How to Calculate Compound Interest in 30 Seconds (With Formula)

The compound interest formula looks scary — but it's actually simple. Follow our step-by-step examples and calculate your growth in under a minute.

8 min read · Published 2026-03-17 · Updated 2026-04-04

How to Calculate Compound Interest by Hand

Work through the compound interest formula step by step, with and without monthly contributions.

  1. Identify your variables — Write down your principal (P), annual rate as a decimal (r), compounding periods per year (n) and time in years (t).
  2. Plug the numbers into the formula — Apply A = P(1 + r/n)^(nt) — divide the rate by the compounding frequency, add 1, raise to the power of n × t, then multiply by the principal.
  3. Add monthly contributions — Add the future value of your deposits: C × [((1 + r/n)^(nt) − 1) / (r/n)] and sum it with the compounded principal.
  4. Check your result — Verify the number with the compound interest calculator and compare interest earned against total contributions.

Frequently asked questions

How do I calculate compound interest by hand?

Use the formula A = P(1 + r/n)^(nt). Identify your principal (P), annual rate (r as a decimal), compounding frequency (n), and time in years (t). Multiply step by step — or use a scientific calculator for the exponent.

What's the difference between APR and APY?

APR is the annual rate without compounding factored in. APY (Annual Percentage Yield) includes the effect of compounding. A 5% APR compounded monthly gives an APY of about 5.12%. Always compare APYs when evaluating savings accounts.

Does compound interest work on stocks?

Stocks don't pay compound interest directly, but reinvesting dividends and capital gains creates a similar compounding effect. A broad index fund averaging 7-10% annually with reinvested dividends compounds your wealth over decades.

How much compound interest can I earn on $10,000?

At 5% compounded monthly: $16,470 after 10 years, $27,126 after 20 years, $44,677 after 30 years. The longer you leave it, the more dramatic the growth becomes.

Run the numbers yourself

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