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Percentage Increase/Decrease Calculator

Apply a percentage increase or decrease to any starting value and instantly see the new total along with the exact amount it went up or down.

Result
230
Amount of change: +30
200 increased by 15% = 230

Quick answer

To increase or decrease a number by a percentage, multiply it by (1 ± percent ÷ 100). The amount of change is value × percent ÷ 100. For example, 200 increased by 15% is 200 × 1.15 = 230 (a change of +30), and 200 decreased by 15% is 200 × 0.85 = 170 (a change of −30). This calculator does both and shows the change.

Formula & method

value × (1 ± percent ÷ 100)

Examples

Example 1: Increase by 15%
Input
200 increased by 15%
Result
230 (change +30)
Why
Amount of change = 200 × 15 ÷ 100 = 30. New value = 200 + 30 = 230, i.e. 200 × 1.15 = 230.
Example 2: Decrease by 15%
Input
200 decreased by 15%
Result
170 (change −30)
Why
Amount of change = 200 × 15 ÷ 100 = 30. New value = 200 − 30 = 170, i.e. 200 × 0.85 = 170.
Example 3: Increase by 100%
Input
50 increased by 100%
Result
100 (change +50)
Why
A 100% increase doubles the value: 50 × 100 ÷ 100 = 50, so 50 + 50 = 100.
Example 4: Decrease by 25%
Input
80 decreased by 25%
Result
60 (change −20)
Why
Amount of change = 80 × 25 ÷ 100 = 20. New value = 80 − 20 = 60, i.e. 80 × 0.75 = 60.
Example 5: Small increase
Input
120 increased by 5%
Result
126 (change +6)
Why
Amount of change = 120 × 5 ÷ 100 = 6. New value = 120 + 6 = 126, i.e. 120 × 1.05 = 126.

When to use this tool

  • Adjusting a price, salary, budget, or quantity up or down by a set percentage.
  • Applying a markup, markdown, raise, or discount and seeing both the new figure and the amount of change.
  • Working out a projected value after an expected percentage rise or fall.
  • Checking homework or estimates that involve increasing or decreasing a number by a percent.

Common mistakes

  • Adding the percent of the new total instead of the original — the change is always a percentage of the starting value, not the result.
  • Thinking a 50% increase then a 50% decrease returns to the start: 100 → 150 → 75, not back to 100, because each step is taken on a different base.
  • Assuming a decrease of 100% leaves the original value; a 100% decrease removes all of it and leaves 0.
  • Confusing percentage points with percent change — going from 10% to 12% is +2 percentage points but a 20% increase.

Frequently asked questions

How do I increase a number by a percentage?

Multiply the number by (1 + percent ÷ 100). For example, to increase 200 by 15%, compute 200 × 1.15 = 230. The amount of increase is 200 × 15 ÷ 100 = 30.

How do I decrease a number by a percentage?

Multiply the number by (1 − percent ÷ 100). To decrease 200 by 15%, compute 200 × 0.85 = 170. The amount of decrease is 200 × 15 ÷ 100 = 30.

What does the amount of change mean?

It is the absolute size of the rise or fall, equal to value × percent ÷ 100. The calculator shows it with a + sign for an increase and a − sign for a decrease.

Why does increasing then decreasing by the same percent not return to the original?

Each percentage is applied to a different base. Increasing 100 by 50% gives 150, then decreasing 150 by 50% gives 75, because the 50% decrease is taken from 150, not 100.

Can the percentage be more than 100?

Yes. An increase over 100% more than doubles the value (a 100% increase doubles it), and a decrease over 100% would push the result below zero, which the tool still computes.

Does the calculator handle decimals and negative starting values?

Yes. You can enter decimal values and percentages, and a negative starting value works too — the change is still value × percent ÷ 100 applied in the chosen direction.

Sources & references

External references open in a new tab. We are independent and not affiliated with these organizations.

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  • ✓ No sign-up required
  • ✓ Runs entirely in your browser — nothing is uploaded.
  • ✓ Formula and method shown above
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Provided “as is” for general information only — results may be inaccurate, so verify before you rely on them. No warranty; use at your own risk.

Built and reviewed by HIFreeTools against the formula shown above and any authoritative references cited on this page. See our methodology and editorial standards.

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