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Compound Interest Calculator

Calculators · Added 5 July 2026

Compound interest pays you interest on your interest. Enter a starting amount, a rate and a time horizon to see how it grows — and add a regular monthly contribution to model a savings plan rather than a one-off deposit.

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Leave blank to model a single lump sum left to grow on its own.

How to use the compound interest calculator

  1. 1Enter the initial principal.
  2. 2Set the annual interest rate and the number of years.
  3. 3Choose how often interest compounds — yearly, half-yearly, quarterly, monthly or daily.
  4. 4Optionally add a monthly contribution, then read the year-by-year table.

Examples

A lump sum left alone

Input
100,000 at 8% p.a., compounded quarterly, for 10 years
Result
Future value 220,804 · Interest earned 120,804

A monthly savings plan

Input
Start 50,000, add 5,000/month at 10% p.a., compounded monthly, for 15 years
Result
Future value about 2,295,000 · Contributions 900,000

About the compound interest calculator

Why time matters more than rate

Compound growth is exponential, which means the final years contribute far more than the early ones. An investment doubling every nine years goes from 1 to 2 to 4 to 8 to 16 — the jump from 8 to 16 in the last period is larger than everything that came before it combined.

The practical consequence is that starting early beats optimising returns. Someone investing 5,000 a month from age 25 to 35 and then stopping entirely often ends up ahead of someone who starts at 35 and contributes for thirty years, purely because the first ten years of growth had three extra decades to compound.

The same maths works against you

Credit card balances compound too, typically monthly at rates between 30% and 45% annually. A balance left to run at 36% doubles in about two years without a single new purchase.

Fees compound as well. A 1.5% annual management charge does not cost you 1.5% — it removes that slice from the base every year, so over thirty years it can consume a quarter of the final balance. When comparing funds, model the fee as a reduction in the rate and re-run the projection.

Frequently asked questions

What is the compound interest formula?
A = P × (1 + r/n)^(n×t), where P is the principal, r is the annual rate as a decimal, n is the number of compounding periods per year and t is the number of years. With regular contributions, each deposit is compounded for the time remaining after it is made, and the totals are summed.
Does compounding frequency make much difference?
Less than most people assume. At 8% over ten years, moving from annual to quarterly compounding adds about 1.8% to the final figure; moving from quarterly to daily adds well under one percent more. Rate and time dominate; frequency is a rounding detail by comparison.
What is the rule of 72?
Divide 72 by the annual percentage rate to approximate the years needed to double your money. At 8% that is nine years; at 12%, six. It is accurate to within a few percent for rates between 5% and 15% and is a useful sanity check on any projection.
Is this adjusted for inflation?
No — the output is a nominal figure. To see purchasing power, subtract expected inflation from your rate and re-run it. A 9% return with 5% inflation grows real wealth at roughly 4%, which is a very different picture from the headline number.

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