CalcMyFin

Saving & Investing

Compound Interest Calculator

See how a starting balance and regular monthly deposits grow with compound interest. The chart shows how much of your future balance comes from your own contributions and how much from growth.

  • Free, no sign-up
  • Private: runs in your browser
  • Updated September 2026

Inputs

$
Amount invested or saved today.
$
Assumed contribution made throughout the year.
%
yr
This changes the mathematical compounding frequency, not the frequency of your contribution.

How to use the compound interest calculator

  1. Enter your starting amount and how much you'll add each month.
  2. Add an expected annual interest rate or return.
  3. Choose how many years and how often interest compounds.

How it's calculated

For a starting principal P, annual rate r compounded n times per year for t years:

A = P(1 + r/n)nt

Monthly contributions are added each compounding period and grow the same way. The calculator steps through every period so the result matches how a savings or investment account actually grows.

Example

Start with $10,000, add $300 a month and earn 7% compounded monthly. After 20 years you'd have about $196,665. You contributed $82,000, so roughly $114,665 came from compound growth.

Tips

  • Time matters more than amount. Starting 10 years earlier can double your final balance.
  • The Rule of 72: divide 72 by your rate to estimate how many years it takes money to double.
  • Automate monthly deposits so saving happens before spending.
  • Investment returns vary year to year. Use a conservative rate for planning.

Frequently asked questions

What is compound interest?
Interest earned on both your original money and the interest it has already earned. Over long periods this snowball effect means growth accelerates, which is why starting early is so powerful.
What rate should I use?
For savings accounts, use the account's APY. For long-term stock investing, many people plan with a conservative 5–7% average, but returns are never guaranteed and can be negative in any given year.
Does compounding frequency matter?
Somewhat. Daily compounding earns slightly more than monthly or yearly at the same rate, but the difference is small compared with the effect of your rate, deposits and time.
How is this different from simple interest?
Simple interest is paid only on the original principal. Compound interest pays interest on interest, so balances grow faster, and the gap widens every year.
Savings Goal calculatorHow much to save each month to hit a goal.Retirement calculatorProject your nest egg and retirement income.Inflation calculatorWhat money will be worth, and what things will cost, later.

Sources

This calculator provides educational estimates only and is not financial, tax, legal or investment advice. Results depend on the assumptions you enter; actual terms from lenders, insurers and tax authorities may differ.