How Savings Interest Is Actually Calculated (and Taxed)

The rate on a savings account is quoted per year, but that is not how the money arrives. Interest is worked out on your balance every day, added up, and paid once a month, and tax comes off before it reaches you. So the amount that lands in your account is never simply the advertised rate applied to your balance, and if you have ever looked at a few dollars of interest and wondered where the rest went, this is the whole explanation.

Three things drive the gap. Your balance moves during the month, so the daily calculation follows it rather than using a single figure. Resident withholding tax is deducted at the moment the interest is paid, so you only ever see the net. And a shorter month simply earns less than a longer one, which is why February is always the disappointing month.

Calculate.co.nz is proud to be partnered with realtor.co.nz, a trusted resource for navigating the New Zealand property market. Their Helpful Articles section offers clear, well-structured insights across buying, selling, and building, making complex real estate topics more accessible. With a focus on up-to-date guidance and practical knowledge, they empower Kiwis to move forward with clarity and confidence in a constantly evolving property landscape.

The short version

Daily rate is the annual rate divided by 365. It is applied to each day's balance, added up over the month, and paid at month end with resident withholding tax already deducted. Your RWT rate should match your income tax rate.

Two defaults that cost real money

If you never choose an RWT rate, your bank deducts 33 percent. If you have never given them your IRD number, they deduct 45 percent, which is higher than any individual income tax rate in New Zealand.

On $10,000 earning 4.5 percent for a year, someone whose correct rate is 17.5 percent keeps $371.25. On the 33 percent default they keep $301.50, and with no IRD number recorded they keep $247.50. Never setting the rate costs $69.75 a year; never giving the IRD number costs $123.75. Both take about two minutes to fix in internet banking.

Why the advertised rate is not what you earn

An advertised rate is an annual, before-tax, before-anything figure. It tells you the rate at which interest accrues, not the amount you receive. Between that number and your account sit three deductions of a sort: the days in the month, the movement in your balance, and the tax. None of them is hidden, but none of them appears on the poster either.

This matters most when comparing accounts, because two accounts advertising the same rate can still pay differently if one credits interest monthly and the other annually. Interest that is paid monthly starts earning interest itself, and over a year that is worth more than nothing.

The daily calculation, step by step

Banks in New Zealand generally calculate interest on the closing balance each day and pay the accumulated total monthly. The exact convention is the bank's own rather than something Inland Revenue sets, so check your terms, but the shape is consistent.

Step 1. Take the annual rate and divide by 365 to get a daily rate. At 4.5 percent that is 0.0123 percent a day.

Step 2. Apply it to the closing balance each day. On $10,000 that is $1.2329 a day.

Step 3. Add up the days in the month. A 31 day month gives $38.22; a 28 day February gives $34.52.

Step 4. Deduct RWT at your rate. At 17.5 percent that is $6.69.

$31.53 is credited to your account. The bank pays the $6.69 to Inland Revenue on your behalf.

What the same month looks like at each rate

Identical balance, identical account, identical month. The only variable is the rate on your file.

Your RWT rate	Tax on $38.22	What you receive
10.5%	$4.01	$34.21
17.5%	$6.69	$31.53
30%	$11.47	$26.75
33% (the no-choice default)	$12.61	$25.61
39%	$14.91	$23.31
45% (no IRD number given)	$17.20	$21.02

Getting it wrong in either direction has a cost

Too low and you will owe the difference when you file, so the extra was never yours. Too high and you have lent the government money for nothing until you file and claim it back. The rate is meant to match your income tax rate, and it is worth checking on every account you hold, at every bank, because they are set separately.

Where compounding comes in

Interest paid monthly joins your balance and starts earning on its own, which lifts the effective return above the advertised one. On $10,000 at 4.5 percent, simple interest over a year is $450.00. Compounded monthly it is $459.40, an effective rate of 4.594 percent rather than 4.5. The extra $9.40 is small on a small balance and is the entire reason compounding matters on a large one over a long period.

