Four percentage calculations on one page: X% of a number, one number as a percentage of another, percentage change between two figures, and increasing or decreasing a number by a percentage. Handy for rent increases, discounts, pay rises and marks.
X% of Y—
X as a % of Y—
% change—
After the % change—
General information only — not financial, tax or legal advice. Figures are estimates from the inputs you enter. Confirm with your adviser, accountant, lender or insurer.
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Prices exclude GST. Illustrative tools only — not legal, tax or financial advice.
The mistakes that catch people
Percentage change is always measured against the starting figure: rent going from $650 to $720 is a 10.8% rise, but from $720 back to $650 is a 9.7% fall. Adding then removing the same percentage doesn't get you back where you started — a 20% discount then a 20% increase leaves you 4% down. And percentage points aren't percentages: a mortgage rate going from 5% to 6% is a one-point rise but a 20% increase in your interest bill.
NZ examples
Rent increases (landlords can increase rent once every 12 months — work out the % so you can compare it with the market), pay rises versus inflation (a 3% rise against 2.5% CPI is a 0.5% real increase), and GST — 15% on top of a price, but to strip GST out you divide by 1.15 (or multiply by 3 and divide by 23), not take 15% off.
Assumptions and sources
Percentage change
(new − old) ÷ |old| × 100.
Rounding
Displayed to two decimal places; calculations are unrounded.
GST
Not applied here — see the GST calculator for the 15% incl/excl conversions.
Last reviewed 6 September 2026. Rates and thresholds change — check the linked sources before relying on a figure.
Frequently asked questions
How do I calculate a percentage increase?
Subtract the old value from the new value, divide by the old value, multiply by 100. From 650 to 720: (720 − 650) ÷ 650 × 100 = 10.77%.
How do I work out 15% of a number?
Multiply by 0.15. 15% of 240 is 36. To add 15% (for example GST), multiply by 1.15 instead: 240 × 1.15 = 276.
How do I take a percentage off a price?
Multiply by (1 − the percentage). 20% off $1,200 is 1,200 × 0.8 = $960. To reverse a discount you divide by 0.8 rather than adding 20% back.
What is the difference between percent and percentage points?
Percentage points measure the gap between two percentages; percent measures relative change. An interest rate moving from 5% to 6% is up one percentage point and up 20 percent.
