Income Volatility Calculator NZ 2026
Reviewed 7 August 2026.
Most self-employed people know their income is uneven and have never measured how uneven. That matters because almost every financial decision you make, from how big a buffer to hold to whether you can commit to a mortgage, rests on a monthly figure, and the monthly figure most people use is the annual total divided by twelve. You cannot spend an average. Rent, power and your own drawings arrive every month regardless of whether that month was a good one, and if half your year lands in a quarter of it then budgeting on the average leaves you short for most of the year and flush for a little of it. That pattern is the single most common source of tax debt among sole traders, because the flush months feel like surplus. This calculator turns twelve months of actual income into three things: a volatility score that can be compared with anyone else's, the spread you are really managing, and the amount of cash it would take to make every month look like the average.
The year in numbers
Month by month
Months below your average are shaded. The shortfall column is what each one would need to reach the average.
| Month | Income | Against average | Shortfall to cover |
|---|
Why the coefficient of variation and not the range
The gap between your best and worst month is the figure people reach for first, and on its own it is misleading, because it depends entirely on two months and ignores the other ten. A year with one freak month and eleven steady ones looks identical to a year that swung wildly throughout. The coefficient of variation uses every month, and because it divides by your own average it is comparable across incomes of any size. A score of 35 percent means the typical month sits about a third away from the average in one direction or the other, whether you earn $50,000 or $200,000.
What the score should change
A low score means you can plan on the average, hold a modest buffer and treat your income much like a salary. A high score means the average is close to useless for budgeting and you should be planning against the low months instead, holding a larger buffer, and being deliberate about reserving tax as money arrives rather than as it is spent. The most common mistake at the high end is treating a strong month as evidence that things have improved. Usually it is just the shape of the distribution, and the correction arrives within the quarter.
The smoothing reserve is the honest buffer number
Adding up what every below-average month falls short by gives you the total cash you would need to circulate through the year to pay yourself the same amount every month. It is a much better starting point for a buffer than a generic months-of-expenses rule, because it is built from your own pattern rather than someone else's. It is also the number to look at before committing to any fixed monthly obligation, since a lender will assess you on the average while your bank account experiences the months.
Worked example
The twelve months on this page total $78,000.00, an average of $6,500.00 a month. The best month is $10,200.00 and the worst is $2,600.00, a spread of $7,600.00, with the worst month coming in at 40.00% of the average. The standard deviation is $2,268.63, which against the average gives a coefficient of variation of 34.90%.
5 of the twelve months fall below the average, and bringing all of them up to it would take $11,300.00 across the year. That is the working capital this income pattern demands, and it is a far more useful buffer target than three or six months of expenses chosen from a rule.
How this is calculated
The average is the total divided by twelve. The median is the midpoint of the twelve months once sorted, which is shown alongside the average because a large gap between the two is itself a sign of skew. The standard deviation is the population standard deviation: each month's difference from the average is squared, those squares are averaged, and the square root is taken. The coefficient of variation is that standard deviation divided by the average, as a percentage. The smoothing reserve is the sum of the shortfalls in every month that came in below the average.
Related NZ calculators
- Self-Employed Emergency Fund Calculator to size the buffer this implies
- Irregular Income Tax Smoothing Calculator for holding back tax as income arrives
- Seasonal Business Cashflow Calculator if the pattern repeats each year
- Salary Plus Self-Employed Tax Calculator if you also have PAYE income
- Cash Runway Calculator for how long the business lasts on what it holds