Practical guide
Percentage of a number, ratio and percentage change — how do they differ?
“Percentage” can mean part of a number, a relationship between values or change over time. Each question uses a different formula and assigns different roles to A and B.
Percentage of a number
To find A% of B, convert the percentage into A ÷ 100 and multiply by B. The result has the same unit as B.
For example, 15% of PLN 200 is PLN 30.
One value as a percentage of another
The question “A is what percentage of B” uses B as denominator: A ÷ B × 100%. B cannot be zero.
If 30 people from a group of 120 selected an answer, the share is 25%.
Percentage change
Change compares B − A with starting value A. Growth from 100 to 120 is 20%, but decline from 120 to 100 is about 16.67%.
For a negative starting base we use the absolute value of A in the denominator; this is an explicit convention, not an investment return. Percentage change from A = 0 is undefined.
Percentage points describe a difference between percentage values and are not the same as percentage change.
Price before an increase or decrease
Recover an original price by dividing the final price by 1 + rate/100 for an increase or 1 − rate/100 for a decrease. PLN 80 after a 20% decrease means PLN 100 originally.
A 100% decrease cannot be uniquely reversed. Price workflows require nonnegative values and decreases no greater than 100%.
Frequently asked questions
What does this guide explain about “Percentage of a number”?
To find A% of B, convert the percentage into A ÷ 100 and multiply by B. The result has the same unit as B. For example, 15% of PLN 200 is PLN 30.
What does this guide explain about “One value as a percentage of another”?
The question “A is what percentage of B” uses B as denominator: A ÷ B × 100%. B cannot be zero. If 30 people from a group of 120 selected an answer, the share is 25%.
What does this guide explain about “Percentage change”?
Change compares B − A with starting value A. Growth from 100 to 120 is 20%, but decline from 120 to 100 is about 16.67%. For a negative starting base we use the absolute value of A in the denominator; this is an explicit convention, not an investment return. Percentage change from A = 0 is undefined. Percentage points describe a difference between percentage values and are not the same as percentage change.
What does this guide explain about “Price before an increase or decrease”?
Recover an original price by dividing the final price by 1 + rate/100 for an increase or 1 − rate/100 for a decrease. PLN 80 after a 20% decrease means PLN 100 originally. A 100% decrease cannot be uniquely reversed. Price workflows require nonnegative values and decreases no greater than 100%.
Calculators covered by this guide
Percentage calculator
Calculate a percentage of a number, percentage ratio or percentage change.