Interest Calculator NZ: Simple, Compound & After-Tax Interest
Calculate how much interest a balance earns, compare simple versus compound interest, estimate the effect of New Zealand RWT, or work backwards from a starting and final balance to an annual interest rate.
Reviewed 10 August 2026 · New Zealand dollars · RWT options checked against Inland Revenue
Direct answer
How much interest will my money earn?
Enter the starting amount, annual interest rate and time period. For compound interest, choose how often interest is credited. If you select an RWT rate, the calculator also estimates the amount left after withholding tax is deducted from each interest credit.
Calculate interest
Enter a valid starting amount, annual rate and time period.
Your interest estimate
Interest rate calculator: work backwards from a final balance
If you know what a balance grew from and to, use this second calculator to estimate the annual compound rate that would produce that result over the period. This is useful for checking an implied return, not for predicting future rates.
The final balance must be positive and the time period must be greater than zero.
Simple interest vs compound interest
| Method | How it works | Formula | Common use |
|---|---|---|---|
| Simple interest | Interest is calculated only on the original principal. | I = P × r × t | Quick comparisons and products that explicitly use simple interest. |
| Compound interest | Interest is added to the balance, so later periods can earn interest on earlier interest. | A = P(1 + r/n)^(nt) | Savings, deposits and investments where returns are periodically credited. |
P = principal, r = annual rate as a decimal, n = compounding periods per year, t = years and A = final amount.
Worked NZ interest example
At 5% a year compounded monthly, $10,000 left for 5 years grows to about $12,833.59 before tax, producing about $2,833.59 gross interest. If a constant 33% RWT rate is applied to each interest credit, the estimated final balance is about $11,820.69. The calculator above lets you replace those assumptions with your own rate and RWT setting.
This is a calculation example, not a current bank rate. Actual products can calculate and credit interest differently, and fees or changing rates can alter the result.
How RWT affects interest in New Zealand
Resident withholding tax (RWT) is commonly deducted from interest paid to New Zealand residents. Inland Revenue currently lists individual RWT rates of 10.5%, 17.5%, 30%, 33% and 39% where an IRD number has been supplied. If you do not choose a rate, 33% is generally the default.
This calculator applies the selected RWT rate to each interest amount before it is added to the balance. That gives a more realistic after-tax compounding estimate than simply subtracting tax once at the end.
Official sources: Inland Revenue — using the right RWT rate and Resident withholding tax.
When to use this calculator instead of another NZ Calculator tool
Interest Calculator NZ FAQs
How do I calculate interest on $10,000?
Enter $10,000 as the starting amount, then add the annual rate and time. A 5% rate does not always mean exactly $500 each year because compound interest can add interest to the balance during the year.
Does monthly compounding earn more than annual compounding?
With the same quoted annual nominal rate, more frequent compounding generally produces a slightly higher gross final balance because interest is added sooner.
What is an interest rate calculator?
It can either calculate interest from a known rate or work backwards to estimate the rate implied by a starting balance, final balance and time period. This page supports both.
Does this calculator use current bank interest rates?
No. You enter the rate yourself, so the result can be used with a rate from any bank, credit union, lender or investment product. Check the provider for the current rate and how it compounds.
Is RWT the same as my final income tax?
Not necessarily. RWT is withholding during the year. Your final tax position can differ depending on your total income and circumstances.