Percentage Calculator
Three jobs, one calculator: find a percentage of a value, work out what share one number is of another, or calculate the increase or decrease between two figures.
Formula and method
Percentage of a value: (percentage ÷ 100) × value.
Part as a percentage of a whole: (part ÷ whole) × 100. Undefined when the whole is zero.
Percentage change: ((new − original) ÷ original) × 100. Undefined when the original value is zero - there's nothing to compare against.
Every calculation runs at full precision. Only the numbers you see on screen are rounded.
Worked example
A supplier quotes you €50 and offers 20% off for early settlement.
20% of 50 = (20 ÷ 100) × 50 = 10, so you save €10.
Now the trap: going from 80 to 100 is ((100 − 80) ÷ 80) × 100 = 25% up, but going back from 100 to 80 is only a 20% drop. Same two numbers, different answer, because the base changes each time.
FAQ
Frequently asked questions
How do I work out a percentage of a number?
Divide the percentage by 100 and multiply by the number. 20% of €50 is (20 ÷ 100) × 50 = €10.
How do I work out what percentage one number is of another?
Divide the part by the whole and multiply by 100. 25 out of 50 is (25 ÷ 50) × 100 = 50%.
How is percentage change calculated?
Subtract the original value from the new value, divide by the original value, then multiply by 100. Going from 80 to 100 is ((100 − 80) ÷ 80) × 100 = 25%.
Why won't the calculator accept a starting value of zero?
Percentage change and percentage-of-a-whole both divide by the original value. You can't divide by zero, so instead of guessing at an answer the calculator tells you it's undefined.
Can I use negative numbers and decimals?
Yes, both work fine. The arithmetic runs at full precision behind the scenes; only what you see on screen gets rounded.
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