Math Runs locally Ready to use Built-in examples No tracking

Percentage Calculator

Three common percentage modes with formulas and worked examples. All calculations run locally in your browser.

Three percentage questions cover most everyday arithmetic: what is 15% of 200, what percentage is 45 of 60, and how much did something change from 80 to 100. Each is its own mode here, answering as you type and running entirely in the browser — useful for checking a discount, a split or a report figure before committing to it.

The third mode handles a simple change; multi-period growth, reversing an original value and similar questions belong to the percent change calculator, which labels the base of each calculation. When answering "what percentage is A of B", remember that the denominator is B — the comparison basis. State it explicitly when the figure goes into a report, because readers assume different bases by default.

%
—

—

—

How to use

  1. Choose a mode: percent of a number, one number as a percentage of another, or percentage change.
  2. Enter the values; the result updates as you type.
  3. Read the result and, for the second mode, note which number is the base.
  4. For compound or reverse questions, switch to the percent change calculator.

How it works

Take a percentage of a number

Percentage of a number: result = value × percent ÷ 100. For example, 20% of 85 is 85 × 20 ÷ 100 = 17 — a 20% tip on an 85 bill is 17.

Find what percent one number is of another

Share of a whole: percent = part ÷ whole × 100. For example, what is 30 out of 150: 30 ÷ 150 × 100 = 20%.

Percentage increase and decrease

Percentage change: rate = (new − old) ÷ |old| × 100. For example, 50 rising to 75: (75 − 50) ÷ 50 × 100 = +50%, a 50% increase.

Code example

JavaScript Three modes, computed directly 

const partOf = (pct, whole) => whole * pct / 100;      // 15% of 200
const shareOf = (part, whole) => part / whole * 100;   // what % is 45 of 60
const change = (a, b) => (b - a) / Math.abs(a) * 100;  // rate of change

partOf(15, 200);    // 30
shareOf(45, 60);    // 75
change(80, 100);    // 25

Python The same three modes in Python 

part_of = lambda pct, whole: whole * pct / 100
share_of = lambda part, whole: part / whole * 100
change = lambda a, b: (b - a) / abs(a) * 100

part_of(15, 200)    # 30.0
share_of(45, 60)    # 75.0

FAQ

What is the difference between percent and percentage points?

A percentage point is the arithmetic difference between two percentages. If a rate rises from 3% to 5%, it rose 2 percentage points — not 2%.

What does a negative percentage change mean?

A decrease. Falling from 80 to 60 is a change of −25%, i.e. down 25%.

How do I mentally compute a 15% tip?

Take 10% (shift the decimal point one place left), then add half of it (5%). For 46: 10% is 4.6, plus 2.3 — a 6.9 tip.

How do I convert a percentage to a decimal?

Divide by 100: 20% = 0.2, 12.5% = 0.125. To go the other way, multiply by 100 and append the % sign. Converting to a decimal before substituting into a formula avoids most arithmetic slips.

Why does the formula multiply by 100?

Because "part ÷ whole" yields a decimal like 0.2; multiplying by 100 merely rewrites it in percent form (20%) — the value itself is unchanged. Only "share of a whole" and "change rate" need the ×100; taking a percentage of a number uses value × percent ÷ 100 directly.

Can this calculator handle discounts, tax and growth?

All of them. Discounts use "percent of a number" (original × discount); tax uses the same mode (amount × rate); growth uses "percentage change" ((new − old) ÷ old). The three modes cover nearly every everyday percentage scenario.

How many decimals should the result keep?

For shopping and quotes, 2 decimals (down to the cent) is enough; displayed percentages read well at 1–2 (like 33.3%); statistics and experiments deserve 3–4 — or carry full precision throughout and round only at the last step to avoid accumulating error.