Compound Interest Calculator

Compound interest is interest earned on interest already earned, and it is the reason a modest amount saved consistently over decades outperforms a large amount saved late. The mechanism is simple — each period's interest joins the principal and starts earning too — but the effect is exponential rather than linear, which is genuinely hard to intuit. Most people substantially underestimate long-run growth as a result. This calculator takes a starting balance, an optional monthly contribution, an annual rate, a compounding frequency and a time horizon, then shows the final balance, how much of it you contributed, and how much the interest generated on its own. That last split is the one worth looking at: over long horizons the interest component routinely exceeds everything you put in.

How this is calculated

The lump sum grows by A = P × (1 + r/n)^(n×t), where P is the principal, r the annual rate, n the compounding periods per year and t the years. Monthly contributions are handled by the future value of an annuity, which sums each deposit's own growth over the time it has left to compound. The two are added for the final balance.

Worked example on $10,000 at 7% compounded annually for 20 years, with no contributions: 10,000 × 1.07²⁰ = $38,697. You contributed $10,000; compounding produced the other $28,697.

Add $200 a month to the same scenario and the final balance rises to roughly $126,000. Your total contributions are $58,000, so interest supplied around $68,000 — more than you put in. That crossover is what people mean by letting money work.

Time matters far more than rate

Doubling the return from 7% to 14% on a 20-year horizon multiplies the outcome by about 3.5. Doubling the horizon from 20 to 40 years at 7% multiplies it by about 15. Time is the dominant variable, and it is the one you cannot buy back later.

This is the argument for starting early with whatever you can rather than waiting until you can afford more. Someone investing $200 a month from age 25 to 35 and then stopping ends up ahead of someone investing the same amount from 35 to 65, purely because the first ten years had thirty more years to compound.

Compounding frequency is mostly a distraction

Moving from annual to daily compounding on $10,000 at 7% over 20 years takes the result from $38,697 to $40,489 — a 4.6% improvement. Real, but trivial next to the effect of contributing more or waiting longer.

The rule of 72 is the useful mental shortcut here: divide 72 by the annual return to get the approximate years to double. At 7% that is a bit over ten years. At 3% it is twenty-four. Inflation compounds against you by exactly the same mechanism, which is why a nominal return below the inflation rate loses money in real terms no matter how impressive the balance looks.

How to use the compound interest calculator

Enter your starting balance. Whatever you already have invested. Zero is fine if you are starting from contributions alone.
Add a monthly contribution. Regular deposits usually dominate the outcome over long horizons. Try changing this before changing the rate.
Set the rate and time horizon. Use a realistic long-run return rather than a recent one. Then look at the interest-earned row, not just the total.

Frequently asked questions

How often should interest compound for the best return?

More frequent compounding helps, but the effect is modest: $10,000 at 7% for 20 years yields $38,697 compounded annually versus $40,489 daily. Contribution amount and time matter far more.

What is the rule of 72?

Divide 72 by your annual return to estimate the years needed to double your money: at 7%, roughly 72 ÷ 7 ≈ 10.3 years.

What return rate should I assume?

Use a long-run figure rather than recent performance. Broad equity markets have historically returned somewhere around 7% a year after inflation over multi-decade periods, though with severe variation along the way. For a cash savings account, use the actual advertised rate. Assuming a rate you cannot achieve is the fastest way to make a plan that fails.

Does inflation affect these results?

Yes, and the figures here are nominal. Inflation compounds against you by the same mechanism, so a balance that looks large in thirty years buys less than the number suggests. Subtract your inflation assumption from the return rate to model growth in today's money instead.