How to Calculate Percentage Increase and Decrease

Percentage increase and decrease come up constantly — a price rise, a pay bump, a discount, a drop in your monthly bill. The formula is simple once you see it, and our Percentage Calculator does it instantly, but it's worth understanding so you can spot when a number is being spun.

The formula

To find a percentage change, take the difference between the new and old values, divide by the old value, and multiply by 100:

change % = (new − old) ÷ old × 100

A positive result is an increase; a negative one is a decrease.

Worked examples

Your rent goes from $1,000 to $1,150. The change is (1150 − 1000) ÷ 1000 × 100 = 15% increase. Your electricity bill drops from $200 to $170. That's (170 − 200) ÷ 200 × 100 = −15%, a 15% decrease. Same percentage, opposite direction.

The mistake almost everyone makes

The single most common error is dividing by the wrong number. You must divide by the starting value, not the ending one. If a price rises from 80 to 100, that's a 25% increase (20 ÷ 80), but if it then falls from 100 back to 80, that's only a 20% decrease (20 ÷ 100). The same $20 move is a different percentage depending on where you started, because percentages are always relative to their base.

Why 50% off then 50% on doesn't cancel

This trips up shoppers and investors alike. Start at $100. A 50% discount takes you to $50. A later 50% increase adds 50% of $50, which is $25, landing you at $75 — not back at $100. Successive percentage changes compound against a shifting base, so they never simply cancel out.

Let the tool check your work

Our Percentage Calculator has a dedicated "% change" mode that spells out the direction and size of the change in a plain sentence, so you can sanity-check any figure you're given.