About ratios and proportions
This ratio calculator solves a proportion of the form A : B = C : D. Fill in any three values and leave one blank, and it finds the missing number by cross-multiplication, then shows the ratio in its simplest whole-number form. It runs in your browser; nothing is uploaded.
How Ratio works
The formula
In a true proportion the cross-products are equal: A × D = B × C. Rearranging gives the missing term — for example D = (B × C) ÷ A. Two ratios are equivalent when they reduce to the same simplest form.
Worked example
Solve 3 : 4 = 9 : D. Cross-multiply: D = (4 × 9) ÷ 3 = 36 ÷ 3 = 12. So 3 : 4 = 9 : 12, which both simplify to 3 : 4.
Common uses
- Solve a proportion for a missing value
- Scale a recipe up or down
- Find an equivalent ratio
- Work out a missing ingredient amount
- Convert a ratio to its simplest form
- Scale drawings or models
- Adjust mixing ratios
- Check if two ratios are equal
Frequently asked questions
How do I solve a ratio?
Enter three of the four values in A : B = C : D and leave one blank — the tool cross-multiplies to find it.
What is cross-multiplication?
For A : B = C : D it means A × D = B × C; solving for the blank gives the answer.
Does it simplify ratios?
Yes — it shows the ratio reduced to its smallest whole numbers.
Can I scale a recipe with it?
Yes — set the known ratio and the new quantity, and solve for the missing amount.
Is my data uploaded?
No — it calculates in your browser.
Is it free?
Yes, completely free.