Percentage Calculator.

Guide

About the Percentage calculator

A percentage calculator handles the four or five percentage questions that come up constantly in everyday life, school, shopping, and business. Pick an operation, enter your two numbers, and get the answer with the calculation explained.

What is A% of B?

This is the most common percentage question — finding a portion of a number. The formula is:

Result = B × (A ÷ 100)

Example: What is 25% of 200? → 200 × 0.25 = 50. This is how you work out a tip, a discount amount, a tax, or a commission. A 15% tip on an 80 bill is 80 × 0.15 = 12.

A is what percent of B?

This reverses the question — you have two numbers and want to know what proportion one is of the other. The formula is:

Result = (A ÷ B) × 100

Example: 25 is what percent of 200? → (25 ÷ 200) × 100 = 12.5%. This is how you work out a test score (45 out of 60 = 75%), a market share, or how much of a budget a cost represents.

Percentage change from A to B

This measures how much a value has gone up or down, as a percentage of the starting value:

Result = ((B − A) ÷ A) × 100

A positive result is an increase; a negative result is a decrease. Example: a price rising from 200 to 250 is ((250 − 200) ÷ 200) × 100 = +25%. A price falling from 200 to 150 is −25%. This is the right tool for price changes, salary rises, investment returns, and growth rates.

A key subtlety: percentage change is not symmetric. Going from 200 to 250 is a +25% change, but going back from 250 to 200 is a −20% change — because the base you divide by is different each time. Always divide by the starting value.

Increase or decrease A by B%

This applies a percentage change to a number:

- Increase: Result = A × (1 + B ÷ 100) - Decrease: Result = A × (1 − B ÷ 100)

Example: increasing 200 by 25% gives 250; decreasing it by 25% gives 150. Use the decrease mode for "percent off" sale prices — an item marked down 30% from 80 costs 80 × 0.70 = 56.

Why "subtract the percentage" can mislead

A common error is to add a percentage and then subtract the same percentage expecting to return to the start. If you increase 100 by 10% you get 110; decreasing 110 by 10% gives 99, not 100 — because the 10% is taken from a larger base the second time. This calculator's separate increase and decrease modes make each step explicit so you avoid the trap.

Everyday uses

- **Shopping:** discount amounts ("25% off"), final sale prices, and sales tax or VAT on top. - **Tips and service charges:** a percentage of the bill. - **Grades and scores:** marks out of a total, expressed as a percentage. - **Finance:** interest, returns, growth rates, and percentage changes in prices or salaries. - **Statistics:** what share one figure is of a total, and how figures change over time.

Tips for mental math

To find 10% of a number, move the decimal one place left (10% of 250 = 25). To find 5%, halve the 10% figure. To find 20%, double the 10% figure. To find 15% (a common tip), add the 10% and 5% figures. These shortcuts let you estimate percentages quickly without a calculator, and this tool gives you the exact figure when you need it.

How to use this calculator

Choose the operation that matches your question, enter the two numbers (labelled A and B to match each operation's wording), and read the answer and the plain-language description of the calculation. Switch the operation to ask a different question of the same numbers — for example, find 25% of 200, then ask what percent 25 is of 200, to see how the questions differ.