Proportions in everyday life
A ratio compares two amounts: 3 : 4 means 3 parts of one thing for every 4 of another. When two ratios are equal, A : B = C : D, you have a proportion, and the missing value is found by cross-multiplying: D = B × C ÷ A.
Proportions scale recipes (2 cups of flour for 3 eggs → how much flour for 5 eggs?), maps (1 : 50,000), screen sizes, mixing ratios for paint, concrete or fuel, and photos you want to resize without distortion.
Worked example
A 16 : 9 video 1,920 pixels wide must be 1,920 × 9 ÷ 16 = 1,080 pixels tall. A concrete mix of 1 : 2.5 cement to sand with 6 buckets of cement needs 6 × 2.5 = 15 buckets of sand.
Simplifying ratios
1.5 : 2.5 is easier to read as 3 : 5: multiply by 10 to remove the decimals, then divide by the greatest common factor.
How this works
The page solves the proportion A : B = C : D by cross-multiplication, D = B × C ÷ A. To simplify A : B, it multiplies both terms by the same power of ten to remove decimals, then divides them by their greatest common divisor, giving the smallest whole-number ratio. The decimal form is A ÷ B. A and B must not be zero. Everything runs in your browser.
Frequently asked
How do I solve a proportion?
Cross-multiply: if A/B = C/D, then D = B × C ÷ A.
How do I simplify a ratio?
Divide both numbers by their greatest common factor; remove decimals first by multiplying both by 10, 100…
What size is 16:9 at 1920 pixels wide?
1,080 pixels tall (1920 × 9 ÷ 16).
Is a ratio the same as a fraction?
Closely related: 3 : 4 corresponds to the fraction 3/4 of the second amount, or 3/7 of the total.
Sources
Reviewed Oct 17, 2026 · test vectors fixtures/calculators/math/ratio-calculator/