Compound Interest Calculator
How a lump sum and regular deposits grow over time.
With $10,000 to start plus $500 a month at 7% a year, you'd have $300,851 after 20 years. You put in $130,000; $170,851 is interest.
FV of initial: P x (1+r)^n. FV of contributions: PMT x ((1+r)^n - 1) / r. Total = both.
Rates checked September 2026 against the ATO.
How to use this compound interest calculator
Enter what you're starting with, how much you'll add each month, the yearly return you expect and how many years you'll keep going. The calculator shows your final balance, how much you put in, how much came from interest, and the growth multiple (final balance ÷ money put in).
Interest is compounded monthly, and each deposit is added at the end of the month. The return is assumed to be the same every year.
- Tax on interest or investment earnings isn't taken off. Savings interest is taxed at your marginal rate, which lowers the real return.
- Fees aren't included. Take them off the return you enter, for example 7% less 0.5% in fees is 6.5%.
- Inflation isn't accounted for, so the final balance is in future dollars.
Example: $10,000 plus $500 a month at 7%
Start with $10,000, add $500 a month, and earn 7% a year:
- After 10 years: $106,639, of which $36,639 is interest.
- After 20 years: $300,851, of which $170,851 is interest. You put in $130,000.
- After 30 years: $691,150, of which $501,150 is interest.
Why time matters more than you'd think
In the example, the balance nearly triples between year 10 and year 20, and more than doubles again by year 30. The later years do most of the work because you earn interest on interest you've already earned.
Saving $500 a month from nothing at 7% gives $260,463 after 20 years, but $609,985 after 30. The extra 10 years add $60,000 of deposits and about $290,000 of growth.
The rate matters too: the same $500 a month for 20 years grows to $194,062 at 4.5% instead of $260,463 at 7%.
The rule of 72
A quick way to estimate how long money takes to double: divide 72 by the yearly return. At 7%, money doubles in about 10 years (72 ÷ 7). At 4.5%, it takes about 16 years.
Check it above: $10,000 with no monthly deposits at 7% grows to $20,097 after 10 years.
What return should I use?
It depends on where the money is. Savings accounts and term deposits pay a set rate that changes with interest rates. Shares and ETFs have averaged higher returns over long periods, but go up and down from year to year and can fall. The term deposit vs ETF calculator compares the two after tax.
For retirement savings, super is often the better place because earnings are taxed at 15% or less. See the super calculator.
Frequently asked questions
Each month, interest is added to your balance, and next month's interest is worked out on the new, larger balance. The formula for a lump sum is P × (1 + r)^n, where r is the monthly rate and n is the number of months. Regular deposits are added on top.
At 7% a year compounded monthly, about $260,000, of which $120,000 is your deposits and about $140,000 is interest. At 4.5% it's about $194,000.
Divide 72 by the yearly return to estimate how many years it takes to double your money. At 7%, about 10 years; at 6%, about 12.
Yes. Interest from savings and term deposits is income, taxed at your marginal rate each year. Investment earnings are taxed too, and earnings inside super are taxed at 15% or less. This calculator doesn't take tax off.
A little, but not much. Daily compounding earns slightly more than monthly at the same rate. The rate itself and how long you save make far more difference.