Percent of a number, percent change, and percent difference
Three percentage questions cover most everyday math, and this calculator handles each one with live results as you type. "X% of Y" multiplies a percentage against a value, which is the operation behind tips, sales tax, commission, and test scores. "X is what % of Y" works in reverse: give it a part and a whole, and it reports the part as a percentage of the whole, useful for grades, market share, battery level, and progress tracking.
The third mode, percent change, measures how much a value grew or shrank between two points: the difference is divided by the starting value and shown as a signed percentage, colored green for increases and red for decreases so the direction is obvious at a glance. Note the asymmetry that trips many people up: going from 80 to 100 is a 25% increase, but falling back from 100 to 80 is only a 20% decrease, because each change is measured against a different starting value.
Results are computed instantly on every keystroke and each mode has a one-click copy button for pasting into email, reports, or spreadsheets. For related money math, the tip calculator applies percentages to restaurant bills, the discount calculator stacks percentage markdowns with sales tax, and the loan calculator turns percentage rates into monthly payments.
Convert the percentage to a decimal by dividing by 100, then multiply by the number. For example, 25% of 200 is 0.25 times 200, which equals 50. The first mode on this page does exactly that arithmetic with any values you enter, including decimals.
Percent change equals the difference between the new and old values, divided by the old value, times 100. If a price rises from 80 to 100, the change is 20 divided by 80, or 25%. The calculator colors increases green and decreases red, and the sign makes direction explicit.
Percent change compares a new value against a specific old starting value, so direction matters: 80 to 100 is +25%, while 100 to 80 is -20%. Percent difference compares two values against their average, giving one directionless number. Use percent change when one value is clearly the earlier or original one.
A change over 100% simply means the value more than doubled. If a quantity grows from 40 to 100, that is a 150% increase, because the 60-point gain is 1.5 times the starting value. Doubled exactly is +100%; tripled is +200%. Negative changes bottom out at -100%, which means the value fell to zero.
Divide the final amount by one plus the rate as a decimal. If a price with 20% tax included is 120, the pre-tax original is 120 divided by 1.2, which is 100. Avoid the common mistake of subtracting 20% of 120; the percentage was computed on the smaller original, not the final total.