Ratio Calculator
Simplify any ratio to its lowest terms, such as 12:18 = 2:3, including decimals, or solve a proportion such as 5 : 3 = 500 : ? to scale a recipe or a mix.
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Simplify a ratio
Solve a proportion
Fill in three values and leave one empty: A : B = C : D.
How do I simplify a ratio or solve a proportion?
To simplify a ratio, divide both parts by their greatest common divisor: 250 : 150 becomes 5 : 3. To scale it, solve a proportion by cross-multiplying: a 5 : 3 flour-to-water recipe with 500 g of flour needs 3 × 500 ÷ 5 = 300 g of water. Enter your numbers and the calculator does both, including decimals.
- To simplify, enter both parts of the ratio under “Simplify a ratio”.
- To solve a proportion, fill in three of A, B, C and D and leave the unknown one empty.
- Read the simplified ratio or the missing value.
Formulas & Methods
Simplify
Example: 12 : 18: both divide by 6, giving 2 : 3
Decimals
Example: 1.5 : 2.5 = 15 : 25 = 3 : 5
Missing value
Example: 5 : 3 = 500 : D gives D = 3 × 500 ÷ 5 = 300
Key features
Lowest terms
Any ratio is reduced to its simplest whole-number form.
Decimals and negatives
Decimal ratios are turned into whole numbers first, and a minus sign stays on the front: −6 : 9 simplifies to −2 : 3.
Solve for any value
Leave any one of the four values empty, and the calculator finds it.
Common uses
- Scale a recipe or mix to a new amount
- Check a model, map or drawing scale
Example: reading a map scale
On a 1 : 25,000 map, 1 cm stands for 25,000 cm. Solve 1 : 25,000 = 4 : D and D is 100,000 cm, so 4 cm on the map is 1 km on the ground. The same proportion scales a recipe: if 500 g of flour needs 300 ml of water, 750 g needs 450 ml.
Frequently Asked Questions
Why does −6 : −9 come out as 2 : 3?
Two minus signs cancel, so −6 : −9 is the same ratio as 6 : 9, which simplifies to 2 : 3. When only one part is negative, the minus sign is shown on the first part: 6 : −9 becomes −2 : 3.
Can I enter a ratio with three parts, such as 2 : 3 : 5?
No. Both parts of the calculator work with two-part ratios. For three or more parts, find the largest number that divides every part and divide them all by it: 10 : 15 : 25 becomes 2 : 3 : 5.
Can ratios have decimals?
Yes. Multiply both parts by the same power of ten to make them whole, then simplify: 0.75 : 0.25 = 75 : 25 = 3 : 1.
What’s the difference from the aspect ratio calculator?
The aspect ratio calculator works in whole pixels for images and screens. This one handles any numbers, including decimals, and any missing value.
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