How to Calculate Percentage: A Complete Guide

Calculating percentages is one of the most useful everyday math skills. Whether you're working out a restaurant tip, comparing sale prices, or checking your exam score, percentages come up constantly.

The Basic Formula

The core percentage formula is simple:

Percentage = (Part / Whole) × 100

For example: if you scored 42 out of 50 on a test, your percentage is (42 ÷ 50) × 100 = 84%.

Three Common Percentage Calculations

1. What is X% of a number?

To find 20% of 150: multiply 150 by 0.20 = 30.

Formula: Result = (Percentage / 100) × Number

2. What percentage is X of Y?

To find what percentage 30 is of 150: (30 ÷ 150) × 100 = 20%.

Formula: Percentage = (X / Y) × 100

3. Percentage change

To calculate a price increase from $80 to $100: ((100 - 80) / 80) × 100 = 25% increase.

Formula: % Change = ((New - Old) / Old) × 100

Real-World Examples

Shopping discounts: A $120 jacket is 30% off. Discount = 30% × $120 = $36. Final price = $84.

Tips: A $45 dinner with an 18% tip. Tip = 18% × $45 = $8.10. Total = $53.10.

Tax: A $200 item with 8% sales tax. Tax = 8% × $200 = $16. Total = $216.

Common Mistakes to Avoid

• Confusing base values: "20% off" and "20% more" use different bases.
• Reversing the formula: Finding what percentage A is of B is different from what percentage B is of A.
• Percentage points vs percentages: A rate rising from 4% to 6% is a 2 percentage point increase, but a 50% increase in the rate itself.

Use the MindMath Percentage Calculator

Skip the manual math entirely. The MindMath percentage calculator handles all three modes instantly — percentage of a number, percentage one number is of another, and percentage change. Just enter your values and get the answer in seconds.