Percentage of a value
result = value × percent / 100Use this mode for discounts, tax amounts, commissions, progress and any question in the form “What is X% of Y?”
One value as a percent of another
percent = part / total × 100This answers questions such as “30 is what percent of 120?” The total must be non-zero.
Percentage change
changePercent = (newValue - oldValue) / |oldValue| × 100Using the absolute starting value keeps the sign tied to increase versus decrease. If the starting value is zero, the conventional percentage-change formula is undefined and the calculator says so explicitly.
Precision
The engine retains JavaScript numeric precision during the calculation and rounds primarily for display. Inputs may include decimals and negative values where the selected formula has a meaningful result.
Frequently asked questions
How do I find a percentage of a number?
Multiply the number by the percentage written as a decimal. For example, 20% of 80 is 80 × 0.20 = 16.
How do I find what percent one number is of another?
Divide the part by the total and multiply by 100. The total cannot be zero because division by zero is undefined.
How is percentage change calculated?
Subtract the starting value from the ending value, divide by the absolute starting value, then multiply by 100. A positive result is an increase and a negative result is a decrease.
What happens when the starting value is zero?
A conventional percentage change from zero is undefined because the formula divides by the starting value. The calculator shows an explanatory state instead of Infinity.
Can this calculator be used for discounts?
Yes. Use the “percentage of a value” mode to find the discount amount, then subtract that amount from the original price.