How this calculator works
- Step 1Whole numbers1.5 : 2.25 → 150 : 225Decimals are scaled by a power of ten.
- Step 2Common factorHCFFind the largest number dividing both.
- Step 3Divide2 : 3Divide both terms by it.
A ratio compares two amounts of the same kind. Dividing both terms by the same number keeps the ratio unchanged, so the calculator divides by the largest one possible.
Worked example: a Full HD screen
- Simplify
A ÷ HCF : B ÷ HCF
- Ratio1920 : 1080
- HCFlargest common factor120
- Simplest form1920 ÷ 120 : 1080 ÷ 12016 : 9
That is the 16 : 9 shape used by most screens and videos. As a decimal it is 1.777778.
Solving a proportion
- Resize a video1280 × 7201920 : 1080 = 1280 : x keeps the 16 : 9 shape.
- Scale a recipe7.5 cupsRice to water 2 : 3, for 5 cups of rice.
To solve A : B = C : x, multiply B by C and divide by A.
Assumptions and limits
- Positive termsFrom 0.000001 to 1,00,00,00,000.
- Six decimalsTerms with more decimal places are rejected.
- Same unitsConvert both terms to one unit first.
- Two termsThree-part ratios are not supported.
Related number tools
Ratios, fractions and percentages are three ways to compare parts.
To work with the same parts as fractions, use the fraction calculator. For screen and image proportions, try the aspect ratio calculator, and for common factors, the LCM and HCF calculator.
Common mistakes
Mistake: Comparing 1 m with 50 cm as 1 : 50.
Instead: Use one unit: 100 cm : 50 cm = 2 : 1.
Mistake: Swapping the terms.
Instead: Keep the order: rice : water is not water : rice.
Mistake: Adding to both terms to scale.
Instead: Multiply or divide both terms.
Checking the method
A separate script re-works every golden value with Python's exact fractions.
Read how we test our calculators, or browse more maths calculators.