Percentage

The Percentage Calculator handles the four percentage questions that come up most often: "What is X% of Y?", "X is what percent of Y?", "Increase Y by X%", and "What is the percent change from A to B?".

Each variation is a small rearrangement of the same underlying formula. The calculator works the algebra out so you do not have to.

What it calculates

• X% of Y — for discounts, tips, sales tax, etc.
• X is what % of Y — for things like "I scored 73 out of 80, what percent is that?".
• Percent change from A to B — for tracking growth, price changes, or rate of change.
• Increase / decrease Y by X% — for applying markups or markdowns.

The underlying formulas

• X% of Y → X / 100 × Y
• X is what % of Y → X / Y × 100
• Percent change from A to B → (B − A) / A × 100
• Increase Y by X% → Y × (1 + X / 100)
• Decrease Y by X% → Y × (1 − X / 100)

Worked examples

• 15% of 240 = 0.15 × 240 = 36.
• 73 is what % of 80? = 73 / 80 × 100 = 91.25%.
• Price went from $120 to $150 — what's the % change? = (150 − 120) / 120 × 100 = 25% increase.
• Decrease 80 by 15% = 80 × 0.85 = 68.

When this is useful

Shopping (sales, discounts), grading and scoring, financial calculations (returns, taxes), and anywhere you need to compare two numbers as a ratio. Percent change is particularly useful for tracking growth over time or comparing budget vs actual.

Watch the base

A common error is calculating percent change using the wrong base. The base is always the starting value, not the ending one. A 50% increase followed by a 50% decrease does not return to the original value — it ends 25% lower. Order matters.

Frequently asked questions

Related tools

• Tip Calculator
• Compound Interest
• Investment ROI