Savings & investment growth
Compound interest calculator
Put in a starting balance, a rate, and whatever you add each month. You get the ending balance, how much of it was interest, what inflation does to it, and the year-by-year path. No sign-up, nothing leaves your browser.
Balance after 20 years
$232,643
Your $10,000 plus $120,000 of contributions becomes $232,643 in 20 years. Interest did 44.1% of the work.
Money you put in
$130,000
$10,000 to start, $120,000 added
Interest earned
$102,643
44.1% of the final balance
Year by year
Where the balance sits at the end of each year, and how much of that year's gain came from interest rather than deposits.
| Year | Opening | Added | Interest | Closing |
|---|
Does compounding frequency matter?
Same money, same rate, same contributions. Only the compounding interval changes.
| Compounded | Final balance | Vs yearly | Effective rate |
|---|
How long to double your money
Exact answer
13.9 years
Solved from the compounding maths at your rate and frequency.
Rule of 72
14.4 years
The mental shortcut: 72 divided by the rate. Close enough for rates between roughly 5% and 12%.
With your deposits
1.7 years
When the balance first passes twice your starting amount.
What compound interest actually is
Interest earns interest. That is the whole idea. Simple interest pays you on your original deposit forever, so 1,000 at 5% pays 50 every year until you close the account. Compound interest pays you on the balance, which includes last year's interest, so year two pays 52.50 and year twenty pays 126.35.
The standard formula for a lump sum with no further deposits:
A = P (1 + r/n)nt
- A
- the amount you end up with
- P
- your starting principal
- r
- the annual rate as a decimal, so 5% is 0.05
- n
- how many times a year it compounds
- t
- years
Worked through with real numbers
Take 10,000 at 5% for 20 years, compounded monthly. Then r/n is 0.05 ÷ 12, which is 0.0041667. Add 1 and you get 1.0041667. The exponent nt is 12 × 20, so 240. Raise 1.0041667 to the 240th power and you get 2.71264. Multiply by 10,000 and the answer is 27,126.40.
Only 10,000 of that was ever yours to deposit. The other 17,126.40 is interest, and interest on interest, which is why the curve above bends upward instead of climbing in a straight line.
Where contributions change the maths
Once you add money on a schedule, each deposit compounds for a different length of time. A deposit made in year one gets nineteen more years of growth than one made in year twenty. This calculator tracks every deposit on its own timeline, which is also why the timing switch (start or end of period) shifts the answer slightly.
Two caveats apply to any calculator, this one included. It assumes the rate holds steady for the whole term, which no real savings account does. And it ignores tax, which matters outside a tax-sheltered account. Use the rate range to see how wrong a steady-rate assumption can be.
Where you are changes the answer
Not the maths, but the wrapper you put the money in and the rates on offer.
Questions people actually ask
What does $10,000 grow to in 20 years?
At 5% compounded monthly with no further deposits, 10,000 becomes 27,126.40. At 7% it becomes 40,387.39, and at 3% it only reaches 18,207.55. Two percentage points either side of 5% swings the answer by more than the original deposit, which is the single most important thing to take away from this page.
What is $1,000 at 6% after 2 years?
Compounded yearly: 1,123.60. You earn 60 in year one, then 63.60 in year two because the second year's interest is calculated on 1,060 rather than 1,000.
Compounded monthly the same deposit reaches 1,127.16. The extra 3.56 is what more frequent compounding buys you over two years, which is not much. Over thirty years it is a different story.
How long does it take to double money at 8%?
The Rule of 72 says 72 ÷ 8 = 9 years. The exact answer with yearly compounding is 9.01 years, and with monthly compounding it drops to 8.69 years. The shortcut is impressively good in that range. It drifts at the extremes: at 1% the rule says 72 years when the real answer is 69.7, and at 25% it says 2.88 when the truth is nearer 3.11.
Is daily compounding much better than monthly?
Barely. On 10,000 at 5% over 20 years, monthly compounding gives 27,126.40 and daily gives 27,180.96. That is a gap of about 55, or 0.2%. Chasing daily compounding while ignoring a rate that is half a point lower is a bad trade. Rate first, frequency second.
What do monthly contributions add over 30 years?
Starting from zero at 7% compounded monthly, 500 a month for 30 years reaches 609,985. You deposited 180,000 of that. The remaining 429,985 is growth. Cut the term to 20 years and the same 500 a month gives 260,463, of which only 140,463 is growth. The last decade does an outsized amount of the work, which is the real argument for starting early.
Why does my bank's number differ from this?
Usually one of four things. Your bank quotes an annual equivalent rate (AER or APY) that already includes compounding, so entering it with monthly compounding double-counts. Or the account pays a bonus rate that expires after twelve months. Or interest is credited on a fixed date rather than on your anniversary. Or tax is deducted at source.
If you have an AER or APY figure, set compounding to yearly and enter it as-is. If you have a nominal rate, set the frequency the account actually uses.
Should I include inflation?
For anything longer than about five years, yes. At 2.5% inflation, money loses roughly a fifth of its purchasing power over ten years and nearly two fifths over twenty. A balance of 100,000 in 2046 does not buy a 2026 house. Tick the today's-money box and read the smaller number as the honest one.
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