Everyday math
Calculate the percentage decrease from an original value to a reduced value.
Enter your numbers to see the answer.
Percentage Decrease Calculator guide
A percentage decrease tells you how much a value has fallen relative to its starting point. Enter the original value and the reduced value, and the calculator divides the drop by the original and shows the result as a positive percentage. It suits discounts, weight loss, a fall in monthly usage, a smaller invoice, or any pair where the earlier figure is the reference and the later one is lower. Because the original value is the denominator, a fall from 100 to 80 is 20%, while the climb back from 80 to 100 is 25%; the two directions are not mirror images. If your new value is larger than the original, the result turns negative, which simply signals growth; use the percentage increase calculator for that. This page is not for comparing two independent measurements, where the percentage difference calculator is the right fit.
Percentage decrease (%) = (original value − new value) ÷ original value × 100.
Using the default inputs of 100 (original) and 80 (new): the value fell by 100 − 80 = 20. Since the base for a decrease is the original, the ratio is 20 ÷ 100 = 0.2, and 0.2 × 100 = 20%. The page reads “20% decrease from 100 to 80” and lists the absolute change (20) and the original value (100). Read as a discount, a $100 item marked down 20% sells for $80. Compare the return trip: climbing from 80 back to 100 is 20 ÷ 80 = 25%, because the base has shrunk to 80.
The two fields must share a unit, and the unit disappears in the result. Enter plain numbers only, no % signs or currency symbols. The answer is a percentage of the original value, not of the new value, so a 20% decrease and a 20% increase do not cancel each other.
The result follows directly from the numbers you entered, so its accuracy is the accuracy of those inputs. The most common source of a surprising answer is a mismatch between the question you meant to ask and the one the calculator answered, so re-read the field labels if the figure looks wrong. The formula is shown above so you can check the arithmetic yourself. On this page the figure rests entirely on original value and new value, so start there if the percentage decrease calculator returns something you did not expect.
Good to know: entering the values in the wrong fields flips the sign and changes the size (100 → 80 is −20%, 80 → 100 is +25%). A decrease can never exceed 100% for a positive quantity; anything larger means a typo. Successive markdowns multiply rather than add: 20% off then 10% off leaves 72% of the price, a 28% total decrease, not 30%.
Results are planning estimates. Check the units, product coverage, carrier rules, or local rates before making a purchase or financial decision.
How it works
Calculate the percentage decrease from an original value to a reduced value. This tool explains the calculation so you can adjust the assumptions to match your situation.
Worked example
With Original value = 100 · New value = 80 → 20 % decrease (from 100 to 80). Change an input above and this example updates with your numbers.
Common questions
20%. The value dropped by 100 − 80 = 20, and 20 ÷ 100 × 100 = 20%. If the same item later returned from 80 to 100, that would be a 25% increase because the denominator becomes 80.
Subtract the new value from the original, divide by the original, and multiply by 100. Keep the original in the denominator even though the new value is the number you have most recently.
No. 100 reduced by 20% is 80, and 80 increased by 20% is 96, not 100. To reverse a 20% decrease you need a 25% increase, because the second step works from a smaller base.
Not for quantities that cannot go below zero, such as prices or weights. A result over 100% means the original and new values were entered in the wrong fields or the new value is negative.
It means the value actually rose rather than fell. Entering original 80 and new 100 shows −25%, which is really a 25% increase. Switch to the percentage increase calculator for a positive reading of the same change.