Investment growth
Compound interest calculator
See what a starting balance and a regular contribution could grow to, and how much of the result is your own money rather than growth.
- No sign-up or email
- Runs in your browser
- Assumptions you can edit
- Print or save as PDF
If I keep adding to this every month, what could it be worth?
This calculator separates the two things that make a balance grow: the money you add, and the return earned on it. Most compound interest calculators show only the final number, which makes a long projection look like investment performance when much of it is simply your own contributions.
$
$
years
%
Before tax and fees. This is an assumption you choose, not a forecast.
Money paid in at the start of a period earns one extra period of growth.
%
Used only for the today's-dollars figure.
Projected balance
$100,134
You paid in
$70,000
$10,000 to start
Growth
$30,134
Before tax and fees
In today's dollars
$78,224
After assumed inflation
| Year | Opening | Paid in | Growth | Closing | In today's dollars |
|---|---|---|---|---|---|
| 1 | $10,000 | $6,000 | $785 | $16,785 | $16,375 |
| 2 | $16,785 | $6,000 | $1,203 | $23,988 | $22,832 |
| 3 | $23,988 | $6,000 | $1,647 | $31,635 | $29,376 |
| 4 | $31,635 | $6,000 | $2,119 | $39,754 | $36,015 |
| 5 | $39,754 | $6,000 | $2,620 | $48,374 | $42,755 |
| 6 | $48,374 | $6,000 | $3,151 | $57,525 | $49,604 |
| 7 | $57,525 | $6,000 | $3,716 | $67,241 | $56,567 |
| 8 | $67,241 | $6,000 | $4,315 | $77,556 | $63,654 |
| 9 | $77,556 | $6,000 | $4,951 | $88,507 | $70,870 |
| 10 | $88,507 | $6,000 | $5,627 | $100,134 | $78,224 |
Estimate only, based on the figures and assumptions you entered. It is general information, not financial advice, and it does not take your objectives, financial situation or needs into account. Model version 1.1.0, not independently reviewed.
Reading the result
What the numbers mean
Your contributions and growth are shown separately. Over short terms most of the balance is money you put in; growth only overtakes contributions well into a long projection.
In today's dollars restates the final balance at the inflation rate you chose. A balance decades away buys less than the same number does now, and this is the column that says by how much.
The result is before tax and fees. Both reduce a real return, and neither is modelled here.
The assumed return is a number you entered. Change it and the projection changes — that sensitivity is the point, not a flaw.
Worked example
An example, start to finish
Someone with $10,000 invested adds $500 a month for 10 years, assuming a 6% annual return compounding monthly and 2.5% inflation.
- Starting balance
- $10,000
- Contributions over 10 years
- $60,000
- Total paid in
- $70,000
- Projected balance
- $100,134
- Of which growth
- $30,134
- Projected balance in today's dollars
- $78,224
Growth is under a third of the final balance. In today's dollars the $100,134 is worth about $78,224 — roughly $22,000 of the headline figure is inflation rather than purchasing power.
Methodology
What this calculator assumes
The return you enter is applied evenly for the whole term. Real returns vary year to year, and a sequence of poor early years produces a different result from the same average arriving in a different order.
Contributions are constant. They are not indexed to inflation or to pay rises.
Contributions and compounding are placed on their own schedules, so a fortnightly contribution into a monthly-compounding balance is modelled on the real dates rather than rescaled.
No tax on earnings, no contribution tax, no account or investment fees, no transaction costs.
The today's-dollars column discounts the final balance by the inflation rate you enter, compounded annually.
Limitations
What it does not do
This is not a forecast, a product comparison or a recommendation. It projects the arithmetic consequence of the assumptions you enter.
A negative return is accepted as an input, but the model applies it evenly for the whole term. That is not how a market loss usually behaves.
Savings accounts and investment portfolios are not the same risk. This calculator does not distinguish between them — the return you enter has to carry that judgement.
Tax treatment differs sharply between an investment held personally, in super, or through a trust or company. None of that is modelled.
About this model
Built by the Souffle team. Model version 1.1.0, in effect from 2026-09-10. It has not been independently reviewed, and no review badge is shown for that reason. The calculation runs entirely in your browser — nothing you type is sent to us or to anyone else, and nothing is stored.
Questions
Common questions
Why is the projected balance lower than other compound interest calculators?
Most default to contributions at the start of each period, which earns one extra period of growth. This calculator defaults to the end of the period and lets you change it under Assumptions. Check the timing before comparing two tools.
Does this include tax and fees?
No. The result is before both. Fees in particular compound against you in the same way returns compound for you, so a projection that ignores them overstates a long-term balance.
What return should I use?
That is a judgement about risk, timeframe and what the money is invested in, and it is exactly the kind of question a licensed adviser is for. This calculator does not suggest a figure.
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