Compound Interest Calculator
Enter a starting balance, a rate, a timeline and a monthly contribution. The result updates as you type, and every page on this site shows the formula behind it.
What compound interest actually does
Compound interest is interest earned on interest already earned. The distinction sounds pedantic and turns out to be the whole story, because it makes growth accelerate rather than accumulate. A savings account paying simple interest adds the same amount every year forever. A compounding account adds more each year than the last, because the balance it is calculated on keeps getting bigger.
P is the starting amount, r the annual rate as a decimal, n the number of compounding periods per year and t the number of years. Recurring deposits need a second term, the ordinary-annuity formula, which the formula guide derives step by step.
Left alone at 7%, $10,000 becomes about $20,097 after ten years, $40,387 after twenty, $81,165 after thirty and $163,114 after forty. Each decade adds more than the one before it, from the same untouched deposit. Nothing was added; only time passed.
Why starting date beats deposit size
This is the table that changes how most people think about the problem. Every row is the same $200 a month at 7%, compounded monthly, stopping at 65. Only the start age changes.
| Start at | Years of contributing | Total contributed | Balance at 65 | Multiple |
|---|---|---|---|---|
| Age 25 | 40 years | $96,000 | $524,963 | 5.5× |
| Age 30 | 35 years | $84,000 | $360,211 | 4.3× |
| Age 35 | 30 years | $72,000 | $243,994 | 3.4× |
| Age 40 | 25 years | $60,000 | $162,014 | 2.7× |
| Age 45 | 20 years | $48,000 | $104,185 | 2.2× |
Starting at 25 rather than 35 means contributing $24,000 more — and finishing with $280,968 more. The extra decade returns roughly 12 dollars for each additional dollar contributed, because those first deposits are the ones with the most time left to compound.
The same logic runs in reverse and is worth saying plainly: if you are 45, the ten years you did not have are gone, and the lever that remains is the contribution. That is what the monthly investment calculator is for.
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Core calculators
There are 39 calculators in total — APY and effective rate, doubling time, the Rule of 70 and 72, savings goals, debt versus investing, and the FIRE tools. The full list is organised by what you are trying to work out.
Before you trust any projection
Numbers published on this site are recomputed independently of the site's own JavaScript and checked against the closed-form formulas before release. The process, and how corrections are handled, is on the editorial policy page.
A constant annual return is a modelling convenience. Real markets deliver an average through years that look nothing like it, and taxes, fees and inflation all reduce what you keep. The disclaimer sets out the limits in full.
If a result here looks wrong compared with another calculator, the cause is almost always compounding frequency or contribution timing rather than an error — compounding frequency and contribution timing explain both, with the numbers. If you think something here is wrong, tell us; corrections are the most welcome kind of email this site gets.