How do you find a missing value in a ratio?

Set the two ratios equal as A:B = C:D and cross-multiply. With three values known, the fourth is found by multiplying the diagonal pair and dividing by the remaining value.

What are equivalent ratios?

Equivalent ratios represent the same relationship, like 1:2 and 5:10. Scaling both terms by the same factor produces an equivalent ratio.

How do I scale a ratio up or down?

Choose Enlarge to multiply both terms by the factor, or Shrink to divide both terms by it. Both keep the ratio equivalent.

Does the calculator simplify the result?

No. It shows the scaled or solved ratio as calculated. You can reduce it to lowest terms separately if needed.

Online Ratio Calculator

What Is This Tool?

The Ratio Calculator does two things. In proportion mode, enter any three values of A:B = C:D and it finds the missing fourth by cross-multiplication — for example, 3:4 = 600:800. In scaling mode, it enlarges or shrinks a ratio by a factor, so 250:280 scaled up by 2.5 becomes 625:700.

How to Use This Tool?

For a proportion, choose the Ratio tab and enter any three of the four values.
Leave the unknown value blank and click Calculate to find it.
For scaling, choose the Ratio Scaling tab and enter the ratio.
Pick Enlarge or Shrink, set the scale factor, and calculate.

Key Features

Solves a proportion A:B = C:D for the missing value.
Scales a ratio up by multiplying both terms.
Scales a ratio down by dividing both terms.
Accepts whole numbers and decimals.
Shows the full ratio so the relationship stays clear.

Examples

3:4 = 600:800 solves the missing term as 800.
1:2 = 5:10 confirms the missing term is 10.
250:280 enlarged by 2.5 gives 625:700.
12:8 shrunk by 4 gives 3:2.

Common Use Cases

Scaling recipe ingredients up or down.
Resizing images or layouts while keeping proportions.
Converting map scales and model dimensions.
Mixing paints, solutions, or materials in proportion.
Solving proportion problems in math and science.

Tips & Best Practices

In proportion mode, fill in exactly three of the four values.
Use non-zero values so the proportion can be solved.
Keep both terms in the same units before scaling.
Use Shrink with a factor greater than one to make a ratio smaller.
Simplify the result separately if you need it in lowest terms.

Limitations

Proportion mode needs exactly three non-zero values.
You cannot shrink a ratio by a factor of zero.
Results are not automatically reduced to lowest terms.
It works with two-term ratios, not three-term ratios.

Frequently Asked Questions

How do you find a missing value in a ratio?: Set the two ratios equal as A:B = C:D and cross-multiply. With three values known, the fourth is found by multiplying the diagonal pair and dividing by the remaining value.
What are equivalent ratios?: Equivalent ratios represent the same relationship, like 1:2 and 5:10. Scaling both terms by the same factor produces an equivalent ratio.
How do I scale a ratio up or down?: Choose Enlarge to multiply both terms by the factor, or Shrink to divide both terms by it. Both keep the ratio equivalent.
Does the calculator simplify the result?: No. It shows the scaled or solved ratio as calculated. You can reduce it to lowest terms separately if needed.

Key Terminology

Ratio: A comparison of two quantities, written as A:B.
Proportion: A statement that two ratios are equal, such as A:B = C:D.
Cross-multiplication: A method to solve a proportion by multiplying terms diagonally.
Equivalent ratios: Ratios that express the same relationship, like 1:2 and 5:10.
Scale factor: The number a ratio is multiplied or divided by to resize it.

Quick Knowledge Check

In 2:3 = 4:?, what is the missing value?

A ratio compares:

Two quantities

Three sums

A single number

Scaling 1:2 up by a factor of 3 gives:

3:6

1:6

4:5