Math calculator
Percentage Decrease Calculator
Find the percentage and absolute decrease between an original value and a new value.
Percentage decrease
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Two boxes, and a label that switches on its own
Enter an original value and a new value. The calculator subtracts new from original, divides by the original value's absolute size, and multiplies by 100. It always displays a positive percentage, but the heading above it changes automatically: if the new value ends up smaller, it reads "Percentage decrease"; if the new value is actually larger, the heading switches to "Percentage increase" instead of showing a confusing negative number.
Below the headline figure, the tool also lists the absolute decrease or increase — the plain difference between the two values, not a percentage — plus a summary line showing the original value pointing to the new one.
The original value can never be 0, since the formula divides by it; entering 0 there produces the message "Percentage change is undefined when the original value is zero" instead of a result.
The formula and a worked example
Dropping from 250 to 180: the difference is 250 − 180 = 70, and dividing by the original value's absolute size gives 70 ÷ 250 = 0.28, so the percentage decrease is 28%. The absolute decrease shown alongside it is simply 70.
Run the numbers the other way — original 80, new 100 — and the formula gives (80 − 100) ÷ 80 × 100 = −25. Because that comes out negative, the label flips to "Percentage increase" and the displayed figure is the positive 25%, since the value actually grew rather than shrank.
The denominator mistake
The single most common error when computing percentage decrease by hand is dividing by the new value instead of the original one. For the 250-to-180 example, that mistake looks like 70 ÷ 180 × 100 ≈ 38.89% — noticeably higher than the correct 28% — because 180 is a smaller number than 250, so dividing by it inflates the result. The rule to remember: the denominator is always the value you started from, never the value you ended up at.
It's also worth keeping the percentage and the absolute change separate in your head. A 28% decrease and a decrease of 70 units are two different facts about the same change — the percentage tells you the relative size of the drop, while the absolute figure tells you the raw amount.
Negative starting values
Because the formula divides by the absolute value of the original number, a negative starting point still produces a sensible result rather than a sign error. Going from −50 to −80 (a value that has dropped further into negative territory) gives (−50 − (−80)) ÷ |−50| × 100 = 30 ÷ 50 × 100 = 60%, correctly reported as a 60% decrease, since −80 represents a smaller quantity than −50 on the number line.
Where percentage decrease gets used
Any before-and-after comparison where the number goes down fits this formula:
- Retail markdowns – a $250 jacket marked to $180 is a 28% price decrease
- Weight tracking – dropping from 200 lb to 185 lb is a (200 − 185) ÷ 200 × 100 = 7.5% decrease
- Population decline – a town shrinking from 12,000 to 10,800 residents is a 10% decrease
- Stock price drops – a share falling from $64 to $48 is a (64 − 48) ÷ 64 × 100 = 25% decrease
- Fuel efficiency loss – mileage falling from 32 mpg to 28 mpg is a 12.5% decrease
- Subscriber counts – a channel losing followers from 5,000 to 4,250 is a 15% decrease
Frequently asked questions
How do you calculate the percentage decrease from 250 to 180?
Subtract the new value from the original: 250 − 180 = 70. Divide that by the original value: 70 ÷ 250 = 0.28. Multiply by 100 to get 28%.
Why does a "percentage decrease" calculator sometimes show "Percentage increase"?
Because you can enter any two numbers, and if the new value turns out higher than the original — like going from 80 to 100 — the change is actually a rise, so the label switches to "Percentage increase" and reports 25% instead of a misleading negative decrease.
What's the most common mistake people make with this formula?
Dividing by the new value instead of the original. For the 250-to-180 example, dividing by 180 instead of 250 gives roughly 38.89% instead of the correct 28% — always divide by the value you started from.
Why can't the original value be zero?
The formula divides by the original value, and division by zero is undefined. The calculator shows "Percentage change is undefined when the original value is zero" rather than attempting the calculation.
How does the calculator handle a negative original value?
It divides by the absolute value of the original, so the sign doesn't break the formula. Going from −50 to −80 gives a 60% decrease, since −80 is a smaller quantity than −50 even though both numbers are negative.
Is percentage decrease the same thing as the amount decreased?
No — they're related but different figures. The amount decreased is the plain subtraction, 70 units in the 250-to-180 example. The percentage decrease, 28%, expresses that same 70-unit drop relative to the starting value of 250.