Compound interest calculator
What your savings grow to — after RWT, fees and inflation, not just the headline.
Uses IRD's RWT rates, checked 8 November 2026. Runs in your browser — nothing you type is sent anywhere. Sources: IRD RWT rates.
Why this calculator shows less than the others
Most compound interest calculators assume you keep every cent of interest. In New Zealand, RWT comes off your interest before it's added, so less money is left to compound. Over decades that gap grows. Fees on investment funds do the same, and inflation shrinks what the final amount will buy.
The blue line is what you'd actually have. The orange line is what that would buy in today's prices.
Compound interest example: what tax takes
$10,000 to start, $200 added each month, 5% a year compounded monthly, with tax taken at 30% on the interest each year. 'No tax' is what most calculators show.
| Time | You put in | No tax | After 30% tax | Lost to tax |
|---|---|---|---|---|
| 10 years | $34,000 | $47,527 | $42,870 | $4,657 |
| 20 years | $58,000 | $109,333 | $89,491 | $19,842 |
| 30 years | $82,000 | $211,129 | $155,615 | $55,514 |
Common questions
How is interest taxed in New Zealand?
Banks take resident withholding tax (RWT) from your interest before paying it. Your RWT rate should match your income band: 10.5%, 17.5%, 30%, 33% or 39%. If you haven't given your bank your IRD number, they take 45%.
What's the compound interest formula?
A = P × (1 + r/n)^(n×t), where P is the starting amount, r the yearly rate, n how many times a year interest is added and t the number of years. Regular deposits are added on top.
Does daily or monthly compounding make much difference?
Very little. $10,000 at 5% for a year earns $511.62 compounded monthly and $512.67 compounded daily. The rate and the time you leave it matter far more.
What about KiwiSaver or PIE funds?
PIE funds are taxed at your prescribed investor rate (PIR) of 10.5%, 17.5% or 28%, and charge fees. Pick the closest tax rate and add the fund's fee under 'Compounding, fees and inflation'. Returns aren't guaranteed like a savings rate.