The Money Decoded

Calculators

Compound Interest Calculator

Educational content only — not financial advice

A 25-year-old who saves $5,000 a year for ten years and then stops finishes with more money at 65 than a 35-year-old who saves the same $5,000 a year for thirty years straight. That's compound interest. Plug your own numbers in below — starting balance, annual rate, years, optional monthly contribution — and you'll see why time matters more than rate over long horizons.

Set to 0 for a single starting deposit only.

Final balance

$92,480.05

Total contributed
$34,000.00
Interest earned
$58,480.05

The formula behind the calculator

Compound interest is calculated on both the original principal and on previously-earned interest:

A = P × (1 + r/n)^(n × t)
  • A is the final balance.
  • P is the principal — the starting balance.
  • r is the annual interest rate, expressed as a decimal (7% becomes 0.07).
  • n is the number of compounding periods per year (12 for monthly, 365 for daily, 1 for annually).
  • t is the time in years.

When monthly contributions are added, the calculator uses the future-value-of-an-annuity formula on top of the principal growth:

FV_contributions = PMT × [((1 + i)^k - 1) / i]

where PMT is the monthly contribution, i is the monthly rate (r ÷ 12), and k is the number of months (12 × t). The two pieces add together to give the final balance shown above.

A worked example

Take $10,000 invested at 7% annually, compounded monthly, for 20 years, with a $100 monthly contribution.

  • Principal future value: $10,000 × (1 + 0.07/12)^(12 × 20) ≈ $40,387
  • Contributions future value: $100 × ((1 + 0.07/12)^240 - 1) / (0.07/12) ≈ $52,093
  • Final balance: $92,480
  • Total contributed: $10,000 starting + $100 × 240 months = $34,000
  • Interest earned: $92,480 − $34,000 ≈ $58,480

Compounding produced more interest than the household contributed. That gap widens dramatically the longer the time horizon — which is why financial educators emphasise starting early rather than starting bigger.

Pair this calculator with the explainer

For the conceptual background — why compound interest produces exponential rather than linear growth, why time matters more than rate, and how the same mechanism makes high-interest debt corrosive — see our companion piece: What Is Compound Interest Explained Simply.

One useful exercise: run two scenarios side by side — your current monthly contribution at your current age versus an additional $50 a month starting today. The gap at 65 is almost always larger than people expect, and it's the cleanest argument for raising the contribution by even a small amount this month rather than waiting.

Frequently asked questions

What's the difference between simple and compound interest?

Simple interest is calculated only on the original principal — $100 at 5% earns $5 every year. Compound interest is calculated on principal plus accumulated interest — $100 at 5% earns $5 in year one, $5.25 in year two (5% of $105), and so on. Over short periods the difference is small; over decades it's enormous.

Does compounding frequency really matter?

It matters more at higher interest rates than at lower ones. A 4% rate compounded monthly produces about 4.07% effective annual return; daily compounding pushes that to roughly 4.08%. Most savings accounts and credit cards compound daily; many investment returns compound monthly or quarterly.

Should I include monthly contributions in the calculation?

Yes if you'll actually be contributing each month. Regular contributions are usually responsible for more of a long-term balance than the starting deposit, especially over multi-decade horizons. Set the contribution to $0 if you only want to model the growth of an existing lump sum.

What's a realistic interest rate to use?

For long-term stock-market returns, 7% is the commonly cited inflation-adjusted average over many decades. For high-yield savings accounts, 4% is roughly the 2026 norm. For credit card debt, 18–25% APR is the typical range. The right number depends on what you're modelling.

Sources