How this calculator works
How our Budget Builder works.
No projections, no compounding — this tool is honest arithmetic on the numbers you type in. Here’s exactly what it does with them.
What it calculates
You enter your income — one or more lines, each weekly, fortnightly, monthly or yearly — and a list of spending lines, each with its own frequency. The tool shows your monthly income, monthly spending, the surplus or shortfall between them, and an estimated savings rate.
The method
Everything is converted to a monthly average. A weekly amount is multiplied by 52 and divided by 12, fortnightly by 26 ÷ 12, and yearly by 1 ÷ 12. So $550 a fortnight in rent shows as 550 × 26 ÷ 12 = $1,191.67 a month. This is the accurate long-run average — slightly higher than “×2” for fortnightly amounts, because most months contain a bit more than four weeks.
Surplus = monthly income − monthly spending. A negative number means you’re spending more than you earn. The “left over” figure is displayed in the frequency of your first income line (per week, fortnight, month or year); the underlying maths is always monthly.
The savings rate counts two things as saving: any line whose name includes “saving”, “invest” or “KiwiSaver”, plus whatever surplus is left unallocated. Formally: savings rate = (surplus + savings-type lines) ÷ income. The idea is that money you haven’t committed to spending is money you could be saving — but do note that an unallocated surplus only becomes savings if it actually gets saved.
A worked example
Take-home pay of $1,000 a week, with spending lines that average $2,727 a month:
| Monthly | |
|---|---|
| Income (1,000 × 52 ÷ 12) | $4,333 |
| Spending (all lines averaged) | $2,727 |
| Surplus | $1,607 |
| Savings rate (incl. $217 savings line) | 42% |
The assumptions — read these before relying on any number
- Enter your take-home pay. The tool does no tax calculation, so income should be what actually lands in your account — after PAYE, KiwiSaver contributions and student loan deductions.
- Everything is a monthly average. Real months are lumpy: a month with five paydays, or the one where the car registration lands, will look different. Put irregular costs in as yearly amounts and the average stays honest.
- Lines are classified by their name. Anything containing “saving”, “invest” or “KiwiSaver” counts toward your savings rate instead of pure spending. Name lines accordingly — “Investment property costs” would be counted as saving, which may not be what you mean.
- Unallocated surplus counts as potential saving. The savings rate assumes leftover money is saveable. If it tends to evaporate, your real rate is lower — tracking actual account balances is the honest check.
- Nothing is verified, stored or sent to us. The tool takes your numbers at face value, entirely in your browser.
What this calculator is — and isn’t
This tool re-arranges the numbers you give it into a monthly view. It doesn’t recommend any product, provider or plan, and it doesn’t know your circumstances, so it isn’t financial advice under the Financial Markets Conduct Act 2013. A budget snapshot is a starting point — the useful conversation is what to do with a surplus (or how to close a shortfall), and that depends entirely on your situation.
That conversation is what we do. If you’re running a deficit, especially, please don’t sit on it — book a free chat, or contact MoneyTalks (0800 345 123) for free, confidential help from financial mentors.
Sow Financial Limited (FSP1008822) is a licensed financial advice provider. Our public disclosure statement sets out our licence status, how we’re paid, our complaints process, and our dispute resolution scheme.
Calculator methodology last reviewed: July 2026. If we change the method or assumptions, we’ll update this page.