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Proportion Calculator

Solve a missing value in a proportion A/B = C/D using cross multiplication.

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

Ratios and cross multiplication

Introduction

The Proportion Calculator solves one missing value in A / B = C / D using cross multiplication.

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.

Formula and method guide

Solve for A

A = (B x C) / D

Use this when the first numerator is missing.

Solve for B or C

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

Use this when one middle value is missing.

Solve for D

D = (B x C) / A

Use this when the second denominator is missing.

Variables

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.

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

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