Proportion Calculator (a/b = c/d)
Solve a proportion a/b = c/d by entering any three values and leaving one blank — runs 100% in your browser with no upload.
Fill in any three values and leave one field blank to solve a/b = c/d.
2 / 4 = 3 / 6 d = 6
a × d = b × c d = (4 × 3) ÷ 2 = 6
🔒 Calculated in your browser — nothing is uploaded.
What this proportion calculator does
A proportion says that two ratios are equal: a/b = c/d. This calculator lets you enter any three of the four values and leave the remaining field blank, and it instantly solves for the missing number. It is the classic "rule of three" you learned in school, done automatically. Everything updates live as you type, so you can change any value and watch the answer recompute immediately — handy for scaling recipes, converting units, resizing images while keeping the aspect ratio, or working out map distances.
How the missing value is found
The tool cross-multiplies. Because a/b = c/d is equivalent to a·d = b·c, each unknown has a simple formula: a = b·c/d, b = a·d/c, c = a·d/b, and d = b·c/a. For example, with a = 2, b = 4 and c = 3, the missing d equals b·c/a = 4·3/2 = 6. Below the answer the calculator also shows the worked steps — the cross-multiplication a·d = b·c and the exact division used — so you can follow (or teach) how the missing value was reached. The calculator checks that exactly one field is blank and guards against dividing by zero, so it never returns a misleading result.
Frequently asked questions
How do I use this proportion calculator?
Type the three values you know into the a, b, c, and d boxes and leave the one you want to find empty. The answer appears instantly below, and you can copy it with one click.
What is the rule of three?
The rule of three solves a proportion a/b = c/d for one unknown by cross-multiplying: a·d = b·c. This tool applies that rule automatically for whichever field you leave blank.
Does it show the steps?
Yes. Under the result the tool prints the rule-of-three working — the cross-multiplication a·d = b·c and the division that produced the missing value — so you can check the reasoning or use it to explain the method.
Is anything I enter uploaded to a server?
No. Every calculation runs locally in your browser with JavaScript. The numbers you type are never sent, stored, or logged anywhere.