Ratio Calculator
Find the missing number in A:B = C:D as you type. Switch to Advanced to simplify, scale, split a total, convert to fractions and percents, work out aspect ratios or scale a recipe.
The shaded box is worked out for you. Decimals and fractions work too: 0.75, 1/2, 1 1/2.
Use a colon between parts. Any number of parts, with decimals or fractions.
Rounded shares are nudged so they still add up exactly to the total.
Part to part, like 3 boys to 5 girls.
What ratio is this size?
Adds the real width and height of the screen, and its sharpness (pixels per inch) for pixel sizes.
Resize to a ratio
Start each line with the amount, in any unit: 1 1/2 cups, 250 g, 2 tbsp, 3 eggs. Lines without a number stay as they are.
How your ratio is worked out
A ratio compares amounts. Multiply or divide every part by the same number and the ratio stays the same.
- 1
Write it as fractions
2/3 = 10/?
- 2
Cross multiply
3 × 10 = 30
- 3
Divide
30 ÷ 2 = 15
- 4
The answer
2:3 = 10:15
Read more: the formulas, a worked example and common ratios
A ratio compares two or more amounts: 2 : 3 means "2 of the first for every 3 of the second". Multiplying or dividing every part by the same number gives an equivalent ratio, so 2 : 3, 4 : 6 and 10 : 15 all mean the same thing. Every mode on this page uses that one idea.
The formula
A : B = C : D means A ÷ B = C ÷ D, so A × D = B × C
missing D = B × C ÷ A · missing C = A × D ÷ B
simplest form = each part ÷ greatest common divisor
one share = total ÷ (sum of the parts) × that part
part as a fraction of the whole = part ÷ (sum of the parts)
new height = new width × ratio height ÷ ratio width
Worked example
Solve 2 : 3 = 10 : ?. Write it as fractions, 2/3 = 10/?, and cross multiply: 3 × 10 = 30. Divide by the number left, 2: 30 ÷ 2 = 15. So 2 : 3 = 10 : 15, and the second ratio is the first one times 5.
More examples, one for each mode
| Mode | Example | Working |
|---|---|---|
| Simplify | 1.5 : 2.25 : 3 | × 4 to clear the decimals = 6 : 9 : 12, ÷ 3 = 2 : 3 : 4 |
| Scale | 2 : 3 × 4 | 2 × 4 : 3 × 4 = 8 : 12 |
| Split a total | 1,000 in 2 : 3 : 5 | 2 + 3 + 5 = 10 parts, 1,000 ÷ 10 = 100 a part, so 200, 300 and 500 |
| Ratio to fraction and % | 3 : 5 | 3 + 5 = 8, so 3/8 = 37.5% and 5/8 = 62.5% |
| Aspect ratio | 1920 × 1080 | Greatest common divisor 120: 1920 ÷ 120 = 16, 1080 ÷ 120 = 9, so 16 : 9 |
| Resize | 16 : 9 at 1280 wide | 1280 × 9 ÷ 16 = 720 high |
| Recipe | 4 servings → 6 | 6 ÷ 4 = 1.5, so 2 cups flour × 1.5 = 3 cups |
Common aspect ratios
| Ratio | As a decimal | Used for | Example size |
|---|---|---|---|
| 16 : 9 | 1.78 | HD and 4K TVs, YouTube, most monitors | 1920 × 1080, 3840 × 2160 |
| 4 : 3 | 1.33 | Old TVs, iPads, many compact cameras | 1024 × 768 |
| "21 : 9" | 2.37–2.39 | Ultrawide monitors (really 64 : 27 or 43 : 18) | 2560 × 1080, 3440 × 1440 |
| 1 : 1 | 1.00 | Square posts and profile pictures | 1080 × 1080 |
| 3 : 2 | 1.50 | DSLR and mirrorless photos, 6 × 4 prints | 6000 × 4000 |
| 16 : 10 | 1.60 | Many laptops | 1920 × 1200 |
| 9 : 16 | 0.56 | Phone videos, Reels, TikTok, Stories | 1080 × 1920 |
| 4 : 5 | 0.80 | Portrait social posts | 1080 × 1350 |
Part to part, or part to whole?
A ratio usually compares parts with each other: 3 boys to 5 girls is 3 : 5. A fraction compares a part with the whole: there are 3 + 5 = 8 children, so 3/8 of them are boys, which is 37.5%. Mixing these up is the most common ratio mistake. The Ratio ↔ fraction ↔ % mode shows both.
Tips
- Ratios have no units, but both parts must be in the same unit before you compare them: 50 cm : 2 m is 50 : 200, which is 1 : 4.
- "1 : n" form is handy for maps and models: a 1 : 25,000 map means 1 cm on paper is 25,000 cm (250 m) on the ground.
- The golden ratio is about 1.618 : 1. Enter 1.618 : 1 in Simplify to see it as whole numbers.
- When scaling a recipe, cooking times and pan sizes don't scale in a straight line, and eggs are best rounded to whole ones.
Screen sizes use the diagonal and the ratio (width = diagonal × ratio width ÷ √(ratio width² + ratio height²)). Real screens can differ slightly because of rounded corners and bezels.
Common questions
How do I solve a ratio with a missing number?
Write both ratios as fractions and cross multiply. For 2 : 3 = 10 : ?, multiply 3 × 10 = 30 and divide by the number left, 2, to get 15. So 2 : 3 = 10 : 15, and the second ratio is the first one times 5.
How do I simplify a ratio?
Divide every part by the greatest common divisor. If there are decimals, multiply first to make whole numbers: 1.5 : 2.25 : 3 times 4 is 6 : 9 : 12, and dividing by 3 gives 2 : 3 : 4. The calculator handles ratios with three or more parts.
How do I split an amount in a ratio?
Add the parts, divide the total by that sum to get one part, then multiply. To split 1,000 in 2 : 3 : 5, there are 10 parts of 100 each, so the shares are 200, 300 and 500. Each share is total ÷ parts × its part.
How do I convert a ratio to a fraction or percent?
A ratio compares parts with each other, while a fraction compares a part with the whole. 3 : 5 has 8 parts in all, so the first part is 3/8 = 37.5% and the second is 5/8 = 62.5%. Mixing these up is the most common ratio mistake.
What is the aspect ratio of 1920 × 1080?
1920 × 1080 is 16 : 9. Both numbers divide by their greatest common divisor, 120, to give 16 and 9. 16 : 9 is used by HD and 4K TVs, YouTube and most monitors. To resize, 16 : 9 at 1280 wide is 720 high.
How do I scale a recipe with a ratio?
Divide the servings you want by the servings in the recipe and multiply every amount by that. Going from 4 servings to 6 is × 1.5, so 2 cups of flour become 3. Cooking times and pan sizes don't scale in a straight line, and eggs are best rounded.
Related calculators
Made by Zulu Calculator, part of AlphaZulu LLC. Formulas are shown in full and rule figures are checked against official sources; see how we build calculators.