Proportion Calculator
Solve for any unknown in a proportion: a / b = c / d. Select which value is unknown, enter the other three, and click solve.
Cross-multiply: a × d = b × c
| Country / Language | Name for Proportion Rule | Decimal Separator | Taught Grade |
|---|---|---|---|
| 🇺🇸 US / 🇬🇧 UK | Cross-multiplication / Proportional reasoning | . (point) | Grade 6–7 |
| 🇩🇪 Germany | Dreisatz (Rule of three) | . or , (both used) | Klasse 6 |
| 🇫🇷 France | Règle de trois / Produit en croix | , (comma) | 6ème |
| 🇪🇸 Spain | Regla de tres / Producto cruzado | , (comma) | 1° ESO |
| 🇮🇹 Italy | Regola del tre / Prodotto incrociato | , (comma) | Scuola media |
| 🇧🇷 Brazil | Regra de três | , (comma) | 6° ano |
Frequently Asked Questions
Why does Europe use commas instead of periods in decimals?
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.
What is the 'rule of three' (Dreisatz / Règle de trois)?
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.