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Proportion Calculator

Solves the proportion a / b = c / x for the unknown fourth term by cross-multiplication (x = b × c / a) and reports the common scale factor.

When to use

Three terms of a proportion are known and you need the fourth, e.g. converting a recipe, map scale or unit rate.

Do not use when: You only want to simplify a ratio (use ratio) or the unknown is a percentage (use percentage).

a / b = c / x ⇒ a × x = b × c ⇒ x = b × c / a; scale_factor = c / a

Parameter	Type	Required	Description
`a`	number	yes	First term (numerator of the known ratio). Must not be 0.
`b`	number	yes	Second term (denominator of the known ratio). Must not be 0.
`c`	number	yes	Third term: the numerator paired with the unknown x. Must not be 0.

Output	Type	Description
`x`	number	The unknown fourth term, b × c / a.
`scale_factor`	number	c / a = x / b: the multiplier from the first pair to the second.
`ratio`	number	The common value of both fractions, a / b = c / x.
`equation`	string	The proportion with x filled in, e.g. '2/3 = 4/6'.

2/3 = 4/x: {"a":2,"b":3,"c":4} → {"x":6,"scale_factor":2,"ratio":0.666667,"equation":"2/3 = 4/6"}

5/8 = 12.5/x: {"a":5,"b":8,"c":12.5} → {"x":20,"scale_factor":2.5,"ratio":0.625}

GET https://tttkmbb.com/api/v1/calculate/proportion?a=2&b=3&c=4

API: GET https://tttkmbb.com/api/v1/calculate/proportion (query parameters) or POST with a JSON body {"inputs": {...}}
Schema: https://tttkmbb.com/api/v1/calculators/proportion · Markdown: https://tttkmbb.com/math/proportion.md · JSON definition: https://tttkmbb.com/math/proportion.json
MCP: server https://tttkmbb.com/mcp, tool run_calculator with calculator_id="proportion"
OpenAPI operationId: solve_proportion
Freshness: static. Authentication: none. Rate limit: fair use (see rate limits).

Rearrange so the unknown is last: a/b = c/x is equivalent to b/a = x/c, c/a = x/b and so on; all give x = b × c / a.

No. For inverse proportion (a × b = c × x) compute x = a × b / c directly.