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

₹30,08,507

Total contributed
₹13,00,000
Interest earned
₹17,08,507

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