying 5% compounded monthly and add $200 at the end of every month for 10 years. The starting deposit grows to $16,470.09 and the 120 deposits grow to $31,056.46, so the balance is $47,526.55. You paid in $34,000, which means $13,526.55 is interest. The APY is 5.116%.
Frequently asked questions
What is the difference between APR and APY?
APR is the stated yearly rate before compounding. APY includes the effect of compounding: 5% compounded monthly is an APY of about 5.116%. If your bank only quotes an APY, enter it here with annual compounding.
Does the compounding frequency make a big difference?
Less than most people expect. $10,000 at 5% for 10 years grows to $16,288.95 with annual compounding and $16,470.09 with monthly compounding — about $181 more. The rate and the length of time matter far more.
How long does it take to double my money?
A quick estimate is the rule of 72: divide 72 by the interest rate. At 6% money doubles in roughly 12 years. Enter your own numbers here for the exact figure.
Are taxes and inflation included?
No. Interest in an ordinary taxable account is usually taxed every year, and inflation reduces what the final balance can buy. Enter an after-tax or inflation-adjusted rate to see those effects.
This tool gives an estimate for general information only. It is not financial, tax, legal or medical advice — check with a qualified professional before you rely on the result.
Last updated: 2026-10-30