Compound Interest Calculator with Inflation

Compound interest calculator

Small amounts, a lot of time. Change the numbers and watch which one moves the result most. It's usually the years.

Starting amount

Added every month

Yearly return: 6%A guess, not a promise. Markets don't pay a fixed rate, and some years are negative.

Years: 20

Inflation per year: 2.5%Used only for the "in today's money" figure

After years you'd have

You put in
Growth
In today's money

Your money Growth

Year	Put in	Growth	Balance

My money plan

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How to use the calculator

Enter a Starting amount (0 is fine) and what you'll put in every month under Added every month.
Set the Yearly return. Use a cautious number. Try three versions (low, middle, high) and plan with the low one.
Choose the number of Years. This slider usually changes the result more than any other.
Set Inflation per year to see the result "in today's money", the honest figure for what the future amount will buy.
Read the total, how much of it you put in and how much is growth. The chart and the year-by-year table show when growth overtakes your own contributions.

What it's good for

Makes the value of starting early visible in seconds.
Shows the hidden cost of fees: lower the return by 1–2% and compare.
Separates your own money from growth, so you see what time actually does.
Converts big future numbers into today's money, which stops the result from looking better than it is.

Where it falls short

Real returns are never smooth. Markets can fall for years; the calculator uses one fixed rate.
It ignores taxes and fees unless you lower the return yourself.
It assumes you never pause or withdraw, which few people manage in real life.
A higher assumed return is not a better plan; it's just a more optimistic guess.

How it's calculated

Formula. Every month: balance = balance × (1 + yearly return ÷ 12) + monthly amount. In today's money = final balance ÷ (1 + inflation)^years.

Worked example. 1,000 to start plus 200 a month at 6% for 20 years grows to 95,718. You put in 49,000; growth is 46,718. With 2.5% inflation that's worth about 58,414 in today's money.

Assumptions.

One fixed return every year; real markets go up and down.
No taxes or fees unless you lower the return yourself.

Formula checked 2026-10-23 and covered by automated tests. See the editorial policy and methodology.

What compound interest really is

Interest earns interest. In year one you earn a return on what you put in. In year two you earn it on your money and on last year's growth. The effect is tiny at first and huge later, which is why the line in the chart bends upwards instead of going straight.

This calculator compounds monthly and adds your monthly amount at the end of each month. Real investments don't grow smoothly: the same average return can come with years of losses in between. Treat the result as "what the maths says if things go roughly as assumed", nothing more.

The rule of 72

Divide 72 by the yearly return to get the years it takes money to double. At 6% that's about 12 years; at 8%, about 9. It's a mental shortcut, and the calculator shows how close it gets to the exact answer.

Worth knowing early

Time beats amount. Starting ten years earlier with less money often ends with more than starting late with more. Try it: halve the monthly amount and add ten years.
Fees compound too, against you. A 2% yearly fee looks small. Set the return 2% lower and look at the difference after 30 years. That gap is what you'd pay.
Inflation is the silent tax. The "today's money" line is the honest one. A big number in 30 years buys less than it sounds like.
Consistency is the real skill. Automatic monthly amounts, kept going through bad years, do most of the work.

Questions

What return should I enter?: Nobody knows future returns. Try a cautious, a middle and an optimistic number and plan around the cautious one. A savings account pays whatever your bank offers; broad stock funds have historically returned more on average, with big swings and no guarantee.
Does this include taxes?: No. Taxes on interest, dividends and gains depend on your country and the account you use, so the real result will be lower.
Why is "today's money" lower?: Prices rise over time. The figure divides the final amount by the total inflation over the period, so you can compare it with what things cost now.

Sources · last checked 24 Oct 2026

Formula: see "How it's calculated" above; tested against the worked example shown there.

Last reviewed 24 Oct 2026

Education, not advice. I'm not a financial adviser and I don't know your situation. These tools show the maths behind common rules of thumb; check important decisions with a qualified professional. Full disclaimer.