Aspect Ratio Calculator_
Four linked fields: put in the original width and height plus one new dimension, and the fourth is calculated live so an image or video keeps its shape. Nine presets cover the ratios people actually use — 16:9, 4:3, 21:9, 1:1, 9:16 and the rest — and a preview box draws the result so you can see the shape, not just read it.
When a resize lands between pixels, this page says so: it shows the exact value, the whole-pixel result, and which direction it was rounded. Switch to Reduce mode to turn any dimensions into their simplest ratio — and when the exact answer is something like 959:540, you also get told it is 16:9 to within a tenth of a percent. Nothing is uploaded; this is arithmetic on numbers you type.
- Ratio
- Decimal (width ÷ height)
- Nearest common ratio
Aspect ratio, in pixels you can actually use
A ratio is a shape, not a size
An aspect ratio is width divided by height, written as two numbers with a colon between them. The single most useful thing to understand about it is that it says nothing about how big something is: 1920 × 1080, 1280 × 720 and 640 × 360 are all 16:9, all exactly the same shape, at wildly different resolutions. That is why a video shot for a phone screen still fills a cinema display, and why "what resolution should this be?" and "what shape should this be?" are two separate questions. Fix the shape and every resolution along that line is yours to pick from.
Resizing an image without squashing it
The arithmetic is one line: new height = new width × original height ÷ original width. A 4000 × 3000 photo dropped into a 1200-pixel-wide column becomes 1200 × 900. Change only one dimension and you stretch the picture — faces widen, circles become ovals, and everyone notices even if they cannot say why. The problem is that this division frequently does not land on a whole pixel: 4000 × 2250 scaled to 1000 wide wants 562.5, and something has to give. Most calculators quietly hand you 563 and move on. This one shows the exact figure alongside the rounded one and names the direction, because a half-pixel absorbed silently in three places is how a layout ends up mysteriously misaligned.
Why some dimensions reduce to ugly numbers
Reduce mode divides both numbers by their greatest common divisor, which is how 1920:1080 collapses to 16:9. The result is only tidy when the dimensions share a large factor. Crop a single pixel off the width and 1918 × 1080 reduces to 959:540 — arithmetically perfect, practically useless, and a common source of "why does my ratio look wrong?". The answer is usually that the file is one or two pixels away from a standard shape, so this page reports the nearest common ratio and how far off you are: 959:540 is 16:9 to within about a tenth of a percent, which is a rounding artefact from an export, not a design decision.
Fitting one shape inside another
Sooner or later a 16:9 video has to go into a 1:1 slot, and there are exactly three options. Letterbox or pillarbox it — scale until the whole frame fits and fill the leftover with bars, keeping every pixel of the original at the cost of screen space. Crop it — scale until the slot is full and lose whatever falls outside, which is what social platforms do by default and why heads get cut off in feed previews. Or stretch it, which fits perfectly and looks wrong to everyone. Choosing between bars and a crop is an editorial call; the calculator's job is the target dimensions once you have made it.
How to use
- 01Enter the original width and height — the dimensions you have now — then type the one new dimension you know. The remaining field fills in immediately and is marked as calculated.
- 02Or start from a preset: click 16:9, 9:16 or any other and the original fields take that shape, so you only need one new dimension to get the matching size.
- 03Watch for the rounding note. If it appears, the exact answer was not a whole pixel — it tells you the true value and which way it went, so you can decide whether to adjust the other dimension instead.
- 04Switch to Reduce to ratio when you have dimensions and want the shape: it gives the exact simplified ratio, the decimal, and the nearest common ratio when they differ. Copy_Result copies whatever is on screen.
Four moments this saves you
A hero image for a fixed column
The design column is 1200px wide and the photo is 4000 × 3000. One field gives the height that keeps it undistorted.
4000 · 3000 · new width 1200
new height 900 (exact, 4:3)
Checking an export before upload
The render says 1918 × 1080 — Reduce mode shows it is not quite 16:9 and by how much.
1918 · 1080
959:540 · ≈ 16:9, 0.1% off
A vertical cut for Reels or Shorts
Click 9:16, type the width your source allows, and read the height to target.
9:16 · new width 1080
1080 × 1920
Verifying a spec's numbers
A comp claims 21:9 but lists dimensions that reduce to something else — the ratio readout settles it.
2560 · 1080
64:27 · decimal 2.370
The ratios worth knowing
| Ratio | Decimal | Where it is used | Typical pixels |
|---|---|---|---|
16:9 | 1.778 | HD video, YouTube, most displays | 1920 × 1080 |
4:3 | 1.333 | Older displays, many camera sensors, iPad | 1024 × 768 |
21:9 | 2.333 | Ultrawide monitors, cinematic video | 2560 × 1080 |
1:1 | 1.000 | Square social posts, avatars | 1080 × 1080 |
9:16 | 0.563 | Stories, Reels, Shorts, TikTok | 1080 × 1920 |
3:2 | 1.500 | DSLR photos, 35mm film frames | 3000 × 2000 |
2:3 | 0.667 | Portrait photography, posters | 2000 × 3000 |
5:4 | 1.250 | Large-format prints, older monitors | 1280 × 1024 |
2.39:1 | 2.390 | Anamorphic widescreen cinema | 2048 × 858 |
Decimal is width ÷ height — above 1 landscape, below 1 portrait, 1 square. Some names are conventions, not reductions: 21:9 in lowest terms is 7:3, a real “21:9” panel is 2560 × 1080 which is 64:27, and 2.39:1 cinema ships as 2048 × 858, or 1024:429.
