Compound interest calculator

What a starting amount and a monthly deposit could grow to, after fees and tax, and what that would buy in today’s money.

After 20 years

$257,434

before tax; $238,319 after tax

You put in $130,000
Growth $127,434
$160,382 in today’s money

Putting in $10,000 now and $500 a month for 20 years at 6% a year, minus 0.2% in fees, you’d have about $257,434. You put in $130,000; the other $127,434 is growth.

After 15% tax on the gains, that’s $238,319. If prices rise 2% a year, it will buy about what $160,382 buys today.

What the final amount is made of

You put in$130,00050%
Growth after tax$108,31942%
Tax$19,1157%

I’m not an accountant or a financial advisor. This is an estimate, for information only. Check with a professional before you decide.

Your numbers

example: change the numbers

Currency

This changes the symbol and the examples. It does not convert your numbers.

Starting amount

What you put in on day one. It can be zero.

Monthly contribution

What you add at the end of every month.

Yearly return

An assumption, not a promise: what the money might earn in a year, before fees.

Yearly fees

Fund expense ratios and account fees, as a share of what you have invested.

Years

How long you keep saving and investing.

Tax on the gains

It depends on your country and account: 0% in a tax-free account, such as a Roth IRA (if you meet its rules) or a UK ISA. In the US, most people pay no more than 15% on long-term gains.

Inflation per year

For the “today’s money” line. The Federal Reserve and the Bank of England both aim for 2% over time.

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Year by year, before tax
Year	Put in	Value	Growth	In today’s money
1	$16,000	$16,758	$758	$16,429
2	$22,000	$23,918	$1,918	$22,989
3	$28,000	$31,505	$3,505	$29,688
4	$34,000	$39,544	$5,544	$36,532
5	$40,000	$48,061	$8,061	$43,531
6	$46,000	$57,086	$11,086	$50,691
7	$52,000	$66,649	$14,649	$58,022
8	$58,000	$76,781	$18,781	$65,532
9	$64,000	$87,517	$23,517	$73,230
10	$70,000	$98,892	$28,892	$81,126
11	$76,000	$110,945	$34,945	$89,229
12	$82,000	$123,715	$41,715	$97,549
13	$88,000	$137,247	$49,247	$106,096
14	$94,000	$151,584	$57,584	$114,882
15	$100,000	$166,776	$66,776	$123,917
16	$106,000	$182,872	$76,872	$133,212
17	$112,000	$199,927	$87,927	$142,781
18	$118,000	$217,999	$99,999	$152,634
19	$124,000	$237,146	$113,146	$162,784
20	$130,000	$257,434	$127,434	$173,246

How it works

Each month, your balance earns one twelfth of the yearly return minus fees (6% − 0.2% = 5.8% a year, about 0.483% a month), and then your monthly contribution is added. The next month, the interest earns interest too: that’s compounding.
The same thing in one line: final = start × (1 + r)^n + monthly × ((1 + r)^n − 1) ÷ r, where r is the monthly rate and n is the number of months.
Tax comes off once, at the end, on the growth (the final value minus what you put in), at the rate you choose. In real accounts, dividends and interest may be taxed every year instead.
“In today’s money” divides the after-tax value by (1 + inflation) once for every year, so you see what it would buy at today’s prices.

Worked example

$10,000 now plus $500 a month for 20 years at 6% a year, with 0.2% in fees: about $257,434. You put in $130,000; $127,434 is growth. After 15% tax on the growth, $238,319. With 2% inflation a year, that buys about what $160,382 buys today.

Sources

Text checked on September 30, 2026

Good to know

The return is an assumption, not a promise. Real returns go up and down, and some years they’re negative; this calculator uses the same rate every year.
Fees compound too. In an SEC example, $100,000 growing 4% a year for 20 years ends near $208,000 with a 0.25% yearly fee and near $179,000 with 1%.
Tax here is one rate on all the growth, at the end. In real life it depends on your country, your account and how long you hold: in the US, gains on investments held more than a year are taxed at 0%, 15% or 20%, and short-term gains as ordinary income.
Nothing here recommends a product or an investment. It shows how the math works with the numbers you choose.

Frequently asked questions

What is compound interest?

Interest on your interest. Each month, the growth is added to your balance, so next month you earn on a slightly bigger amount. Over long periods that snowball can do much of the work: in the example, growth is about half of the final amount.

What return should I assume?

Nobody knows future returns, which is why you choose the number. Try a cautious number and a hopeful one, and look at the gap between them. Subtract fees too: as the SEC points out, small fees make a big difference over time.

What tax rate should I put in?

It depends on where you live and the account. In the US, long-term capital gains are taxed at 0%, 15% or 20%, and most people pay no more than 15%; short-term gains are taxed as ordinary income. Qualified withdrawals from a Roth IRA are tax-free, and in the UK you pay no tax on gains inside an ISA: put 0% for those.

What does “in today’s money” mean?

Prices rise, so $238,319 in 20 years won’t buy what $238,319 buys today. Dividing by 2% inflation a year for 20 years gives about $160,382 of today’s buying power. 2% is the Federal Reserve’s longer-run goal, not a guarantee.

Does it matter if interest compounds monthly or yearly?

A little. This calculator adds growth every month at one twelfth of the yearly rate, so 5.8% a year works out to about 5.96% over a full year. Compounded once a year, the same 5.8% would give a little less.