usefulweb.site Percentage & Ratio Calculator

Everyday calculators

Percentage & Ratio Calculator

X% of Y, % change, reverse %, percentage points, ratios and aspect ratios, with formulas.

Percentage modes: the percentage or the start value (see the question). Aspect ratio: width.

Percentage modes: the base, end or final value. Aspect ratio: height.

For the ratio modes: 2 to 10 positive terms separated by ":" (e.g. 16:9, 2:3:5, 1.5:2).

Split: the total to share out. Scale: the value the chosen term should become.

Scale mode: which ratio term (1 = first) should equal the amount.

Aspect ratio: optional; the matching height is calculated.

Aspect ratio: optional; the matching width is calculated.

Rounding of the displayed answer and of split amounts (2 = cents).

Try an example: What is 15% of 80? · Price rose from 40 to 50: percentage change · Price after 19% VAT is 119: price before tax · Interest rate from 2% to 3%: points vs percent · Split 1,000 by 2:3:5 · 1920 × 1080 aspect ratio, resize to 1280 wide

How it works

Answers every everyday percentage question (X% of Y, X is what percent of Y, percentage change, increase or decrease by a percentage, reverse percentage, percentage points versus percent) and every ratio task (simplify, scale, split an amount, aspect ratios). Each answer shows the formula and the steps.

  • X% of Y = X ÷ 100 × Y. X as a percent of Y = X ÷ Y × 100.
  • Percentage change from X to Y = (Y − X) ÷ |X| × 100; positive is an increase.
  • Increase Y by X% = Y × (1 + X/100); decrease Y by X% = Y × (1 − X/100).
  • Reverse percentage: if Y is the value after a change of X%, the original is Y ÷ (1 + X/100) (use a negative X for a decrease, e.g. −20 for 20% off).
  • Percentage points measure the plain difference between two percentages (3% − 2% = 1 percentage point); percent measures the relative change ((3 − 2) ÷ 2 = 50%).
  • Ratios are simplified exactly: decimal terms are first multiplied by a power of 10 to make whole numbers, then divided by their greatest common divisor.
  • Splitting an amount by a ratio uses the largest-remainder method at the chosen decimal places, so the parts always add up to exactly the amount.
  • Aspect ratios: width:height simplified by the GCD, the decimal width ÷ height, and the nearest well-known ratio within 1%. Resizing keeps the ratio: new height = new width × height ÷ width.

For scripts and AI agents

  • JSON API: POST https://percentageratiocalculator.usefulweb.site/api/v1/calculate (same calculation as this page)
  • Interface description: OpenAPI 3.1 · AI guide: llms.txt
  • MCP server (Streamable HTTP): https://percentageratiocalculator.usefulweb.site/mcp · tool name percentage_ratio_calculator
Example request
curl -s -X POST https://percentageratiocalculator.usefulweb.site/api/v1/calculate \
  -H 'content-type: application/json' \
  -d '{"mode":"percent-of","x":15,"y":80}'
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