Ratio Calculator – Solve, Simplify and Find Aspect Ratios
Find the missing number in a proportion, simplify a ratio of two or three parts, or work out the aspect ratio of a screen, photo or video and resize it without distortion.
For A:B = C:D, cross-multiply: A × D = B × C. Example: 3:4 = x:20 → x = 3 × 20 ÷ 4 = 15. To simplify a ratio, divide every part by the greatest common divisor: 24:36 → both ÷ 12 → 2:3.
How to use the Ratio Calculator
- Solve: fill in three of the four boxes in A:B = C:D and leave the unknown empty. Fill all four to check whether two ratios are equal.
- Simplify: enter two or three parts (decimals are fine). You get the simplest form, the 1:n and n:1 forms, the fraction and each part’s share of the total.
- Aspect ratio: enter a width and height, or tap a preset (16:9, 4:3, 21:9, 1:1, 3:2, 9:16). Type a new width or height to resize while keeping the ratio.
How to work out a ratio between two numbers
Write the two quantities in the same unit, put a colon between them, then divide both by their greatest common divisor (GCD). A class with 12 boys and 18 girls has a ratio of 12:18; the GCD is 6, so the ratio is 2:3. As a share of the total, that is 2/5 = 40% boys and 3/5 = 60% girls.
Dividing an amount in a ratio
Add the parts, divide the amount by that total, then multiply by each part. To split $200 in the ratio 1:2:1, there are 4 parts of $50, so the shares are $50, $100 and $50. Recipes, concrete mixes and paint thinning all use this method.
Solving proportions
If 5 pounds of apples cost $8, what do 12 pounds cost? 5:8 = 12:x, so x = 8 × 12 ÷ 5 = $19.20. The calculator shows each cross-multiplication step so you can copy them into homework.
Common aspect ratios
| Ratio | Example sizes | Used for |
|---|---|---|
| 16:9 | 1920 × 1080, 3840 × 2160 | TVs, monitors, YouTube |
| 9:16 | 1080 × 1920 | TikTok, Reels, Shorts, stories |
| 4:3 | 1024 × 768 | Older screens, iPad, many phone photos |
| 3:2 | 6000 × 4000 | DSLR and mirrorless photos, 6 × 4 in prints |
| 1:1 | 1080 × 1080 | Square social posts |
| 21:9 | 2560 × 1080, 3440 × 1440 | Ultrawide monitors, cinema |
“21:9” is a marketing name: 2560 × 1080 is really 64:27 and 3440 × 1440 is 43:18. The calculator gives the exact ratio and names the closest common one.
Resizing an image
New height = new width × original height ÷ original width. A 1200 × 800 image (3:2) resized to a width of 640 gets a height of 640 × 800 ÷ 1200 = 426.67, so about 427 pixels.
Related tools: Proportion Calculator (Rule of Three), Fraction Calculator and Percentage Calculator.
Frequently asked questions
How do you calculate a ratio?
Divide one quantity by the other, or write them as A:B and simplify by dividing both by their greatest common divisor. 15:25 simplifies to 3:5.
What is the aspect ratio of 1920 × 1080?
16:9. Both numbers divide by 120: 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9.
How do I find the missing number in a ratio?
Cross-multiply and divide. For 2:5 = 6:x, x = 5 × 6 ÷ 2 = 15.
What does a ratio of 1:2:1 mean?
Three quantities in the proportions 1, 2 and 1: the middle one is twice each of the others and makes up half of the total (4 parts).
Is a ratio the same as a fraction?
Not quite. The ratio 2:3 compares two parts; as a fraction of the whole, the first part is 2/5. The fraction 2/3 compares the first part with the second.
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