How do you calculate percentage decrease?
Subtract the new value from the old value, divide by the absolute old value, and multiply by 100.
Percentage Decrease Calculator
Measure how much a value fell, or apply a percent decrease such as a discount.
Choose a decrease mode
Compare old and new values, or reduce a starting value by a percent.
About this calculator
How to use Percentage Decrease Calculator, plus the formulas and logic behind the results.
Tool description
Percentage decrease measures a reduction relative to the original amount, or applies a percent off to compute the new amount.
How to use
- Select from-to mode or decrease-by-percent mode.
- Enter the old value and either the new value or the percent.
- Select Calculate to view the decrease.
- Confirm the decrease amount in the results.
Formulas & reference
From→to: ((old − new) ÷ |old|) × 100By percent: new = old × (1 − percent ÷ 100)Decrease amount = old − new
Decrease examples
- 100 → 80 is a 20% decrease
- Decrease 50 by 10% → 45
- Discounts are percent decreases on price
- Base of zero cannot measure percent decrease
Calculation logic
From-to mode uses (old − new) over |old|. By-percent mode scales by (1 − p/100). Zero old values are blocked for from-to comparisons.
If the new value is higher than the old value in from-to mode, the result is a negative decrease (an increase).
Percentage Decrease Calculator FAQ
Common questions about this calculator and how to use the results.
How do you decrease a number by a percent?
Multiply by (1 − percent/100). For example, 50 × 0.90 = 45 for a 10% decrease.
Is a discount a percentage decrease?
Yes. A 20% off sale decreases the price by 20%.
What if the new value is higher?
From-to mode returns a negative decrease, meaning the value actually increased.
Can percent be over 100 in by-percent mode?
Yes mathematically, but the new value may become negative.
Why use absolute old value?
It keeps the formula defined for negative bases while preserving a clear magnitude.