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Ratio Calculator

Solve ratio and proportion problems with a missing value.

Choose which value to solve in A / B = C / D.

First numerator in A / B = C / D.

First denominator in A / B = C / D.

Second numerator in A / B = C / D.

Second denominator in A / B = C / D. Leave this blank when solving for D.

Status: initial

Results

Awaiting calculation

Calculator guide

Introduction

The Ratio Calculator helps solve equivalent-ratio and proportion problems with one unknown value.

Purpose

Use this calculator for scale recipes, map ratios, classroom proportions, and quick cross-multiplication checks.

Cross multiplication method

For A / B = C / D, cross multiply to get A x D = B x C. Rearrange that equation to solve for whichever value is missing.

Variable explanations

A

First numerator in the proportion.

B

First denominator in the proportion.

C

Second numerator in the proportion.

D

Second denominator in the proportion.

Cross products

The products A x D and B x C.

Formula guide

Solve for A

A = (B x C) / D

  • B, C, and D are known.

Use this when the first numerator is missing.

Solve for B or C

B = (A x D) / C; C = (A x D) / B

  • The unknown cannot make the ratio denominator zero.

Use this when one middle value is missing.

Solve for D

D = (B x C) / A

  • A, B, and C are known.

Use this when the second denominator is missing.

Worked examples

Solve for D

  1. 3 / 5 = 9 / D gives D = (5 x 9) / 3 = 15.

Solve for A

  1. A / 8 = 6 / 12 gives A = (8 x 6) / 12 = 4.

Scale a recipe

  1. 2 cups for 3 servings and x cups for 9 servings gives x = 6 cups.

Common mistakes

Using zero as a denominator

B and D cannot be zero in A / B = C / D.

Cross multiplying the wrong pairs

Multiply A by D and B by C.

Forgetting to divide after cross multiplication

After multiplying the known cross pair, divide by the remaining known value.

Mixing units

Ratios should compare compatible units or clearly converted values.

FAQs

What does this Proportion Calculator do?
It solves a missing value in A / B = C / D using cross multiplication.
What is a proportion?
A proportion is an equation showing that two ratios or fractions are equal.
What is cross multiplication?
Cross multiplication compares A x D with B x C in the proportion A / B = C / D.
How do I solve for D?
Use D = (B x C) / A when A, B, and C are known.
How do I solve for A?
Use A = (B x C) / D when B, C, and D are known.
Can denominators be zero?
No. B and D cannot be zero in a valid proportion.
Can I use decimals?
Yes. Decimal values are supported.
Can I use negative numbers?
Yes, if the proportion makes sense for your problem.
What does A / B = C / D mean?
It means the ratio A to B equals the ratio C to D.
Can this scale recipes?
Yes. Proportions are useful for scaling recipe amounts.
Can this solve map scale problems?
Yes, if you enter corresponding known values in the same proportion order.
Can this solve inverse proportions?
No. This calculator solves direct proportions in fraction form.
How do I check the answer?
Substitute the result back into A / B = C / D and compare both ratios.
Is this the same as a ratio calculator?
It is related, but this page focuses on solving one missing value in a proportion.
Can this solve complex algebraic proportions?
No. It solves one numeric missing value among A, B, C, and D.

References

Related calculators

Last updated and version history

Last updated: 2026-07-10

  • 1.0.0 (2026-07-10): Initial ratio calculator page using the verified proportion engine.