Compound interest explained

Last updated: 2026-08-14

Put $10,000 somewhere earning 7% and leave it for 30 years. It becomes about $76,000. If the same 7% were paid only on the original amount — simple interest — you would finish with $31,000.

The $45,000 difference is compounding, and where it shows up is the part worth understanding.

Growth is not spread evenly

Years	Balance	Gained in that decade
10	≈$19,700	$9,700
20	≈$38,700	$19,000
30	≈$76,100	$37,400

The third decade produces nearly four times what the first did, from the same money at the same rate. Nothing changed except the size of the balance the return was applied to.

This is why starting early matters more than contributing heavily later, and why the last years before you need the money do the most work — which is also why sequence risk matters near the end.

What moves the result

Time is the strongest lever, because it appears as an exponent rather than a multiplier. Ten extra years on this example roughly doubles the outcome.

Rate compounds too, so small differences widen dramatically. Over 30 years, 7% turns $10,000 into $76,100 while 6% gives about $57,400 — one percentage point costing nearly $19,000. This is the entire argument for keeping fees low.

Compounding frequency matters least. Moving from annual to monthly compounding at 7% adds a fraction of a percent to the effective rate. It is real, but it is rounding error next to time and rate.

It runs in both directions

The same mechanism drives credit card debt. A balance at 22% compounds against you at roughly 1.83% a month, and unpaid interest joins the balance that next month's interest is charged on.

That is why clearing high-rate debt usually beats investing: paying off 22% debt is a guaranteed 22% return, and very little offers that reliably.

Run your own numbers

The compound interest calculator handles a focused growth estimate with contributions and compounding frequency. For fees, tax and inflation, use the investment return calculator.

What actually moves the outcome

Three levers drive a compounding result: the amount, the rate and the time. They are not equally powerful, and the one people optimise hardest is usually the weakest.

$10,000 invested once, no further contributions
Rate	After 10 years	After 20 years	After 30 years
4%	$14,802	$21,911	$32,434
6%	$17,908	$32,071	$57,435
8%	$21,589	$46,610	$100,627
10%	$25,937	$67,275	$174,494

Two percentage points of rate roughly doubles the thirty-year outcome. So does an extra decade at the same rate. Chasing return is the harder of the two and the one more likely to go wrong.

Common questions

How much does compounding frequency matter?

Less than people expect. Moving from annual to monthly compounding on a modest rate changes the outcome by a fraction of a per cent a year. The rate and the time horizon dominate.

What is the rule of 72?

Divide 72 by the annual rate to approximate the years needed to double. At six per cent that is about twelve years. It is a mental shortcut, not a precise calculation.

Does compounding work against me on debt?

Exactly the same way, in the other direction. Unpaid credit card interest is added to the balance and then itself charged interest, which is why revolving balances grow so quickly.

Why does starting early matter so much?

Because the last doubling is the largest. Money invested in your twenties gets more doublings than the same amount invested in your forties, and each doubling is bigger than all previous growth combined.

The rest of this series, and the calculators that let you run the idea on your own numbers.

More investing and retirement guides

Try it with your figures

See also all guides, every calculator, and the calculation methodology behind these estimates.