➗ Math
How to Calculate Percentage Change (Increase and Decrease)
Work out the percentage increase or decrease between two numbers, why the original value is the anchor, and how percentage change differs from percentage points.
Updated 21 September 2026 · 2 min read
Prices go up, audiences grow, budgets get cut — and in each case the honest way to describe the shift is as a percentage change relative to where you started. Getting it right is mostly about remembering which number to divide by.
The formula
Percentage change compares the size of a change to the original value, not the new one. Subtract the old value from the new value to get the raw change, divide by the original, and multiply by 100.
A worked example
Say a subscription rises from £120 to £150. The raw change is 150 − 120 = 30. Divide by the original 120 to get 0.25, then multiply by 100: that's a 25% increase. Note that going the other way — from 150 back to 120 — is *not* a 25% fall. The change is still 30, but now you divide by 150, giving a 20% decrease. The starting point sets the size of the percentage, which is why increases and decreases rarely mirror each other.
Increase vs decrease
- If the new value is larger, the result is positive — a percentage increase.
- If the new value is smaller, the result is negative — a percentage decrease.
- If the values are equal, the change is 0% — no change at all.
- You can't take a percentage change from an original of zero: dividing by zero is undefined, so there's no meaningful starting point to compare against.
Percentage change vs percentage points
These two are constantly confused, especially with statistics. If an interest rate or approval share moves from 10% to 15%, that is a rise of 5 percentage points — the plain gap between the two figures. But as a percentage *change* it's a 50% increase, because 5 is half of the original 10. When a headline says a rate "jumped 50%", it usually means percentage change; when it says "up 5 points", it means percentage points.
For everyday jobs — comparing this month to last, a before-and-after price, or a year-on-year figure — the percentage change calculator gives both the percentage and the absolute amount of change in one step. If you're brushing up on the basics, the guide to percentages covers the fundamentals, and working out a sale price is really the same maths dressed differently — see how to calculate a discount.
Frequently asked questions
- How do I calculate a percentage increase?
- Subtract the original value from the new value, divide by the original value, then multiply by 100. From 120 to 150 is (150 − 120) ÷ 120 × 100 = 25% increase.
- Why isn't a decrease the same percentage as the increase?
- Because the original value changes. Going from 120 to 150 divides the change of 30 by 120 (25%), but going from 150 to 120 divides the same 30 by 150 (20%). The starting number is always the anchor, so the two directions give different percentages.
- What's the difference between percentage change and percentage points?
- Percentage points are the plain difference between two percentages — 10% to 15% is a 5-point rise. Percentage change measures that gap relative to the start, so the same move is a 50% increase.