Compound Interest Calculator
Compound interest means you earn interest on your interest. The longer the money stays invested, the faster it grows. This calculator projects the future value of a starting amount with a fixed annual rate, a chosen compounding frequency and optional regular contributions added at the end of each period.
How to use the compound interest calculator
- Enter your starting amount, annual rate and number of years.
- Pick how often interest compounds. Monthly is common for savings accounts.
- Optionally add a contribution made each compounding period. Then compare the total you put in with the interest earned.
Formula
A = P(1 + r/n)nt + C × ((1 + r/n)nt − 1) ÷ (r/n), where P is the starting amount, r the annual rate as a decimal, n the compounds per year, t the years and C the contribution per period.
Worked examples
A lump sum
$10,000 at 7% compounded monthly for 10 years grows to about $20,096.61. That is roughly double, thanks to compounding.
With monthly contributions
$10,000 plus $200 a month at 7% compounded monthly for 20 years reaches about $144,573. You put in $58,000, so about $86,573 is interest.
The power of compounding
Simple interest pays a fixed amount each year on the original deposit. Compound interest pays interest on the growing balance, so the growth itself grows. Over short periods the difference is small; over decades it is dramatic.
Simple versus compound
| $10,000 at 7% | Simple | Compound (monthly) |
|---|---|---|
| 10 years | $17,000 | $20,097 |
| 20 years | $24,000 | $40,387 |
| 30 years | $31,000 | $81,165 |
Time matters more than rate
Starting early is the strongest lever. The same monthly contribution invested for 30 years ends up far larger than if invested for 20, even with a slightly higher rate in the shorter case. The Rule of 72 gives a quick feel: at 6% money doubles in about 12 years, at 9% in about 8.
The other side: debt
Compounding works against you on credit cards and loans. A 20% APR balance grows quickly if you only pay the minimum. Paying high-interest debt is often the best guaranteed “return” available.
What this projection leaves out
Investment returns are not constant, and inflation reduces what future money can buy. Taxes, account fees and market losses all change real results. Use the figures to compare scenarios, not to predict them. This is not financial advice.
Practice problems
Try these yourself first, then check your answer. The answers come from the calculator above.
- $5,000 at 5% compounded yearly for 15 years, no contributions.
Show answer
Future value: $10,394.64
- $0 start, $300 added monthly, 6% compounded monthly for 25 years.
Show answer
Future value: $207,898.19
Common mistakes to avoid
- Expecting a steady return. Real investments fluctuate; this is a projection at a fixed rate.
- Forgetting inflation, taxes and fees, which reduce real growth.
- Mixing up the rate and the periodic rate. Enter the annual rate here; the tool divides it for you.
Frequently asked questions
What is compound interest?
Interest calculated on both the original amount and the interest already earned, so growth accelerates over time.
How does compounding frequency matter?
More frequent compounding gives slightly more growth. Monthly against yearly at 7% adds a small amount; the time invested matters far more.
What is the Rule of 72?
A shortcut: divide 72 by the annual rate to estimate the years to double. At 7% that is about 10.3 years, matching the example above.
Is this financial advice?
No. It is an educational projection. Actual returns vary and are not guaranteed.
Reviewed September 26, 2026 by the SolveCalcPro editorial team. Found a mistake? Tell us; see our editorial policy.