HomeCalculatorsMathPercent, Ratio & ProportionProportion Calculator
Math calculator

Proportion Calculator

Solve A:B = C:D for one missing term. Same engine as Ratio. Cross-multiply A·D = B·C. Not percent change. Not a second engine. Runs locally.

Instant result
Result
—

Enter values to calculate.

Inputs—
Mode—
Formula—
Trust summary CVP VERIFIED · CVP protocol 1.0.0-proposed · Core assurance · all published modes
Input interpretation
Enter values to calculate.
Result
—
Verified scope
all published modes
Assurance
Core
Declared partition coverage
PASS · 8/8 declared partitions (simplify, proportion, scale, split, compare, convert, three-term, invalid-domain) · Matrix
Numerical scope
All six published Ratio modes (simplify, proportion, scale, split, compare, convert), including A:B:C where those modes support it. Independent O2 expected values; exact rational BigInt; Node is runtime host only.
Known limitations
  • Core CVP does not include live graph, viewport, or pointer interaction.
Model
The missing term in A:B = C:D.
Scope
Finite rationals: integers, terminating decimals, or simple fractions a/b.
Verification
Engine tested · Source checked · v1.0.2 · CVP VERIFIED · CVP protocol 1.0.0-proposed · Core assurance · all published modes· View Manifest · CVP overview · Specification
Versions
Calculation 1.0.2 · CVP protocol 1.0.0-proposed · Evidence 2026-09-16.sources-no-wolfram
Verification revision
2026-09-16.sources-no-wolfram · 7/7 property · digest e32c91d652f9
Legacy regression
26/26 tests · Production surface contract 10/10
Trust layers
Verification VERIFIED · Production CURRENT · overall VERIFIED
Reference
O1 model · O2 expected_values · O2 numerical_behavior
Interfaces
PASS · UI (SSR) / REST / MCP — ui-ssr is query-result HTML, not a live browser session.
Supplemental domain review
Not performed
Named expert review
Not performed
CVP suite
13/13 golden · 7/7 CVP boundary · 7/7 invalid · 7/7 property · 2/2 metamorphic · 2/2 round-trip · 3/3 cross-interface · 8/8 CVP contract · Manifest
Sources
Sources
Evidence
11 legacy golden · 7 legacy boundary · legacy regression suite · 13/13 oracle-backed golden · 7/7 invalid · Artifact integrity PASS
This calculator CURRENT · Public schema 1.0.2 matches · Semantic contract ✓ · Production attested · Public/cache ✓ · Origin ✓
Semantic contract
PASS
Full verification

Manifest identity, reference classes, interfaces, suite, and production records.

Formulas

Core equations used by this calculator.

Cross-multiplyA/B = C/D → A·D = B·C
Missing CC = A·D / B
iSame engine as /calc/math/ratio. Exactly one of A, B, C, D is blank. B and D must be nonzero when they are denominators. Simplify, scale, split, compare, and convert stay on /ratio.

How to use

1

Enter three of A, B, C, D

4:7 = x:21 is A=4, B=7, C blank, D=21. The full ratio page stays at /ratio.

2

Read the missing term

x = 4×21/7 = 12. Terms are exact rationals.

Example calculations

Common configurations with formula and result.

ϟ

Missing C

4:7 = x:21

C = A·D / B
12
ϟ

Check a full proportion

4:7 = 12:21

A·D = B·C
84 = 84

Proportion calculator specification

Version 1.0.2 · Engine tested

Calculation status

Review policy · Evidence

Definition
A proportion equates two ratios: A:B = C:D, so A·D = B·C. This page is mode=proportion on math.ratio. Leave exactly one of A, B, C, D blank. Not a second engine. Not percent change. Not percent error.
What it calculates
The missing term in A:B = C:D.
Inputs
  • Three of A, B, C, D
Outputs
  • Missing term
Formula
A·D = B·C
Assumptions
  • Finite rationals: integers, terminating decimals, or simple fractions a/b.
  • Exactly one of A, B, C, D is blank.
  • Not percent change. Not percent error. Not simplify, scale, split, compare, or convert.
Units
  • Dimensionless (or consistent units across terms)
Boundary conditions
  • Not exactly one blank → PROPORTION_ONE_BLANK
  • Zero denominator when solving → undefined
Example
4:7 = x:21 → x = 12
Validation cases

1 published on this page · 26/26 tests · Production surface contract 10/10 · View evidence

  • 4:7 = x:21 → 12
Sources
Calculation version
1.0.2

Background

Interpretation and common distinctions.

What is a proportion?

A proportion says two ratios are equal: A:B = C:D, which is the same as A/B = C/D. Cross-multiply to get A · D = B · C.

This page only solves that equation. Simplify, scale, split, compare, and convert stay on Ratio. Percent change stays on percentage change.

Other calculators in this family: Percent Error Calculator, Percentage Calculator, Percentage Change Calculator, Ratio Calculator . Explore all Percent, Ratio & Proportion.

→ Percent Error Calculator Compute percent error between experimental and accepted values for lab and measurement checks. Interactive calculations run locally; shared URLs and REST use the same engine. Free — no sign-up. › → Percentage Calculator Calculate percentages, percentage of a number, and “what percent is X of Y” with clear arithmetic steps. Runs locally in your browser. Try it free. › → Percentage Change Calculator Calculate the percentage increase or decrease from an initial value to a final value. You can also solve for the initial or final value. Free online — no sign-up. › → Ratio Calculator Simplify ratios and scale quantities that stay in proportion. Runs locally in your browser. Free online tool — no sign-up. No account needed. ›
}

Frequently asked questions

Key distinctions behind the calculation.

Is this a second ratio engine?

No. It is mode=proportion on math.ratio, the same engine as /calc/math/ratio.

Is this percent change?

No. Percent change is (V₂ − V₁) / |V₁|. That page is /calc/math/percentage-change. A proportion equates two ratios.

Can I leave more than one field blank?

No. Exactly one of A, B, C, D is blank. Two blanks is PROPORTION_ONE_BLANK.

Where does this run?

Locally in the browser by default. REST and MCP call the same ratio engine.