Solve any proportion a:b = c:d for the missing value, with the cross-multiplication shown.
Solving for d: if 2/4 = 3/d, then d = 6. Leave the maths to the calculator — just enter any three values.
A proportion states that two ratios are equal. Cross-multiplying gives a × d = b × c, so the missing term is always a single multiplication and division away.
missing value d
How to use this tool
Type a, b and c from your proportion a:b = c:d.
d is shown big, with both cross products beside it.
Equal cross products prove the proportion balances.
The a:b ratio is reduced to its simplest whole-number form where possible.
Formula & worked example
If 2/4 = 3/d, then d = 4 × 3 ÷ 2 = 6, and both cross products are 12.
Limitations — read this before you rely on it
This solves direct proportions only. Inverse proportions (more workers, less time) need the opposite setup: a × b = c × d, not a/b = c/d.
Frequently asked questions
An equation stating two ratios are equal, written a:b = c:d or a/b = c/d.
Cross-multiply: a × d = b × c, then divide to isolate the missing term, d = b × c ÷ a.
Multiplying diagonally across the equals sign. In a true proportion the two cross products are always equal.
No: a/b would divide by zero. Enter a non-zero value for a.
If 2 kg costs $4, then 3 kg costs $6 — the proportion 2/4 = 3/6, solved as d = 6.