Calculator
Compound interest, live.
Drag the sliders. The curve redraws as you go. Then scroll down to see what waiting costs.
- After 40 years
- $656,203
- You put in
- $120,000
- Growth
- $536,203
Drag the line or use the slider.
82% of that final number is growth. You never deposited it.
Assumes a steady 7% a year, compounded monthly, deposits at the end of each month. No taxes, fees or inflation. Real returns are uneven and can be negative.
The expensive part
Starting at 22 vs 32
Same $250 a month. Same 7%. Both stop at 65. The only thing that changes is when you start.
What the wait costs
Waiting means $30,000 less deposited. But the balance ends $433,017 lower. The other $403,017 is growth that never had time to happen.
To catch up starting at 32, it would take about $530 a month instead of $250.
Start at 22
$819,032at 65
Start at 32
$386,016at 65
Drag the line or use the slider.
How to read this
The solid line is your balance. The dotted line is what you actually put in. The space between them is growth. Early on the lines hug each other. Later they split, and the gap gets wide fast. That bend is compounding.
Three things that move the number
- Time. Slide years from 20 to 40 and watch. More years add more deposits and more time for growth at a positive rate.
- The amount. Start with $0 and double the monthly amount. That doubles the result.
- The rate.Small changes add up over decades. It's also the one you don't control.
What it can't tell you
This is a steady-rate model. Real life isn't steady. Some years are up, some are down, and the order matters. Use it to understand the shape of the curve, not to predict a balance.
FAQ
Compound interest, quick answers
What is compound interest?
It's growth on top of growth. Your money earns a return, then that return earns its own return. Over long stretches most of the final balance can come from growth, not from what you deposited.
How does this calculator do the math?
It compounds monthly. Each month the balance grows by one twelfth of the yearly rate you set, then your monthly contribution is added. It repeats that for every month in the timeline.
What return should I use?
That's your call, and nobody knows the future number. Try a few. A low rate, a middle one and a high one show you a range instead of a single guess. A savings account and a stock fund behave very differently, and investments can lose money.
Why does starting ten years earlier matter so much?
Starting 10 years earlier adds 120 deposits. Those deposits also get more time to grow. At 0% return, the entire gap is deposits.
Does this include taxes, fees or inflation?
No. It's the plain math. Taxes depend on the account type, fees depend on what you hold, and inflation means future dollars buy less than today's.
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