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Math tool

Proportion Calculator

Solve the missing fourth value in a:b = c:x using three positive values and a transparent cross-product formula.

Free · Private · Runs in your browser

Fourth proportional solver

Solve a:b = c:x

Enter three positive values to solve for the fourth. Values and results stay in your browser.

Known values

Values must be positive finite numbers from 0.000000000001 through 1,000,000,000,000.

Your fourth value will appear here after you submit valid values.

Formula and calculation method

For a:b = c:x, cross multiplication gives a × x = b × c. Dividing by a gives x = (b × c) ÷ a. For example, 2:3 = 4:x produces x = (3 × 4) ÷ 2 = 6.

Conventions and boundaries

  • Inputs: three positive finite numbers.
  • Bounds: each value is from 0.000000000001 through 1,000,000,000,000.
  • Precision: the internal calculation uses floating-point arithmetic; the decimal convenience display rounds to ten places.
  • Meaning: the calculator does not verify that the quantities have compatible units or real-world meaning.

Limitations

This tool solves only the form a:b = c:x. It does not convert units, accept zero or negative values, infer unknown units, or provide financial, medical, legal, engineering, or other professional advice.

Worked examples

  • 2:3 = 4:6.
  • 2.5:5 = 7.5:15.
  • 3:5 = 12:20.

Version and verification

Calculation version 1.0.0; formulas and deterministic fixtures last verified 2026-08-19. Automated tests cover ordinary and decimal fixtures, cross-product consistency, bounds, and invalid values.