Aspect Ratio Calculator
Simplify a resolution and resize while keeping proportions exact.
Last updated
Your details
Leave blank and fill the new height instead.
Result
- Aspect ratio
- 16 : 9
- As a decimal
- 1.7778
- Megapixels
- 2.07
- Height at 1,280 px wide
- 720.00 px
A standard 16:9 ratio
Formula — written step by step
Reducing 1920 × 1080…
ratio = width : height, divided by their GCD
GCD of 1920 and 1080 = 120
1920 ÷ 120 = 16
1080 ÷ 120 = 9
1920 × 1080 = 16 : 9
Every resolution that reduces to the same pair shares a shape. 1280 × 720 and 3840 × 2160 are both 16 : 9, which is why one ratio describes an entire ladder of sizes.
The formula for each shape
-
Reduce a ratio
w : h ÷ GCD(w, h)
Needs the pixel width and height
-
Height from a new width
h₂ = (w₂ × h₁) ÷ w₁
Needs the original ratio and the new width
-
Width from a new height
w₂ = (h₂ × w₁) ÷ h₁
Needs the original ratio and the new height
A worked example
Resizing 1920 × 1080 to 1280 wide
- Original
- 1920 × 1080
- New width
- 1280 px
- h₂ = (w₂ × h₁) ÷ w₁
- h₂ = (1280 × 1080) ÷ 1920
- h₂ = 1,382,400 ÷ 1920
h₂ = 720 px — so 1280 × 720
Rounding the height to a whole pixel is what causes the thin black bars you sometimes see down the edge of a video. Sticking to standard resolution ladders avoids it.
How to use this calculator
- Enter the original width and height in pixels to see the ratio in lowest terms.
- To resize, type either a new width or a new height — the matching dimension is worked out for you.
- Check the table of common resolutions to find a standard size that shares your ratio.
Why 16:9 became universal
Widescreen television settled on 16:9 as a compromise between the 4:3 of old broadcast and the wider 2.35:1 of cinema. It is now the default for monitors, phones in landscape, and nearly all streaming video.
Ratios are conventionally written in lowest terms. 1920×1080 reduces to 16:9 by dividing both by 120 — the greatest common divisor.
Resizing without distortion
To scale an image without stretching, both dimensions must change by the same factor. Fix one and the other follows from the ratio, which is what the new width and height fields do.
Fractional results should be rounded to whole pixels. Rounding both independently can shift the ratio by a hair; for video encoding, round to even numbers, since most codecs require dimensions divisible by two.
Letterboxing and cropping
Displaying content at a different ratio than the screen forces a choice. Letterboxing adds black bars and preserves the whole frame; cropping fills the screen and discards the edges. Cinema at 2.39:1 shown on a 16:9 television is the everyday example of the first.
Cropping is destructive, so it matters what was framed near the edges. Broadcasters have historically shot with "safe areas" in mind precisely so that a frame survives being cropped to a different ratio.
Pixels, resolution and ratio
Resolution and aspect ratio are independent. 1920x1080 and 1280x720 are both 16:9 despite differing sharply in pixel count. Ratio describes shape; resolution describes detail.
Most codecs require even pixel dimensions, and some require multiples of four or sixteen. When resizing, round to an even number rather than accepting a fractional result from the ratio arithmetic.
Formula & methodology
What this calculator assumes
- Assumes square pixels, which is true of essentially all modern displays but not of older anamorphic video formats.
- Common ratios are recognised by name — 16:9, 4:3, 21:9 (actually 64:27), 1.85:1 and 2.39:1 — because reducing by GCD alone gives unhelpful ratios for cinema formats.
- Resized dimensions are rounded to whole pixels, so an exact ratio is not always achievable at a given width.
Sources
- ITU-R BT.709-6, Parameter values for the HDTV standards for production and international programme exchange, International Telecommunication Union, 2015 — the 16:9 image format.
Common aspect ratios
| Ratio | Decimal | Where you meet it |
|---|---|---|
| 16 : 9 | 1.778 | HD and 4K video, most monitors and TVs |
| 16 : 10 | 1.600 | Many laptop and productivity displays |
| 4 : 3 | 1.333 | Older broadcast, projectors, iPad |
| 3 : 2 | 1.500 | Most digital camera sensors, 35 mm film |
| 1 : 1 | 1.000 | Square social posts and profile images |
| 21 : 9 | 2.333 | Ultrawide monitors, cinematic crops |
21 : 9 is a marketing name rather than an exact reduction — the usual 2560 × 1080 panel is precisely 64 : 27.
Frequently asked questions
What ratio should I use for social media?
1:1 or 4:5 for feed posts, 9:16 for stories and short-form video, 16:9 for landscape video. Vertical formats dominate mobile-first platforms.
What aspect ratio is a phone screen?
Modern phones are typically 19.5:9 or 20:9 — considerably taller than the 16:9 that dominated a decade ago, which is why older video letterboxes on them.
Why is cinema wider than television?
Widescreen cinema formats were introduced in the 1950s specifically to differentiate the theatrical experience from television, and the convention stuck.
Same ratio at common sizes
| Name | Width | Height |
|---|---|---|
| 4K UHD | 3840 | 2,160 |
| 1440p | 2560 | 1,440 |
| 1080p | 1920 | 1,080 |
| 720p | 1280 | 720 |
| 480p | 854 | 480 |