Compound Interest Calculator
What a starting amount plus regular deposits grows into, and how much of it is interest.
Compound interest is growth that earns its own growth. It is the reason a modest monthly deposit beats a large one made late, and the reason a long horizon matters more than a clever rate.
Enter what you start with, what you add each month, the return you assume and the number of years, and this calculator shows the final balance and how much of it you contributed yourself.
At a glance
- What you enter
- Starting amount, Monthly deposit, Assumed annual return, Years
- What you get
- Balance after the period, Growth from compounding
- Formula shown
- Yes - every step, right below your results.
- Cost and privacy
- Free, no account needed, and nothing you type is stored.
See worked examples ↓ Read the FAQ ↓ Share this calculator ↓
Results are estimates for general information only - always verify with a professional or official source before acting on them.
How to use the Compound Interest Calculator
- 1 Enter Starting amount
- 2 Enter Monthly deposit - Enter a negative amount to model a monthly withdrawal instead of a deposit.
- 3 Enter Assumed annual return
- 4 Enter Years
- 5 Press Calculate. You get Balance after the period, Growth from compounding, and every intermediate step stays visible under “How is this calculated?”
- 6 Change one number at a time to see what moves the result - that is usually the most useful thing a calculator can tell you.
Worked examples
Real numbers run through this calculator. Open any example to load it into the form above and change it.
$5,000 saved plus $200 a month for 20 years
A long horizon at an assumed 7% annual return.
You enter
- Starting amount
- $5,000.00
- Monthly deposit
- $200.00
- Assumed annual return
- 7%
- Years
- 20
You get
- Balance after the period
- $124,986.77
- Growth from compounding
- $71,986.77
The balance reaches about $124,987. You put in $53,000, so roughly $71,987 of the total is growth from compounding: more than half of the final balance was never deposited. By year ten the growth is about $15,900, so most of it arrives in the second decade, once the balance is large enough for the rate to matter.
$100,000 invested at 5% while taking $422 a month out
A withdrawal run over ten years with $50,000 left at the end.
You enter
- Starting amount
- $100,000.00
- Monthly deposit
- -$422.00
- Assumed annual return
- 5%
- Years
- 10
You get
- Balance after the period
- $98,898.79
- Growth from compounding
- $49,538.79
Withdrawals total $50,640 over the ten years and the balance ends near $98,900, so the growth more than covers what you took out. The negative deposit is what makes this a withdrawal run rather than a savings run, and the 5% return is assumed to hold for the whole decade, which a real portfolio would not do.
Guides that go with it
What Will $100,000 Be Worth in 10 Years?
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Frequently asked questions
Is the assumed annual return a realistic figure?
It is an input, not a forecast, and it is the biggest source of error in the result. A long-run average for a diversified share portfolio is often quoted in the region of 5% to 8% a year before inflation, but any single period can be far above or below that, and a bad decade early on changes the outcome completely.
A practical approach is to run the same numbers at a low, middle and high rate. If the plan only works at the highest one, the plan needs more contributions or more time rather than a better assumption. The calculator compounds a fixed monthly rate, which real markets never deliver.
Does this show what my money will really be worth?
No. The balance is a cash figure with no allowance for inflation, taxes or fees. At 3% inflation, money in twenty years buys roughly half of what it buys today, so a final balance can look impressive while representing much less spending power.
Fees matter in the same direction: a fund charging 1% a year reduces the effective return you should enter. If you want the honest version, lower the annual return by your fee and read the balance as today's money rather than future money.
What happens if I stop making deposits partway through?
The result would be lower, and the loss is larger than the missing deposits alone, because each deposit that never happens also loses all the interest it would have earned. That is why an early pause costs far more than a later one on the same schedule.
To model it, split the period into two runs: one for the years you contribute, then feed the resulting balance in as the starting amount with a deposit of zero for the remaining years. The difference between the two results is what the pause costs.
Could I use this to see how long a lump sum would last if I withdraw from it?
You can, with a negative monthly deposit. Enter the lump sum as the starting amount, put the monthly withdrawal as a negative number and set the return you expect to earn while the money is invested. The balance after the period is what would be left.
Remember that the order of returns matters for withdrawals in a way it does not for deposits: a poor first year while you are taking money out does more damage than the same poor year later. A constant assumed rate hides that risk entirely.
Terms used here
Short, plain-language definitions of the words this calculator uses.