Percentage Calculator — Portion, Increase, and Decrease
Calculate a percentage of an amount and see that amount after the same percentage increase or decrease.
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How to use it
Enter a base amount and percentage to find the portion, increase, and decrease.
Good to know
Results use standard floating-point arithmetic; round appropriately for your use.
Calculate the portion and two related amounts
Enter a base amount and a percentage. With an amount of 80 and a percentage of 25, the result is 25% of 80 is 20. The tool also shows an Increase of 100 and a Decrease of 60.
The three calculations use the same base amount: portion = amount × percentage ÷ 100; increase = amount × (1 + percentage ÷ 100); decrease = amount × (1 − percentage ÷ 100). This is useful for checking a discount, markup, or quick allocation before entering it elsewhere.
Do not treat a decrease as the reverse of an increase
A 25% increase from 80 reaches 100, but a 25% decrease from 100 reaches 75. Percent changes use their current base, so equal up and down percentages do not usually cancel each other. Use a percentage-change calculation when you need to compare an old value with a new one.
Round for the real-world use
The calculator uses standard floating-point arithmetic and normally rounds non-integer results to six decimal places. Very small nonzero results can display in JavaScript’s scientific notation. Currency, tax, invoices, and regulated calculations can have their own rounding rules, so round at the step required by the relevant policy rather than assuming the displayed value is final. Negative and percentages above 100 are accepted mathematically; check that they match your intended situation.