📐 Knowledge · Maths

How to Calculate
Percentage

Three types of percentage calculations explained — with formulas, a free calculator, and real-world examples.

Three Percentage Formulas

% of a Number
Result = (P/100) × N
20% of 80 = 0.20 × 80 = 16
What % is X of Y?
P% = (X/Y) × 100
45 of 60 = (45/60)×100 = 75%
% Change
% = ((New–Old)/Old) × 100
50→65 = (15/50)×100 = +30%
💡 Mental shortcut: To find 10%, just move the decimal point one place left. 10% of 340 = 34. Then double for 20% = 68, halve for 5% = 17.

Percentage Calculator

Result
16
20% of 80 = 16
(20 / 100) × 80 = 16
Percentage
75%
45 is 75% of 60
(45 / 60) × 100 = 75%
Percentage Change
+30%
Increase of 30%
((65 – 50) / 50) × 100 = +30%

Step-by-Step: % of a Number

1

Convert the percentage to a decimal

Divide by 100. Example: 35% → 35 ÷ 100 = 0.35

2

Multiply by the number

0.35 × 200 = 70. So 35% of 200 = 70.

3

Check your answer

Reverse check: 70 ÷ 200 × 100 = 35%. ✅

Worked Examples

Test Score

Got 42 out of 50
(42/50) × 100
= 84%

Discount

30% off $120
0.30 × 120 = $36 off
Pay $84

Price Rise

From $40 to $52
((52–40)/40) × 100
+30% increase

Sales Tax

8% tax on $75
0.08 × 75 = $6
Total: $81

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Frequently Asked Questions

How do I calculate 20% of a number quickly? ▾
Find 10% by moving the decimal left one place, then double it. Example: 20% of 85 → 10% = 8.5 → 20% = 17.
What's the difference between percentage and percentage points? ▾
If interest rates go from 4% to 6%, that's a 2 percentage point increase, but a 50% increase in relative terms. "Percentage points" measures absolute difference; "percent change" measures relative change.
How do I reverse a percentage (find original before % was added)? ▾
Divide by (1 + percentage). Example: a price is $108 after a 8% tax → original = 108 ÷ 1.08 = $100. Or for a 20% markup: original = price ÷ 1.20.
How do I calculate a percentage decrease? ▾
Use the same percentage change formula: ((New – Old) / Old) × 100. If Old = 80 and New = 60, result = ((60–80)/80) × 100 = –25%. The negative sign means a 25% decrease.