Aspect Ratio:
Table of Contents
Aspect ratio, simplified
Enter pixel width and height and this calculator returns the simplified ratio (W:H). Use it to verify that a video is in 16:9 before uploading, to size social media assets, or to scale an image without distorting it.
The formula
The aspect ratio is the simplified form of the width-to-height fraction. You find it by dividing both numbers by their greatest common divisor (GCD):
ratio = (W / GCD) : (H / GCD)
- W — width in pixels (or any consistent unit).
- H — height in same units.
- GCD — the greatest common divisor of W and H, found via Euclid’s algorithm.
Worked example
Using the default 1920×1080:
- Find GCD(1920, 1080). Euclid: 1920 mod 1080 = 840; 1080 mod 840 = 240; 840 mod 240 = 120; 240 mod 120 = 0. GCD = 120.
- Divide: 1920 / 120 = 16, 1080 / 120 = 9.
- Ratio = 16:9.
1920×1080 (“Full HD”) is therefore the same shape as 1280×720 (“720p”) and 3840×2160 (“4K UHD”) — just at different resolutions.
Scaling while preserving the ratio
To resize without distortion, pick one new dimension and compute the other:
H′ = W′ × (H / W)
W′ = H′ × (W / H)
Example: scaling a 1920×1080 image to a new width of 1200 px gives H′ = 1200 × (1080 / 1920) = 675 px.
Common aspect ratios
| Ratio | Common resolution | Where it’s used |
|---|---|---|
| 16:9 | 1920×1080 / 3840×2160 | HDTV, 4K UHD, YouTube, most monitors |
| 21:9 | 2560×1080 / 3440×1440 | Ultrawide monitors, cinematic widescreen |
| 4:3 | 1024×768 / 1600×1200 | Traditional TV, older monitors, iPad portrait |
| 1:1 | 1080×1080 | Instagram square, profile photos |
| 9:16 | 1080×1920 | TikTok, Instagram Reels, mobile vertical video |
| 3:2 | 3000×2000 | DSLR sensors, traditional 35 mm photo prints |
| 4:5 | 1080×1350 | Instagram portrait feed posts |
| 2.39:1 | cinema mastering | Anamorphic widescreen (“Cinemascope”) |
| 1.85:1 | cinema mastering | Theatrical “flat” projection |
| 5:4 | 1280×1024 | Legacy 17″ LCD monitors |
Letterboxing vs pillarboxing
When the source ratio doesn’t match the display ratio, you have three choices:
- Letterbox — bars at top and bottom (wider source on a narrower display).
- Pillarbox — bars at left and right (narrower source on a wider display).
- Crop — chop off the parts that don’t fit. Use carefully — you lose pixels.
Never stretch to fill; it distorts the image visibly.
Common applications
- Social media. Each platform has its preferred ratios; mismatched uploads get center-cropped or letterboxed.
- Video production. Frame for the delivery ratio from the start — reframing in post loses resolution.
- Web design. Responsive images use CSS aspect-ratio to reserve layout space before the image loads, preventing cumulative layout shift.
- Photography prints. Standard print sizes have fixed ratios (4×6 = 3:2, 5×7 = 7:5, 8×10 = 5:4); reframe in post or expect to be cropped at the lab.
Limitations & gotchas
- Pixel rounding can give an “off by one” result — 1366×768 is technically 683:384 but is treated as 16:9 (1366 should really be 1365.333).
- Some sensors and displays use non-square pixels; the displayed aspect ratio differs from the pixel-count ratio.
- This calculator uses Euclid’s algorithm for GCD — results are exact integer ratios, not the “market name” (so 256:135 may not be relabelled as the common 17:9 marketing term).
Sources & references
- Wikipedia, “Aspect ratio (image).”
- ITU-R BT.709, the HDTV standard defining 16:9 broadcast video.
- SMPTE (Society of Motion Picture and Television Engineers), industry standards for cinema mastering ratios.
FAQs
For every 16 units of width there are 9 units of height. It’s a proportion, not an absolute size — a 16:9 image could be 1280×720, 1920×1080 or 3840×2160. All three share the same shape, just at different resolutions.
Pick one new dimension (say a new width W′) and compute the matching height: H′ = W′ × (H / W). Equivalently, multiply both width and height by the same scale factor. If you change only one dimension without rescaling the other, the image stretches or squashes.
Because phones are held vertically. TikTok and Reels are designed for 9:16 (vertical), while standard YouTube video is 16:9 (horizontal) — the same proportions, rotated 90 degrees. Posting a horizontal clip to a vertical-first platform leaves big bars or forced crops at top and bottom.
Not without losing or adding pixels. Your options: pillarbox (add black bars left and right, preserving the original frame), crop (cut off the top and/or bottom to fill the wider frame), or reframe shot-by-shot. Stretching the image to fit is the one option to avoid — it visibly distorts faces and objects.
By default, iPhones record video in 16:9 (matching HD/4K TV standards). The native still-photo aspect is 4:3 on the main camera, with a 16:9 option in settings. Cinematic mode on newer iPhones records 2.4:1, the same ultra-wide ratio used in modern film.
Letterboxing adds black bars at the top and bottom — used when a wider source (e.g. 2.39:1 cinema) is shown on a less-wide screen (e.g. 16:9 TV). Pillarboxing adds black bars at the left and right — used when a narrower source (e.g. 4:3) is shown on a wider screen (e.g. 16:9). The bars preserve the original aspect ratio at the cost of unused screen area.