Aspect ratio calculator
Enter a size, get the ratio — or resize at the same ratio.
How do I find the aspect ratio of a size?
Divide the width and height by their greatest common divisor: 1920 × 1080 ÷ 120 = 16:9.
How do I resize while keeping the ratio?
New height = new width × ratio height ÷ ratio width. At 16:9, a 1280-pixel-wide image is 720 pixels high.
Is 21:9 really 21:9?
Not exactly — it is a marketing name. The CTA-861 video standard uses 64:27 (2.37:1), which is what a 2560 × 1080 monitor has; 3440 × 1440 is 43:18 (2.39:1). A true 21:9 would be 2.33:1.
What are common aspect ratios?
16:9 for HD and 4K TVs and video, 4:3 for older TVs and many tablets, 3:2 for many photos, 1:1 for square posts, and 9:16 for vertical phone video.
Why is my result not a simple ratio?
Some sizes do not reduce to small numbers — 1366 × 768 is 683:384. The calculator also shows the decimal form, 1.78:1, which is easier to compare.
How do I crop a photo to a different aspect ratio?
Keep the full width and compute the height the new ratio needs: width × ratio height ÷ ratio width. A 3:2 photo of 6000 × 4000 cropped to 16:9 keeps 6000 pixels across and needs 6000 × 9 ÷ 16 = 3375 pixels of height, so 625 rows go. If that height is more than the photo has, keep the full height and compute the width instead.
What if my width or height has decimals?
Simplifying works on whole numbers, so scale both sides first. A page 8.5 by 11 inches becomes 17 by 22 when both are doubled, which is already in lowest terms: 17:22, or 0.7727. Both sides must use the same unit.
Which aspect ratios do the video standards define?
ITU-R Recommendation BT.709 specifies HDTV as 16:9 with 1920 × 1080 square pixels. The CTA-861 standard for digital video interfaces names four picture aspect ratios: 4:3, 16:9, 64:27 and 256:135.
The formulas and what they assume
An aspect ratio is width to height, w:h. A ratio compares two quantities in the same unit and is simplified by dividing out common factors (OpenStax Prealgebra 2e, section 5.6). For whole numbers the calculator divides both sides by their greatest common divisor, so 1920 × 1080 becomes 16:9 (divisor 120, decimal 1.7778). To resize, it uses the fact that equal ratios form a proportion whose cross products are equal (section 6.5): new height = new width × h ÷ w. The ratio describes the shape, not the resolution: 1280 × 720 and 1920 × 1080 share one ratio. Square pixels are assumed, as in BT.709.
Worked example: a 16:9 frame 1000 pixels wide
The height is 1000 × 9 ÷ 16 = 562.5. Pixels are whole, so round to 562 or 563: the frame is then 1.7794:1 or 1.7762:1 against the exact 1.7778:1, a difference under 0.1%.
Common mistakes
Swapping width and height, which turns 16:9 into the portrait 9:16 (1.7778 becomes 0.5625). Scaling with the rounded decimal: 1920 ÷ 1.78 gives 1078.7, not 1080, so keep the ratio as a fraction. Mixing units, such as inches against pixels. And taking a marketing name literally: “21:9” screens are usually 64:27 or 43:18, as the question above shows.
Sources: OpenStax, Prealgebra 2e, sections 5.6 “Ratios and Rate” and 6.5 “Solve Proportions and their Applications”; ITU-R Recommendation BT.709-3 (parameter table, aspect ratio 16:9, square pixels); CTA-861 (picture aspect ratios). Related tools: ratio calculator, proportion calculator, PPI calculator and TV size calculator. How we check our calculators.