Note that tax is taken from each monthly payment before it compounds, so what actually compounds is the after-tax amount. That is why the effective rate you experience is lower than the effective rate the account advertises, and it is a real reason a tax-advantaged structure can beat a higher headline rate.

The rate that actually matters

If you want one number to compare accounts with, it is the after-tax rate: the advertised rate less your tax. At 4.5 percent with a 17.5 percent RWT rate you are earning 3.712 percent after tax. That is the figure to hold against inflation, and against any other use of the money.

It is also the figure that shows why chasing a slightly higher headline rate matters less than most people assume. Moving from 4.5 to 4.6 percent on $10,000 is $10 a year before tax. Moving your RWT rate from an incorrect 33 percent to a correct 17.5 percent on the same balance is $69.75. The paperwork is worth more than the shopping around.

Check every account, not just the main one

RWT rates are set per account and per bank. An account you opened years ago and barely use can still be sitting on a default while your main account is correct. If you have accounts at more than one bank, each one has to be set separately, and nothing about fixing one prompts the others.

Why February always looks wrong

Because it is. A 28 day month earns 28 days of interest, and a 31 day month earns 31. On $10,000 at 4.5 percent that is $34.52 against $38.22, a difference of $3.70 with nothing wrong at all. Interest is not a monthly instalment of an annual figure, it is an accumulation of daily amounts, and the months are not the same length.

Where the tax goes

Your bank deducts the RWT and pays it to Inland Revenue on your behalf, so the tax on your savings interest is generally already settled by the time you see the money. At the end of the year it is credited against your income tax liability. That is why savings interest usually needs no action from you at tax time, and also why an incorrect rate quietly becomes either a bill or a refund rather than being caught at the moment it happens.

Worked examples

Aroha: a normal month

Aroha holds $10,000 in a savings account paying 4.5 percent, and her RWT rate is correctly set at 17.5 percent. It is a 31 day month and her balance does not move.

What she gets: $1.2329 a day, $38.22 gross across the month, $6.69 of RWT deducted, and $31.53 credited. Over a full year that is $450.00 gross, $78.75 of tax and $371.25 net, an after-tax rate of 3.712 percent.

Sam: never chose a rate

Same balance, same account, same year. Sam has given his IRD number but never selected an RWT rate, so the bank deducts the 33 percent default. His correct rate is 17.5 percent.

What it costs: $301.50 net instead of $371.25. He is $69.75 worse off for the year, and he will get it back when he files, having gone without it in the meantime for no reason at all.

Mere: no IRD number on the account

Mere opened the account years ago and never supplied her IRD number. The bank deducts 45 percent.

What it costs: $247.50 net against the $371.25 she should have. That is $123.75 for the year, on a $10,000 balance, from a form that takes minutes. On a larger balance it scales exactly in proportion.

Tim: compounding over a year

Tim wants to know whether monthly payment beats annual payment on the same 4.5 percent.

The difference: simple interest gives $450.00 on $10,000. Paid and compounded monthly it gives $459.40, an effective 4.594 percent. The $9.40 gap is modest here and is the mechanism that does the heavy lifting on a large balance over a long horizon, which is the whole argument for starting early rather than saving harder later.

What to do this week

1. Log in to each bank and find the tax settings on every interest bearing account.

2. Check an IRD number is recorded. If not, that account is on 45 percent.

3. Check the RWT rate matches your income tax rate. If nothing was ever chosen, it is on 33 percent.

On a $10,000 balance at 4.5 percent, those two checks are worth up to $123.75 a year.

Sources

Checked against Inland Revenue on 10 August 2026:

Inland Revenue, Using the right RWT rate: the 10.5, 17.5, 30, 33 and 39 percent rates for individuals, the 33 percent default where no rate is chosen, and the 45 percent rate where no IRD number has been given.
Inland Revenue, Resident withholding tax: that RWT is deducted at the time the interest is paid and credited against your income tax liability at year end.