Standard sizes by context
| Context | Dimensions | Ratio |
|---|---|---|
| HD video (720p) | 1280 × 720 | 16:9 |
| Full HD (1080p) | 1920 × 1080 | 16:9 |
| QHD (1440p) | 2560 × 1440 | 16:9 |
| 4K UHD | 3840 × 2160 | 16:9 |
| 8K UHD | 7680 × 4320 | 16:9 |
| Ultrawide monitor | 3440 × 1440 | 43:18 (≈ 21:9) |
| Square social post | 1080 × 1080 | 1:1 |
| Portrait social post | 1080 × 1350 | 4:5 |
| Story / Reel / Short | 1080 × 1920 | 9:16 |
| YouTube thumbnail | 1280 × 720 | 16:9 |
| A-series paper (A4, A3…) | 2480 × 3508 at 300dpi | 1:√2 (≈ 1:1.414) |
| Common laptop display | 1440 × 900 | 8:5 (16:10) |
Paste any of these into Reduce mode to see the arithmetic. Social platforms change their preferred sizes periodically — the ratio is the stable part, so target the shape and scale to whatever pixel size the platform currently wants.
Tips & best practices
- Decide the shape first, then the resolution — they are separate choices, and fixing the shape leaves every resolution on that line available.
- When a result lands between pixels, nudge the dimension you control by one rather than accepting the rounding — a neighbouring width often divides cleanly.
- Export at an exact multiple of the target ratio when you can — clean numbers survive every later resize.
- For social platforms, target the ratio rather than the exact pixel count: the ratios stay put for years while the recommended pixel sizes drift.
- If an image looks subtly wrong but you cannot say why, run its dimensions through Reduce mode — the ratio usually reveals a one-dimension resize.
Gotchas — where ratios mislead
A rounded dimension is no longer the same ratio
Scaling 1920 × 1080 to 1000 wide wants 562.5 pixels of height. Whichever whole number you pick, the result is fractionally off 16:9 — 1000 × 563 is 1.776, not 1.778. Invisible in one image, cumulative across a grid of them — which is why this page shows the exact value rather than hiding the choice.
Ratio and resolution are independent
1920 × 1080 and 1280 × 720 are the same shape at different sizes; 1920 × 1080 and 1920 × 1200 are different shapes at similar sizes. Judging quality from a ratio, or shape from a pixel count, gets this backwards — and "HD" describes resolution, not proportion.
Letterboxing and cropping are the only honest fits
Putting a 16:9 frame in a 1:1 slot means either bars (keep everything, lose space) or a crop (fill the space, lose edges). Stretching fits without either cost and looks wrong to every viewer. Automated feeds crop by default, so check what falls outside before publishing.
Pixel aspect ratio is not display aspect ratio
Most formats use square pixels, so the two match. Some video standards do not: DV PAL stores 720 × 576 with non-square pixels that display as 16:9, so the stored dimensions reduce to 5:4 while the picture is widescreen. If a file plays wider than its numbers suggest, the pixel aspect ratio is the reason.
Anamorphic content lies about its shape until it is unsqueezed
Anamorphic capture squeezes a wide image onto a narrower sensor and relies on playback to stretch it back. Until that happens the raw frame looks vertically stretched, and its stored dimensions describe the container rather than the picture. Calculate with the unsqueezed dimensions or every number downstream is wrong.
Technical details
- Solving
- Any three of original width/height and new width/height solve the fourth by cross-multiplication at full float precision
- Rounding
- Reported, never silent: the exact value, the whole-pixel result, and the direction and size of the change are all shown when they differ
- Reduction
- Euclidean GCD, with decimal inputs scaled by a power of ten first, so 1.5 × 1 reduces to 3:2
- Nearest ratio
- Dimensions within 1% of a known ratio report it with the percentage difference — 1918 × 1080 is exactly 959:540 and approximately 16:9
- Presets
- 16:9, 4:3, 21:9, 1:1, 9:16, 3:2, 2:3, 5:4 and 2.39:1, sharing one definition with the reference table so they cannot drift apart
- No images involved
- Nothing is uploaded, decoded, or resized — this is arithmetic on the numbers you type. File-size units belong to a planned byte converter, icon output to a planned favicon generator
- Input
- Positive numbers, decimals accepted, whitespace trimmed; zero, negative and non-numeric values each get a message naming the field.
FAQ
How do I calculate an aspect ratio?
Divide width by height for the decimal, then reduce both numbers by their greatest common divisor for the ratio: 1920 ÷ 1080 is 1.778, and dividing both by 120 gives 16:9. Reduce mode above does both at once and tells you when the tidy answer is only approximate.
How do I resize an image and keep its proportions?
Multiply the new width by the original height, then divide by the original width — that is the height that preserves the shape. Enter the three numbers you have above and the fourth appears; if it is not a whole pixel, the rounding note tells you rather than letting it slide.
What is 16:9 in pixels?
Any pair where width ÷ height is about 1.778 — commonly 1280 × 720, 1920 × 1080, 2560 × 1440 or 3840 × 2160. There is no single answer: pick the resolution your target needs and the shape stays identical.
Why did my dimensions reduce to something like 959:540?
Because they share no large common factor — usually the file is a pixel or two off a standard size after a crop or an export. That is arithmetically correct but not what anyone means; the nearest-common-ratio line tells you it is 16:9 to within a tenth of a percent, which is almost certainly what was intended.
What is pixel aspect ratio?
The shape of an individual pixel. Modern formats use square pixels, so the frame ratio follows straight from the dimensions. Some broadcast video standards store non-square pixels and rely on the player to stretch them, which is why a 720 × 576 file can display as widescreen despite its numbers.
Which ratio should I use for social media or YouTube?
Landscape video and thumbnails are 16:9; feed squares are 1:1; portrait posts are usually 4:5; and stories, Reels and Shorts are 9:16. Target the ratio rather than a fixed pixel count — platforms adjust their recommended sizes far more often than they change shape.