Ratio Calculator

What a ratio means

A ratio compares two quantities: 3 : 2 means "3 of the first for every 2 of the second". Ratios have no units of their own, and multiplying or dividing both sides by the same number gives an equivalent ratio, so 3 : 2, 6 : 4 and 1.5 : 1 all describe the same relationship. The calculator does three jobs: it simplifies A : B, solves a proportion A : B = C : D when one of C or D is missing, and splits an amount in the ratio A : B.

Simplifying a ratio

Divide both sides by their greatest common divisor. A Full HD screen is 1920 × 1080 pixels; the GCD of 1920 and 1080 is 120, so the aspect ratio is 1920 ÷ 120 : 1080 ÷ 120 = 16 : 9. A 1440 × 900 laptop display simplifies to 8 : 5, usually written 16 : 10.

If the ratio contains decimals, first multiply both sides by 10, 100 or 1000 to make whole numbers: 1.5 : 2.5 becomes 15 : 25, which simplifies to 3 : 5; 0.25 : 0.6 becomes 25 : 60, or 5 : 12.

Solving for a missing value

A proportion A : B = C : D is true when the cross products match: A × D = B × C. So the missing value is

D = C × B ÷ A    or    C = D × A ÷ B

The default values above solve the video example. If you fill in all four boxes, the calculator instead checks whether the two ratios are equivalent by comparing the cross products.

Splitting an amount in a ratio

To share a total in the ratio A : B, add the parts, find the value of one part, then multiply:

first share = total × A ÷ (A + B)  ·  second share = total × B ÷ (A + B)

Splitting 600 in the ratio 2 : 3: there are 2 + 3 = 5 parts, each worth 600 ÷ 5 = 120, so the shares are 2 × 120 = 240 and 3 × 120 = 360. Check: 240 + 360 = 600. Splitting 250 ml of concentrate in the ratio 1 : 4 (1 part syrup to 4 parts water) gives 50 ml and 200 ml.

Ratios, fractions and percentages

A ratio of 2 : 3 is not the fraction 2/3 of the whole. With 2 : 3 there are 5 parts in total, so the first share is 2/5 (40%) and the second is 3/5 (60%). The fraction 2/3 is the first share divided by the second. Mixing these up is the most common ratio error. To work with the fractions directly, use the fraction calculator; to turn shares into percentages, see the percentage calculator.

Where ratios appear

• Screens and photos: 16 : 9 video, 4 : 3 and 3 : 2 camera sensors, 9 : 16 phone stories.
• Mixing: two-stroke fuel at 50 : 1 means 1 part oil to 50 parts petrol; the two-stroke oil mix calculator converts that to millilitres for any tank size. Concrete, mortar and paint thinning use part ratios too; for three-part mixes such as 1 : 2 : 3, divide the total by 6 and multiply by each part.
• Cooking: rice to water, coffee to water (often around 1 : 15 to 1 : 17 by weight for filter coffee), and scaling recipes.
• Maps and models: a 1 : 25,000 map means 1 cm on paper is 25,000 cm (250 m) on the ground.
• Finance: debt-to-income and current ratios compare two amounts in the same way.

Common mistakes

• Swapping the order. 2 : 3 and 3 : 2 are different ratios; keep the quantities in the order stated in the question.
• Mixing units. Convert both quantities to the same unit first: 500 ml : 2 l is 500 : 2000, or 1 : 4, not 250 : 1.
• Adding to both sides. Adding the same number to both sides changes the ratio; only multiplying or dividing keeps it equivalent.

Frequently asked questions

Sources

• Common Core State Standards for Mathematics, Ratios and Proportional Relationships (grades 6–7).
• SMPTE ST 2036-1, Ultra High Definition Television image parameter values (16:9 image formats).

Last reviewed 8 October 2026. Results are estimates; see our disclaimer.