Tech Tips

How to Calculate Percentage (5 Common Cases with Formulas)

Percentage math shows up everywhere — discounts, tips, tax, grades, salary hikes. Here are the five cases you actually need.

1. X is what % of Y?

Formula: (X / Y) × 100

Example: 45 is what percent of 180? → 25%

2. What is X% of Y?

Formula: (X / 100) × Y

Example: 18% of 2,500 → 450

3. Percentage change

Formula: ((new − old) / old) × 100

Example: 50,000 → 62,000 → 24% increase

4. Discount price

Formula: original × (1 − discount%/100)

Example: 1,200 with 25% off → 900

Or use [Discount Calculator](/calculators/discount).

5. Add tax or tip

Formula: amount × (1 + rate/100)

Example: 850 + 18% GST → 1,003

Or use [GST/VAT Calculator](/calculators/gst-vat) and [Tip Calculator](/calculators/tip).

Quick mental math

• 10% — move the decimal one place left
• 5% — half of 10%
• 20% — 10% × 2
• Percentage flip — X% of Y = Y% of X (8% of 50 = 50% of 8 = 4)

Common mistakes

• Adding percentages naïvely — a 20% raise + 20% cut leaves you 4% down, not even
• % change vs % points — 5% → 7% is 2 percentage-point but 40% relative
• Using the wrong base

Skip manual math — use the [Percentage Calculator](/calculators/percentage).

5 min read