Percentage Calculator – Percent Change, Increase

Percentage calculator formulas, and when each one applies

Three questions get called “percentage” and each takes a different formula. Picking the wrong one is the most common way a percentage goes wrong in a spreadsheet, so it is worth knowing which of the three you are actually asking:

Section 1 — taking a slice

A tip, sales tax, a deposit, a commission, progress against a quota. You know the whole and you want part of it.

Section 2 — comparing two numbers

A test score, a conversion rate, one line item against a budget. You know both numbers and you want the ratio between them.

Section 3 — measuring a move

A price change, headcount, traffic month over month. Whichever value came first is always the divisor, which is what makes the answer directional.

One shortcut worth knowing: to find a percentage of a number, p% of n always equals n% of p. If 18% of 50 looks awkward, flip it — 50% of 18 is 9, and you did it in your head.

Reverse percentage: working back to the original number

This is the calculation people get wrong most often, because instinct says to subtract that same percentage again. You have to divide instead. If a price of £78 is what remains after a 30% increase, the original was 78 ÷ 1.30 = £60 — not 78 − 30%, which would give £54.60. Going the other way, if $90 is a price after 25% off, the original was 90 ÷ 0.75 = $120.

That asymmetry explains a result which looks like a bug. Take 100, add 10% to get 110, then take 10% off: you land on 99, not 100. Your increase was 10% of 100 and your decrease was 10% of 110, so a percentage increase or decrease of the same size is never the same amount. To undo a 10% rise you need a 9.09% cut. Stacked discounts behave this way too — 25% off then a further 20% off is 40% off in total, not 45%, because that second cut applies to an already reduced price.

To reverse anything with this percentage calculator, use section three: put your known figure in as the new value, try an original, and adjust until the percentage change matches what you were told. Or use the discount calculator, which reverses a sale price directly. If the same rate applies period after period rather than once, that is compounding, and the compound interest calculator is the right tool.

Percentage points are not percent change

When a rate moves from 20% to 25%, two different true statements describe it: it rose by 5 percentage points, and it rose by 25%. One is a plain gap between two percentages. The other is relative change, 5 ÷ 20 × 100, and that is what section three of this percentage calculator computes.

This matters whenever whatever is being measured is itself a percentage — interest rates, unemployment, market share, tax bands. “Approval climbed 25%” and “approval climbed 25 points” describe very different weeks. Comparing two percentages? Say points. Measuring growth? Say percent.

Percentage change in Excel and Google Sheets

Same formula, same in both. With the old value in A2 and the new value in B2, percentage change is =(B2-A2)/A2 with the cell formatted as Percentage, or =(B2-A2)/A2*100 if you want a plain number. =B2/A2-1 is the same sum written shorter.

Two things catch people out. A #DIV/0! means the old value is zero, and percentage change from zero is undefined — no formula fixes that, because there is no base to compare against. And if A2 and B2 are already percentages, subtracting them with =B2-A2 gives percentage points, not percent change; the full formula above gives the relative move.

Common percentage calculations

Worked-out pages for frequently searched sums, each with a reference table.

• 10% of 100
• 15% of 100
• 20% of 100
• 25% of 100
• 30% of 100
• 40% of 100
• 5% of 1,000
• 10% of 1,000
• 15% of 1,000
• 20% of 1,000
• 1% of 1,000
• 10% of 50
• 15% of 50
• 20% of 50
• 18% of 50
• 20% of 60
• 15% of 60
• 10% of 200
• 15% of 200
• 20% of 200
• 25% of 200
• 30% of 200
• 20% of 80
• 25% of 80
• 40% of 50
• 7% of 100
• 8% of 100
• 3% of 100,000
• 6% of 50,000
• 2% of 100,000
• 1% of 1,000,000