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Compound Interest Calculator

See how a starting balance and steady contributions grow with compounding over time.

Inputs

What you start with today.

Added every month.

Yearly rate before compounding (%).

How long the money stays invested.

Compounding Frequency

How often interest is added to the balance.

Result

Principal
$0
Contributions
$0
Interest Earned
$0
Growth Multiple
—
Future Value
$0

Deposits are applied each compounding period. Taxes, fees, and inflation are not included.

Yearly Growth

Enter a starting principal or monthly contribution above to see the growth curve.

How compound interest works

Compound interest pays you on your balance, then pays you again on the interest that balance just earned. Early on the curve looks flat — the growth is real but small. Give it years and the interest starts out-earning your deposits; that crossover is what the green band in the chart above shows.

The three levers

  • Time — the most powerful and the only one you can't buy back. Starting at 20 beats starting at 30 with twice the money.
  • Contribution — steady monthly deposits do most of the heavy lifting in the first decade.
  • Rate — by the Rule of 72, money at 7% doubles about every 10 years.

For service members: this is the general-purpose version of the math behind your TSP. For TSP-specific projections with BRS matching and IRS limits, use the TSP calculator — and remember a 5% contribution there captures the full government match before any of this compounding even starts.

FAQs

What is compound interest?

Interest earned on both your original money and on the interest it has already earned. Each period the balance grows, and the next period's interest is calculated on that larger balance — which is why growth accelerates over time.

Does compounding frequency matter?

Yes, but less than people expect. Monthly compounding earns slightly more than annual at the same stated rate. Time in the market and contribution amount matter far more than frequency.

What's the Rule of 72?

A quick estimate of doubling time: divide 72 by your annual return. At 7%, money doubles roughly every 10 years — so 20 invested years means about two doublings.

How is this different from the TSP calculator?

The TSP calculator models your military pay, BRS matching, and IRS contribution limits. This tool is the general-purpose version for any account — savings, brokerage, or a what-if on any lump sum.
Compound Interest Calculator — Savings Growth Over Time | Quarters