Percentage Increase Calculator — increase or decrease a number by a percent

How it works

Increasing a number by a percentage means adding that share of it back on. Since % is just 0.01, a percent converts to a decimal, and adding it to 1 gives the multiplier that scales the original in one step:

new value = value × (1 + percent ÷ 100)

The (1 + …) is the key: the "1" keeps the whole original, and the decimal percent adds the extra on top. A 15% increase multiplies by 1.15; a 10% decrease uses a negative percent, multiplying by 0.90. This is why the same formula handles both — a decrease is simply an increase by a negative amount.

Worked example

Increase 80 by 15%:

new value = 80 × (1 + 15 ÷ 100) = 80 × 1.15 = 92

The change — the amount added — is 92 − 80 = 12, which is just 15% of 80. Going the other way, 200 decreased by 10% is 200 × (1 − 0.10) = 200 × 0.90 = 180, a change of −20. And a 50% markup on 40 is 40 × 1.5 = 60.

Frequently asked questions

How do I increase a number by a percentage?

Multiply by 1 plus the percent as a decimal. For a 25% increase on 60, that's 60 × 1.25 = 75. Equivalently, work out 25% of 60 (which is 15) and add it on — same answer, two steps instead of one.

How do I decrease a number by a percentage?

Multiply by 1 minus the percent as a decimal, or enter a negative percent here. A 30% decrease on 90 is 90 × 0.70 = 63. The shortcut is that taking 30% off means keeping 70%, so you multiply by 0.70 directly.

What's the difference between the "new value" and the "change"?

The **change** is the amount added or removed — that's percent of the original (15% of 80 = 12). The **new value** is the original plus that change (80 + 12 = 92). One is the size of the step; the other is where you end up.

Can the result be negative?

Yes, if you decrease by more than 100%. Taking 120% off 50 gives 50 × (1 − 1.2) = −10. Increases have no upper limit — a 200% increase triples the value (×3), because you keep the original 100% and add 200% more.

Why can't I just add the percent to the number directly?

Because a percent is a share *of* the number, not a fixed amount — 15% of 80 is 12, but 15% of 200 is 30. You have to scale the percent by the value first, which is exactly what multiplying by (1 + percent ÷ 100) does in a single step.

How is this different from percentage change?

This page starts with a value and a percent and gives you the **new value**. Percentage change works backwards: you already have the old and new values and want the **percent** between them. They're inverse operations on the same relationship.