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 and spending as separate lines, each in whatever frequency it’s paid or billed — rent monthly, groceries fortnightly, and so on. The tool puts everything on a common footing, shows your income, spending and what’s left over, and lets you view those totals in whichever frequency you choose.
The method
Every line is converted to a monthly average. A weekly amount is multiplied by 52 ÷ 12, a fortnightly one by 26 ÷ 12, a monthly one stays as it is, and a yearly one is divided by 12. So $550 a fortnight in rent counts as 550 × 26 ÷ 12 = $1,191.67 a month — the accurate long-run average, slightly more than “×2” because most months run a little longer than four weeks. This is what lets one line be fortnightly and another monthly, yet still add up correctly.
Your totals show in the frequency you pick. The “Show totals per” control (above the result) converts those monthly figures back into a weekly, fortnightly, monthly or yearly amount — so you can read your position in whatever cycle suits you. The underlying maths is always monthly.
Left over = income − spending. A negative number means you’re spending more than you bring in. An unallocated surplus only becomes savings if you actually save it.
A worked example
A mix of frequencies — $2,000 take-home a fortnight, $1,800 rent a month, $250 groceries a fortnight — with totals shown per month:
| Per month | |
|---|---|
| Income (2,000 × 26 ÷ 12) | $4,333 |
| Rent (already monthly) | $1,800 |
| Groceries (250 × 26 ÷ 12) | $542 |
| Left over | $1,991 |
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.
- Each line uses its own frequency. Set every line to how it’s actually paid or billed — weekly, fortnightly, monthly or yearly. The “Show totals per” control only changes how the totals are displayed, not the underlying maths.
- The monthly figure is an average. Real months are lumpy: one with five paydays, or the month the car registration lands, will look different. Put irregular costs in as yearly amounts and the average stays honest.
- An unallocated surplus is only potential saving. The tool doesn’t assume leftover money gets saved — that’s up to you. Tracking your 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.