How it works
Select the question, then enter A and B. For “A percent of B,” 20 and 150 return 30. For “A is what percent of B,” 30 and 150 return 20%. For “Change from A to B,” 100 and 125 return a 25% increase.
Example: an increase from 100 to 125
Select percentage change, enter 100 as A and 125 as B. The difference is 25, so the change is 25%. Reversing the inputs gives a 20% decrease because the starting amount is different.
Starting at zero
A percentage share needs a nonzero total. A percentage change needs a nonzero starting value. When the starting value is negative, this calculator divides by its absolute value; different reporting conventions can interpret negative bases differently. The result is arithmetic using your inputs, not a verification of a price, tax, or financial return. Results are rounded for display while calculations retain numeric precision.
Formula
A percent of B = A ÷ 100 × B. A as a share of B = A ÷ B × 100. Change from A to B = (B − A) ÷ |A| × 100. Shares require a nonzero B; change requires a nonzero A.
Common questions
Are a 20% increase and a 20% decrease opposites?
They use different starting values. Increasing 100 by 20% gives 120; reducing 120 by 20% gives 96.
Can I calculate a change from zero?
No. The percentage-change formula divides by the starting amount. Describe the absolute change instead.
Is the displayed value exact?
Display values are rounded. Calculations use JavaScript numeric precision and are intended for ordinary estimates, not accounting reconciliation.
Pick the question before entering numbers
A percentage of an amount, a percentage share, and a percentage change answer different questions. If twenty files out of eighty are videos, the share is 25%. If you want to keep 25% of eighty files, the quantity is twenty. If a folder grows from eighty files to one hundred, the change is 25%. The same numbers can appear in different roles, so check the selected mode.
For a smaller video export, suppose the original is 200 MB and the new file is 120 MB. The new file is 60% of the original size, while the reduction is 40%. Calling that a “60% reduction” confuses the remaining share with the amount removed.
Percentage points are different
Moving from a 10% click-through rate to a 15% rate is an increase of five percentage points. Relative to the original 10%, it is a 50% increase. State which measure you mean when comparing rates; the calculator’s percentage-change mode returns the relative change.
A change in a rate also needs context about the underlying counts. Ten clicks from one hundred views and one click from ten views have the same percentage, but not the same amount of evidence. This tool performs arithmetic and does not establish the reliability or cause of a difference.
Keep units and rounding consistent
Use the same units for both amounts. Convert gigabytes and megabytes before calculating a storage change, and avoid mixing a rounded display value with an exact byte count when a small difference matters. The Storage Calculator explains the decimal units used in these tools.
If you are comparing video exports, record the measured sizes and review quality as well as the percentage saved. Our video compression guide explains the practical check. A large reduction is not a success if the recipient cannot read the text or hear the explanation. Keep the original until you have verified that the smaller result still serves its purpose.
