Free Tool
Compound Interest & Savings Growth Calculator
Work out how a starting amount and monthly contributions grow with compound interest, how much comes from growth, and what the balance is worth after inflation.
Balance after 20 years
$155,163
Total you paid in
$77,000
Growth earned
$78,163
In today's money
$94,692
How the balance builds
| After | Paid in | Balance | Share from growth |
|---|---|---|---|
| 5 years | $23,000 | $27,675 | 17% |
| 10 years | $41,000 | $58,261 | 30% |
| 20 years | $77,000 | $155,163 | 50% |
At 6% a year, money left untouched doubles in about 11.6 years.
Estimates are for general education only and are not financial, insurance or tax advice. Calculations run in your browser; nothing you enter is sent to our servers.
What compound interest is
Compound interest means earning a return on your earlier returns as well as on the money you put in. In the first years most of the balance is your own contributions. Later, growth makes up a larger share, which is why time matters more than almost any other input. The table under the result shows this shift by listing how much of the balance came from growth at each point.
How the calculation works
The tool compounds monthly. Each month the balance grows by one twelfth of the annual rate, and then your monthly contribution is added. The result is the standard future value of a lump sum plus the future value of a series of equal monthly payments made at the end of each month. With a rate of zero, the balance is simply everything you paid in.
The figure in today's money divides the final balance by the effect of inflation over the period, so you can judge what the balance could buy. The doubling time uses the same monthly compounding. It is close to the rule of 72, which divides 72 by the annual rate.
Choosing a realistic rate
The rate is the biggest assumption. A savings account or fixed deposit pays a stated rate, which can change. Investments such as shares and bonds have no fixed rate, their returns vary from year to year, and they can lose money. No rate in this tool is a forecast or a promise. A sensible approach is to try a low, a middle and a high rate and plan around the low one.
What the tool leaves out
The result ignores tax on interest or gains, account and fund fees, and changes in your contributions over time. Fees matter a great deal over long periods: you can see their effect by lowering the rate by the annual fee. The tool also assumes the same return every year, while real returns arrive unevenly.
This is an educational estimate, not advice. For the full explanation, read our guide: How Compound Interest Works.