Compound interest calculator

See how a starting amount and a monthly contribution grow when the returns are reinvested. Change any number and the projection updates.

By Michael Afreh, senior software engineer with 12+ years in e-commerce, fintech and banking, and author of The Software Engineer's Guide to Investing. Reviewed 31 October 2026.

In 20 years at 7%, you would have

292,465

Total paid in: 130,000
Interest earned: 162,465
Range 5% – 9%: 229,435 – 375,470

Balance at 7%
Range 5% – 9%
Total paid in

Year-by-year table

Year	Paid in	Interest	Balance	Low – high
0	10,000	0	10,000	10,000 – 10,000
1	16,000	890	16,890	16,636 – 17,144
2	22,000	2,263	24,263	23,604 – 24,930
3	28,000	4,151	32,151	30,921 – 33,418
4	34,000	6,592	40,592	38,603 – 42,669
5	40,000	9,623	49,623	46,670 – 52,753
6	46,000	13,287	59,287	55,139 – 63,744
7	52,000	17,627	69,627	64,033 – 75,725
8	58,000	22,692	80,692	73,371 – 88,784
9	64,000	28,530	92,530	83,175 – 103,018
10	70,000	35,197	105,197	93,471 – 118,533
11	76,000	42,751	118,751	104,280 – 135,445
12	82,000	51,254	133,254	115,631 – 153,878
13	88,000	60,772	148,772	127,548 – 173,971
14	94,000	71,376	165,376	140,062 – 195,872
15	100,000	83,143	183,143	153,202 – 219,744
16	106,000	96,153	202,153	166,998 – 245,765
17	112,000	110,494	222,494	181,484 – 274,127
18	118,000	126,258	244,258	196,695 – 305,042
19	124,000	143,547	267,547	212,666 – 338,740
20	130,000	162,465	292,465	229,435 – 375,470

What compound interest is

Compound interest is interest earned on interest. Each period, the return is added to the balance, so the next period's return is calculated on a bigger number. Early on the growth looks linear; over decades the curve bends upwards, and time in the market matters more than the exact rate.

The formula

For a single deposit, the balance after t years is P × (1 + r/n)^n·t, where P is the starting amount, r the annual rate and n how many times a year interest is added. 10,000 at 7% compounded annually for 20 years becomes about 38,700.

How this calculator works

Monthly contributions are added at the end of each month. The rate you choose is converted to the equivalent monthly rate for your compounding frequency, so a contribution starts earning from the following month. The range applies your variance below and above the estimate, never going under 0%.

What it leaves out

Real returns are not smooth: markets fall, sometimes for years. The projection also ignores taxes, inflation and fees beyond what you take off the rate yourself. Use it to compare plans, not to predict a number.

Frequently asked questions

+Which interest rate should I enter?

For a savings account, use the rate the bank pays today, knowing it can change. For a broad stock index fund, many people plan with 5–7% a year, but that is a long-run assumption, not a guarantee. Enter a return after inflation if you want the result in today's money, and use the variance to see what a weaker or stronger decade does.

+Does compounding more often make a big difference?

Less than most people expect. 10,000 at 5% for 10 years grows to 16,289 with annual compounding and 16,487 with daily compounding. The rate, the monthly amount and the number of years matter far more.

+How long does it take to double my money?

The rule of 72 gives a quick estimate: divide 72 by the annual rate. At 6% money doubles in about 12 years; at 9%, in about 8. Set the monthly contribution to 0 above to see the exact figure.

+Are taxes included?

No. The calculator shows growth before tax, because tax depends on where you live and on your account type.

In Germany (2026 tax year), investment income above the saver's allowance of 1,000 € a year (2,000 € for married couples filing jointly) is taxed at a flat 25% plus the 5.5% solidarity surcharge on that tax, 26.375% in total, plus church tax if you pay it. For equity ETFs, 30% of the gain is tax-free, which brings the effective rate on gains to about 18.46%.

ci / cd for your money

Automate the boring pipeline

The monthly contribution is the part of this chart you control. The Software Engineer's Guide to Investing shows how to deploy your savings on a schedule, so compounding works without you hand-deploying every month.

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Or read chapter one first, free:

Sources

Not financial advice. The results are estimates for education, not a recommendation or a promise of returns. See the disclaimer.