Odd pixel dimensions break video encoders
Codecs encode in blocks and several refuse an odd width or height, which is why the aspect-ratio calculator lists even standard sizes.
Codecs encode in blocks and several refuse an odd width or height, which is why the aspect-ratio calculator lists even standard sizes.
Aspect Ratio Calculator does two jobs. Enter a ratio and one dimension, get the other. And it lists the pixel sizes that actually get used for 9:16, 16:9, 1:1, 4:5, 21:9 and 4:3, so you copy a real size rather than deriving one that ends on a half-pixel — or on an odd edge. The even-rounding is not a UI quirk. It is the encoder constraint the table exists to keep you from hitting.
Blocks, not pixels
Most of the codecs that land in an MP4 — H.264, HEVC, the VP family — do not store a frame as an independent grid of pixels. They encode in macroblocks (classically 16×16) and, on 4:2:0 chroma, subsample colour on a 2×2 grid. An odd width or an odd height leaves a half-block. Some pipelines pad it. Several refuse the frame outright and return a dimension error.
That is why a perfectly good arithmetic answer can be a perfectly useless export size. 1080×1920 is 9:16. 1079×1918 is also 9:16, to two decimal places, and a lot of encoders will not take it. Deriving height as width × 16 / 9 without snapping to even is how a 9:16 brief becomes a rejected encode.
The calculator's FAQ states the rule without dressing it up: most video codecs encode in blocks and expect even pixel dimensions; several refuse an odd width or height outright. Standard sizes are all even for that reason.
Why the calculator rounds to even
When you type a width, the tool solves height as round(n / 2) × 2, floored at 2. Same in the other direction. Change the ratio and the unlocked side snaps again. It does not round to nearest integer. Nearest would still land on odd.
The standard-size table is the other half of the same rule:
| Ratio | Common size | Where it is used |
|---|---|---|
| 9:16 | 1080×1920 | TikTok, Reels, Shorts |
| 16:9 | 1920×1080 | YouTube, landscape |
| 1:1 | 1080×1080 | Square feed |
| 4:5 | 1080×1350 | Instagram portrait |
| 21:9 | 2560×1080 | Anamorphic |
| 4:3 | 1440×1080 | Legacy / iPad |
Every number in that table is even. 1080×1920 is the size the short-form surfaces actually serve; going higher than 1080 wide is rarely worth it, because those platforms re-encode on upload and discard the extra pixels.
If you need the other edge of a custom ratio, type one even side and let the tool emit the other even side. Do not paste a spreadsheet result that came out 1079.
Standard sizes that actually get used
Copying a listed size is cheaper than inventing one. 9:16 at 1080×1920 is the default across TikTok, Reels and Shorts. 16:9 at 1920×1080 is the YouTube landscape default. 4:5 at 1080×1350 is the Instagram portrait box. Those are the dimensions change video aspect ratio is aiming at when you ask the editor for a platform crop — and that editor still wants a source whose edges an encoder will accept.
Safe Area Preview paints typical TikTok, Reels, Shorts and YouTube chrome onto a 1080×1920 or 1920×1080 canvas for the same reason: those are the frames those surfaces show. An odd-width plate that survived an image export will still have to land on one of those even boxes before it is a video.
A derived size that "looks like 9:16" on a monitor is not the same as a size a codec will encode. The table is the list of sizes that have already survived that filter.
Stills can be odd; video usually cannot
Image Resizer sets an exact width and height, or a platform preset. Stills do not have to be even. JPEG, PNG and WebP will write 1079×1918. The resizer's presets happen to be even because they are platform-served sizes — TikTok 1080×1920, Instagram 1080×1080 and 1080×1350, a 1280×720 thumbnail, a 3840×2160 4K frame — but you can type an odd pair if you want a still.
One preset is the tell: X / LinkedIn post is 1200×675. Height 675 is odd. That is fine for a still. Feed that same box to a video encoder and you are back in the refusal case. If the still is a plate you will later animate, size it to an even pair before the video step, not after a failed encode.
The split is the workflow:
- Still that stays a still: exact pixels are allowed, odd included.
- Still that becomes a clip, or any MP4: even × even, preferably a size from the calculator table.
- Platform crop in the editor: pick the ratio, preview the 480p pass, export once. The editor is not a place to discover that the source was 1079 wide.
Do not "fix" an odd video size by stretching one axis by a pixel in a prompt. Stretching 1079 to 1080 is a resize. Do it in the resizer or the calculator, then encode.
FAQ
Why do some dimensions need to be even numbers?
Most video codecs encode in blocks and expect even pixel dimensions; several refuse an odd width or height outright. That is why the listed standard sizes are all even, and why deriving a dimension arithmetically can produce a number a working encoder then rejects.
What ratio should I export vertical video at?
9:16 at 1080×1920 is the default across TikTok, Reels and Shorts. Going higher than 1080 wide is rarely worth it — those platforms re-encode on upload, so the extra pixels are discarded and the upload is only slower.
Can I use an odd size for a thumbnail or a still?
Yes. Image files do not inherit the video-block rule. The resizer will write the exact pixels you type. If that still is going to be animated, snap it to even first.
The calculator changed my height after I typed a width. Is that a bug?
No. The unlocked side is snapped to even so the pair is encodable. If you need a listed platform size, copy it from the table rather than fighting the snap.