Compound Interest Calculator
See how savings and investments grow over time with compound interest and regular contributions, including a visual growth chart. Free, instant, private.
How the projection is actually built
This page does not apply a single closed-form equation. It steps through the projection one month at a time, which is the only honest way to handle regular contributions.
First it works out a monthly growth factor from your chosen compounding frequency: factor = (1 + annual rate / f)f / 12, where f is 1, 2, 4, 12 or 365 depending on the dropdown. Then, for every month in the term, it does two things in order: multiply the balance by that factor, then add your contribution.
The order matters. Your contribution lands at the end of the month, so it does not earn growth during the month you make it. That is the conservative convention, and it means a projection here will sit slightly below a calculator that credits contributions at the start of the period.
The chart records a data point every twelfth month, plus one at the end if your term is not a whole number of years.
Worked example: a superannuation balance
Say you have $45,000 in super, employer and salary sacrifice contributions add $650 a month, and you assume 7 per cent a year compounding monthly over 25 years.
The monthly factor is (1 + 0.07 / 12)1 = 1.00583333. Starting at $45,000, the loop runs 300 times.
Final value: $784,190.42. Total contributions: $240,000.00, which is your $45,000 opening balance plus 300 payments of $650. Growth: $544,190.42.
Look at the split. More than two thirds of the ending figure is growth rather than money you put in, and none of that happens in the first decade. The same inputs stopped at 20 years give $520,345.58, so the final five years of a 25 year run add roughly $264,000, almost all of it growth on money already there.
Remove the contributions entirely and the $45,000 alone reaches $257,643.82. The contributions are doing a lot of work, but so is time.
Compounding frequency is not a rounding detail
People often assume the frequency dropdown barely moves the needle. At a 7 per cent nominal rate it moves it more than you would expect, because 7 per cent compounded monthly is an effective 7.23 per cent a year.
Run the example above with each setting and the final value comes out at $753,211.69 annually, $784,190.42 monthly, and $787,112.04 daily. The gap between annual and monthly is nearly $31,000 on identical inputs.
This matters when you compare products. A rate quoted as a nominal annual figure and a rate quoted as an effective annual figure are not the same number, and the frequency you choose here should match how the institution actually credits interest.
What the projection leaves out
Three things, all of which push the real result downwards.
Fees. Administration and investment fees come out of your return. If a 7 per cent gross return becomes 6 per cent after fees, the example above finishes at $651,369.72 instead of $784,190.42. One percentage point costs about $133,000 across 25 years.
Inflation. The final figure is in future dollars. Deflate $784,190.42 by 2.5 per cent a year for 25 years and it is worth about $422,985 in today's money. That is still a good outcome, but it is not the number on the card.
Tax and volatility. Earnings inside and outside super are taxed differently, and no real investment returns a smooth identical percentage every month. A market that averages 7 per cent does it through good years and bad, and the order those years arrive in changes your result, particularly when you are drawing money out.
So the output is an illustration of how compounding behaves under steady assumptions, not a forecast and not financial advice. For decisions about your super, contributions or investments, speak to a licensed financial adviser who can see your whole situation.
Reading the two lines
The lower line is cumulative contributions, which climbs in a straight line. The upper line is total value, which curves. The vertical distance between them at any point is the growth earned so far. In the early years they are almost touching; the visible fanning apart later in the chart is the whole point of the exercise.
Where to go next
To see the same maths working against you on borrowed money, open the Loan Calculator and compare the interest column. For rate changes, fee percentages and contribution increases, the Percentage Calculator handles the small sums. If you want to know exactly how many years sit between now and a target date, the Age Calculator will count them.
Common questions
Are contributions added before or after growth each month?
After. The balance is grown first and the contribution is added at the end of the month, so a payment earns nothing in the month it is made. This is deliberately the more conservative of the two conventions.
Can I model an annual or fortnightly contribution?
Only by converting it. The contribution field is monthly, so divide a yearly amount by 12, or multiply a fortnightly amount by 26 and divide by 12. The result will be very close but not identical to a true fortnightly schedule.
Why does total contributions include my starting balance?
Because that figure is meant to represent every dollar of yours in the account, so the growth number underneath is genuinely just the earnings. Your opening principal is counted once, at month zero.
What rate should I enter?
That is your assumption to make and there is no correct answer here. Rather than hunting for one number, run the projection two or three times across a range and see how wide the spread of outcomes is. That spread is the real information.
Does anything I enter get sent to a server?
No. Every figure stays inside the page while you use it, and nothing is uploaded, remembered or shared when you change an input.