---
title: "Proportion Calculator — Solve a/b = c/d by Cross-Multiplication"
description: "Solve for any unknown in a proportion a/b = c/d using cross-multiplication. Works for ratios, scaling, map distances, recipe scaling, and currency conversion. The 'rule of three' used worldwide."
url: "https://worldcalculators.org/calculators/proportion/"
canonical: "https://worldcalculators.org/calculators/proportion/"
---

# Proportion Calculator

Solve for any unknown in a proportion a/b = c/d using cross-multiplication. Works for ratios, scaling, map distances, recipe scaling, and currency conversion. The 'rule of three' used worldwide.

## Quick answer

Cross-multiply: a × d = b × c

## Frequently Asked Questions

Most European countries (Germany, France, Spain, Italy, etc.) use a comma as the decimal separator: 3,14 instead of 3.14. Correspondingly, they use a period or space as the thousands separator: 1.000.000 or 1 000 000 for one million. The UK and US use a period as the decimal separator and comma as the thousands separator: 1,000,000 and 3.14. This is why European students learning proportions in English sometimes write "3,14" and confuse English-speaking teachers. ISO 31-0 recommends the thin space as thousands separator to avoid the ambiguity, but only scientific contexts follow this.
The "rule of three" is the classic European method for solving proportions: given three values A, B, and C where A/B = C/x, you find x = (B × C) / A. Taught as early as primary school in Germany, France, Spain, and much of Latin America, it's a fundamental arithmetic skill for everyday scaling: currency conversion (€3 per dollar, how many euros for $15?), recipe scaling, map reading, and unit conversion. In UK and US curricula, the same concept is taught as "proportional reasoning" or "cross-multiplication" without the "rule of three" label.

Source: https://worldcalculators.org/calculators/proportion/
Markdown mirror of the HTML page. Prefer this URL for RAG; the interactive calculator still lives on the HTML page.