Compound Interest Calculator

See how savings grow with regular contributions.

Calculate compound interest with monthly or annual contributions. See year-by-year growth, total interest earned, and what compounding frequency changes.

Written and maintained by Mohit PatelLast checked August 4, 2026How we build these

Starting amount

Currency

Monthly contribution

Added at the end of every month. Leave at 0 for a lump sum only.

Annual return rate

Compounding

Time horizon20 years

Balance after 20 years

$343,778

You contributed$130,000

Interest earned$213,778

Growth multiple2.64×

At 8% your money doubles roughly every 9.0 years (Rule of 72).

Growth year by year

Each column is your balance at the end of that year, split into what you put in and what the compounding earned.

Your contributions
Interest earned

View as table

Balance by year, split into contributions and interest earned
Year	Contributed	Interest	Balance
1	$16,000	$1,055	$17,055
2	$22,000	$2,695	$24,695
3	$28,000	$4,970	$32,970
4	$34,000	$7,932	$41,932
5	$40,000	$11,637	$51,637
6	$46,000	$16,148	$62,148
7	$52,000	$21,531	$73,531
8	$58,000	$27,859	$85,859
9	$64,000	$35,210	$99,210
10	$70,000	$43,669	$113,669
11	$76,000	$53,329	$129,329
12	$82,000	$64,288	$146,288
13	$88,000	$76,655	$164,655
14	$94,000	$90,546	$184,546
15	$100,000	$106,088	$206,088
16	$106,000	$123,419	$229,419
17	$112,000	$142,685	$254,685
18	$118,000	$164,049	$282,049
19	$124,000	$187,684	$311,684
20	$130,000	$213,778	$343,778

How the Compound Interest Calculator works

Compound interest pays you interest on your interest. Over short periods the effect is small; over decades it dominates. This calculator shows the full curve, separating what you contributed from what the compounding actually earned.

Also known as: investment growth calculator · compounding returns calculator · how much will my savings grow

Why compounding accelerates

Simple interest pays on the original amount. Compound interest pays on the original amount plus all previously earned interest, so each period's interest is calculated on a larger base.

The formula is A equals P times one plus r over n, all raised to the power n times t, where P is the principal, r the annual rate, n the compounding frequency and t the years.

The effect is modest early and dramatic later. £10,000 at 7% is £14,026 after five years, £19,672 after ten, and £76,123 after thirty. Most of the growth in the final figure came from interest on interest rather than from the original amount.

The rule of 72

Dividing 72 by the annual percentage rate gives an approximate number of years for money to double. At 6%, twelve years. At 9%, eight.

It is an approximation and it is accurate enough for mental arithmetic across the range of rates people encounter, drifting slightly at very high and very low rates.

Its value is in making the effect of a rate difference intuitive. Two percentage points does not sound like much until you see that it changes the doubling time from eighteen years to twelve, which across a working life is the difference between doubling twice and doubling nearly four times.

Regular contributions

Adding money regularly changes the picture substantially, and each contribution compounds only for the time remaining rather than the full term.

Which means early contributions do far more work than late ones. £200 a month for ten years then nothing for twenty, at 7%, ends up ahead of nothing for ten years then £200 a month for twenty, despite the second involving twice the contributions.

That comparison is the strongest argument available for starting early, and it holds regardless of the amounts involved. Time in the market does more than the size of the contribution over any long horizon.

Inflation, and the number that matters

A nominal return of 7% with inflation at 3% is a real return of roughly 4%. The purchasing power of the final figure is what matters, and nominal projections overstate it substantially over long periods.

Over thirty years at 3% inflation, a pound buys about 41 pence of what it does today. A projection showing £500,000 in thirty years is showing something closer to £206,000 in today's terms.

Which is why long-term projections are more honest in real terms. It is also why the historical equity return figures quoted in investment discussion are usually real rather than nominal, and mixing the two produces projections that are badly wrong.

What the projection cannot include

A constant rate of return. Real investment returns vary year to year, and the sequence matters as well as the average, particularly for anyone drawing down rather than accumulating.

Fees. A platform charge and a fund charge together of 1% a year does not sound like much and reduces a thirty year outcome by roughly a quarter, because the fee compounds against you exactly as the return compounds for you.

Tax, which depends entirely on the wrapper and the jurisdiction. An ISA in the UK or a Roth account in the US grows without tax on gains; an ordinary account does not. Those differences compound too, and none of them appear in the arithmetic.

Where to go next

The Compound Interest question rarely arrives on its own. These are the ones that usually come with it:

SIP Calculator — Project returns on a monthly investment plan.
Loan & EMI Calculator — Monthly payment, total interest, and a full amortization schedule.
Mortgage Calculator — Full monthly cost including tax, insurance, and PMI.
Car Affordability Calculator — Worked back from income, with running costs taken out first.

Sources

Not financial advice. This calculator is for planning and illustration, not financial advice. Real products carry fees, taxes, and terms it does not model. Confirm figures with your lender or a qualified adviser before committing.

Frequently asked questions

What is the compound interest formula?

A = P(1 + r/n)^(nt), where P is the starting principal, r is the annual rate as a decimal, n is how many times per year interest compounds, and t is years. Regular contributions are added with a separate future-value-of-an-annuity term.

How does compounding frequency affect returns?

More frequent compounding earns slightly more. At 8% over 30 years, monthly compounding beats annual by roughly 3-4% of the final balance. It matters, but far less than the rate itself or how long you stay invested.

What is the Rule of 72?

Divide 72 by the annual return to estimate the years needed to double your money. At 8% that is about 9 years; at 12%, about 6. It is a rough mental shortcut, accurate to within a few percent for rates between 6% and 15%.

Does this account for inflation or tax?

No, the result is a nominal figure. To estimate purchasing power, subtract expected inflation (historically 2-3%) from your return rate before entering it, which gives you a real, inflation-adjusted balance.