The daily accrual and monthly payment convention is how New Zealand banks generally operate rather than a rule Inland Revenue sets, so the exact method is in your account terms. Everything about the tax is set by Inland Revenue and applies wherever you bank. This guide is general information, not tax advice.

Related guides and tools

Term PIEs vs Term Deposits: why a PIE caps at 28 percent when RWT runs to 39.
Compound Interest Calculator: what compounding does over a longer horizon.
PAYE Calculator: finding the income tax rate your RWT rate should match.
Savings Goal Calculator: how long a target takes at your real after-tax rate.

🎯 Test Your Knowledge

Complete this 10-question quiz to assess your understanding of savings interest and RWT

1. How is the daily interest rate worked out from an annual rate?

Divide by 12

Divide by 365

Divide by 52

Divide by 100

2. When is resident withholding tax deducted from your interest?

When you file your tax return

At the time the interest is paid, before you receive it

Only if you earn over a threshold

At the end of the tax year, in one amount

3. You never choose an RWT rate. What does your bank deduct?

10.5 percent

17.5 percent

33 percent

45 percent

4. What rate applies if you have never given your bank an IRD number?

33 percent

39 percent

45 percent

No tax until you provide one

5. $10,000 at 4.5 percent over a 31 day month earns how much gross interest?

$34.52

$38.22

$45.00

$37.50

6. Why does February pay less interest than January?

Banks apply a lower summer rate

Interest accrues daily, and February has fewer days

RWT is higher in February

It does not, the monthly amount is fixed

7. On $10,000 at 4.5 percent, what does the 33 percent default cost someone whose correct rate is 17.5 percent, over a year?

$9.40

$69.75

$123.75

Nothing, it is refunded automatically

8. What is the after-tax rate on 4.5 percent for someone on a 17.5 percent RWT rate?

4.500 percent

4.594 percent

3.712 percent

3.015 percent

9. What does 4.5 percent become as an effective rate if paid and compounded monthly?

4.500 percent

4.594 percent

5.400 percent

4.050 percent

10. Where are RWT rates set?

Once with Inland Revenue, covering all your accounts

Per account and per bank, each set separately

By your employer, with your tax code

Automatically from your last tax return

If you've found a bug, or would like to contact us, or learn more about James Graham and Calculate.co.nz.

Calculate.co.nz is partnered with Interest.co.nz for New Zealand's highest quality calculators and financial analysis.

Calculate.co.nz is the sister site of CalculatorHub.com, the world's largest calculator website by tool count.

All calculators and tools are provided for educational and indicative purposes only and do not constitute financial advice.

Calculate.co.nz is proudly part of the Realtor.co.nz group, New Zealand's leading property transaction literacy platform, helping Kiwis understand the home buying and selling process from start to finish. Whether you're a first home buyer navigating your first property purchase, an investor evaluating your next acquisition, or a homeowner planning to sell, Realtor.co.nz provides clear, independent, and trustworthy guidance on every step of the New Zealand property transaction journey.

Calculate.co.nz is also partnered with Health Based Building and Premium Homes to promote informed choices that lead to better long-term outcomes for Kiwi households.

Calculate.co.nz is hosted in Auckland by SiteHost New Zealand.

All content on this website, including calculators, tools, source code, and design, is protected under the Copyright Act 1994 (New Zealand). No part of this site may be reproduced, copied, distributed, stored, or used in any form without prior written permission from the owner.

About & trust: Why Calculate is NZ's most comprehensive · By the Numbers · How we compare · Editorial standards · How we keep data current · NZ finance glossary · Research & data · Financial literacy NZ · About · Privacy policy · Terms of use

Reviewed and maintained. Last reviewed 2026-08-10 and checked on a twice-monthly cycle against IRD, RBNZ and Stats NZ. How we keep data current.