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Tool guide Everyday Math 6 min read

Percentage Calculations: A Practical Guide to Change, Difference and Reverse Percentages

A practical reference for percentage of a number, percentage change, percentage difference and working backwards from a percentage.

By mppm-test Published October 2, 2026 Updated October 2, 2026

Percentage of a number

To find a percentage of a value, convert the percentage to a decimal and multiply it by the value. Twenty percent of 500 is 0.20 multiplied by 500, which equals 100.

Percentage change

Percentage change compares the difference between a new value and an original value with the original value. The starting value matters because it is the denominator.

Percentage difference

When neither number is naturally the starting point, percentage difference often compares the absolute difference with the average of the two values.

Reverse percentages

If you know the final amount after a percentage increase or decrease, work backwards using the multiplier. This is useful for discounts, taxes, growth rates and pricing questions.

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