Enter the original dimensions and one new dimension — it fills in the other so nothing stretches.
Type in one and the other updates to match the original proportions.
Aspect ratio is width divided by height, expressed as the smallest whole-number pair that produces the same result. 1920 × 1080 divides down by 120 to give 16:9. The point of keeping it constant when resizing is that anything else stretches or squashes the content.
To scale without distortion, multiply both dimensions by the same factor. If you know one target dimension, the other follows: new height = new width ÷ ratio.
| Ratio | Decimal | Where you meet it |
|---|---|---|
| 16:9 | 1.778 | HD and 4K video, most monitors and TVs |
| 4:3 | 1.333 | Older TVs and monitors, iPad, many compact cameras |
| 3:2 | 1.500 | 35mm film and most DSLR and mirrorless sensors |
| 1:1 | 1.000 | Square — profile pictures, album art |
| 9:16 | 0.563 | Vertical video — Reels, Shorts, TikTok |
| 16:10 | 1.600 | Many laptops and productivity monitors |
| 64:27 | 2.370 | Ultrawide monitors, marketed as "21:9" |
| 256:135 | 1.896 | DCI 4K cinema |
No mainstream ultrawide monitor is actually 21:9. A 2560 × 1080 panel simplifies to 64:27 — a decimal ratio of 2.370, not the 2.333 that a true 21:9 would give. A 3440 × 1440 panel is 43:18, or 2.389. Both are sold as 21:9 because it reads as a natural step up from 16:9 and the exact figures do not.
This calculator reports what the numbers actually reduce to and names the marketing label alongside, because if you are computing a crop or a render size, the real ratio is the one that matters.
When content and screen have different ratios, something has to give. Letterboxing adds bars to preserve the whole frame — black bars top and bottom when 21:9 cinema content plays on a 16:9 screen, or pillarbox bars at the sides when 4:3 content plays on a widescreen. Cropping fills the screen by cutting content off, which is what "zoom to fill" does and why heads get chopped. Stretching fills the screen by distorting, which is always the wrong answer and always obvious once you notice it.
The same trade-off drives the "safe area" convention in video production: shoot so the important content survives a centre crop, because you cannot control what ratio it eventually plays back at.
1280 × 720, 1920 × 1080 and 3840 × 2160 are all 16:9. They differ in resolution — the pixel count — not in shape. Doubling both dimensions quadruples the pixels: 1080p is about 2.1 megapixels and 4K is about 8.3, four times as many, which is why file sizes and render times scale the way they do.
This is worth keeping straight because "higher resolution" and "wider screen" get conflated constantly. An ultrawide monitor is a different shape; a 4K monitor is a finer grid. They are independent choices.
Social platforms each want different ratios and re-crop what does not fit, usually badly. Instagram feed posts favour 1:1 and 4:5, stories and Reels want 9:16, and YouTube thumbnails are 16:9. Uploading one ratio and letting the platform crop is how key content ends up outside the frame.
For print, remember that standard paper sizes do not match standard sensor ratios. A 3:2 photo does not fit an 8 × 10 inch print, which is 4:5 — something gets cropped, and it is better to decide what than to let a lab decide for you.
Keep the aspect ratio constant by multiplying both dimensions by the same factor. If you know one target dimension, divide or multiply by the ratio to get the other — new height = new width ÷ ratio. Scaling 1920 × 1080 to a width of 1280 gives a height of 720, because both are 16:9. Changing one dimension without the other is exactly what causes stretching.
For every 16 units of width there are 9 of height, a decimal ratio of about 1.778. It is the standard for HD and 4K video and for most monitors and televisions. The numbers are a proportion, not a size — 1280 × 720, 1920 × 1080 and 3840 × 2160 are all 16:9 and differ only in how many pixels make up the same shape.
No. It reduces to 64:27, a decimal ratio of 2.370, where a true 21:9 would be 2.333. A 3440 × 1440 ultrawide is 43:18, or 2.389. Both are marketed as 21:9 because it reads as a tidy step up from 16:9, but neither is that ratio. If you are calculating a crop or a render size, use the real figure — this calculator reports it and names the marketing label alongside.
Aspect ratio is the shape; resolution is the number of pixels filling it. 1280 × 720 and 3840 × 2160 are the same shape at very different resolutions. Doubling both dimensions quadruples the pixel count, which is why 4K is about four times the data of 1080p rather than twice. Screens can differ in either dimension independently — an ultrawide is a different shape, a 4K panel is a finer grid.
Because the content and the screen have different aspect ratios, and letterboxing preserves the entire frame rather than cutting into it. Cinema shot at 2.39:1 shown on a 16:9 television gets bars top and bottom; older 4:3 material on a widescreen gets bars at the sides. The alternatives are cropping, which removes content, or stretching, which distorts it — bars are the option that shows you exactly what was filmed.
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