Why does 21:9 at 1920 pixels wide not come out exact?

21:9 reduces to 7:3, so an exact whole-pixel width must be a multiple of seven. At 1920 wide the ideal height is 822.857… pixels; rounding to 823 changes the output ratio. The calculator distinguishes the requested ratio from the actual output and offers the nearest positive exact size to your supplied dimension.

Why does the tool show 7:3 when I typed 21:9?

They are the same ratio. 21:9 reduces to 7:3 the same way 4/8 reduces to 1/2. Named labels are shown only for an exact ratio match; a marketed or approximate resolution need not share that exact ratio.

What does the even-numbers option do?

It rounds both output dimensions to the nearest positive even number, including the dimension you supplied. The final exactness check uses those actual output pixels. When rounding changes the ratio, the suggested alternative preserves both the ratio and the even grid. Even dimensions can suit some encoder/pixel-format requirements, but do not guarantee H.264/H.265 or other encoding compatibility.

Can I type a decimal like 1.85 instead of a ratio?

Yes. Bare 1.85, 2.39 and 2.35 mean that value against 1. Decimal and scientific notation are reduced exactly from their supplied text, within normalized decimal scales -324 to +324. 2.39 becomes 239:100, and 1e-7 becomes 1:10000000. 16:9, 16/9, 16x9 and 16 9 work too. Size → ratio accepts two positive measurements in the same units; it does not round them into integers first.

What ratio should I use for social video?

Common starting shapes include 9:16 for vertical video, 1:1 for squares, 16:9 for landscape video and 4:5 for portraits. This calculator gives dimensions; it does not verify a platform's current upload, feed or cropping requirements.

What is the difference between 21:9 and 2.39:1?

21:9 is exactly 7:3, or 2.333…:1. 2.39:1 is exactly 239:100 and is wider. Named raster examples can approximate a ratio: 3440×1440 is 43:18, and 4096×1716 is 1024:429. Use the actual dimensions or the explicit intended ratio; the calculator does not infer anamorphic pixel-aspect or lens-squeeze factors.

Can I get the CSS or FFmpeg form?

Yes. The CSS declaration names the requested ratio; a rounded pixel result may differ and its actual ratio is shown separately. The FFmpeg argument scales to the output pixel dimensions. It does not correct sample/pixel aspect ratio, desqueeze footage, crop or encode a file.

Aspect Ratio Calculator

Give it a ratio and one dimension and it returns the other, or give it two dimensions and it tells you the ratio they already make. When whole pixels cannot hold the ratio exactly it says so, instead of quietly handing you a number that is slightly wrong.

Widescreen / HD1,920 × 1,080Output holds the requested ratio exactly

Mode

Ratio

Width

Height

Even numbers: Even pixels

The output dimensions preserve 16:9.

Common ratios

Pixels
1920 × 1080
Output ratio
16:9
CSS aspect-ratio (requested)
aspect-ratio: 16 / 9;
FFmpeg scale filter (output pixels)
-vf scale=1920:1080
Output decimal (rounded to four places)
1.7778

Ratio → size produces positive whole pixels, up to 9,007,199,254,740,991 per dimension. Size → ratio accepts measurements in the same units. Decimal precision and powers are bounded to normalized scales from −324 to +324. Very thin previews are constrained to one screen pixel; the numbers are authoritative. Requested CSS and actual rounded output can differ. FFmpeg text is a scale argument, without pixel-aspect correction or an encoding guarantee.

Selectable complete calculation

Aspect Ratio Calculator
Mode: Ratio → size
Supplied width: 1920
Supplied height: calculated
Supplied ratio: 16:9
Requested ratio: 16:9
Pixel grid: whole pixels
Output preserves requested ratio: yes
Supplied dimension adjusted: no
Nearest exact permitted size: current output
Pixels: 1920 × 1080
Output ratio: 16:9
CSS aspect-ratio (requested): aspect-ratio: 16 / 9;
FFmpeg scale filter (output pixels): -vf scale=1920:1080
Output decimal (rounded to four places): 1.7778
Decimal displays are rounded to four places. This calculates rectangular dimensions; it does not process image/video files, pixel-aspect correction or provider specifications.

Common aspect ratios and their resolutions

Common intended shapes and example resolutions. Some raster examples approximate their named ratio; calculations use the exact entered dimensions. Platform rules and pixel-aspect correction are separate.

Ratio	Decimal	Typical resolution	Where it is used
16:9	1.778	1920 × 1080	YouTube, HD/4K video, most monitors and TVs
9:16	0.563	1080 × 1920	TikTok, Reels, Shorts, Stories (vertical video)
1:1	1.000	1080 × 1080	Instagram square posts, profile pictures
4:5	0.800	1080 × 1350	Portrait example; check current platform crop rules
4:3	1.333	1440 × 1080	Older monitors, iPad, most compact cameras
3:2	1.500	6000 × 4000	Digital SLR sensors, 6×4 inch prints
21:9	2.333	3440 × 1440	Nominal ultrawide label; 3440×1440 is exactly 43:18
2.39:1	2.390	4096 × 1716	Cinema raster example; 4096×1716 is exactly 1024:429
1.85:1	1.850	3996 × 2160	Standard theatrical widescreen
5:4	1.250	1280 × 1024	Legacy displays, 8×10 inch prints

Common questions

Why does 21:9 at 1920 pixels wide not come out exact?: 21:9 reduces to 7:3, so an exact whole-pixel width must be a multiple of seven. At 1920 wide the ideal height is 822.857… pixels; rounding to 823 changes the output ratio. The calculator distinguishes the requested ratio from the actual output and offers the nearest positive exact size to your supplied dimension.
Why does the tool show 7:3 when I typed 21:9?: They are the same ratio. 21:9 reduces to 7:3 the same way 4/8 reduces to 1/2. Named labels are shown only for an exact ratio match; a marketed or approximate resolution need not share that exact ratio.
What does the even-numbers option do?: It rounds both output dimensions to the nearest positive even number, including the dimension you supplied. The final exactness check uses those actual output pixels. When rounding changes the ratio, the suggested alternative preserves both the ratio and the even grid. Even dimensions can suit some encoder/pixel-format requirements, but do not guarantee H.264/H.265 or other encoding compatibility.
Can I type a decimal like 1.85 instead of a ratio?: Yes. Bare 1.85, 2.39 and 2.35 mean that value against 1. Decimal and scientific notation are reduced exactly from their supplied text, within normalized decimal scales -324 to +324. 2.39 becomes 239:100, and 1e-7 becomes 1:10000000. 16:9, 16/9, 16x9 and 16 9 work too. Size → ratio accepts two positive measurements in the same units; it does not round them into integers first.
What ratio should I use for social video?: Common starting shapes include 9:16 for vertical video, 1:1 for squares, 16:9 for landscape video and 4:5 for portraits. This calculator gives dimensions; it does not verify a platform's current upload, feed or cropping requirements.
What is the difference between 21:9 and 2.39:1?: 21:9 is exactly 7:3, or 2.333…:1. 2.39:1 is exactly 239:100 and is wider. Named raster examples can approximate a ratio: 3440×1440 is 43:18, and 4096×1716 is 1024:429. Use the actual dimensions or the explicit intended ratio; the calculator does not infer anamorphic pixel-aspect or lens-squeeze factors.
Can I get the CSS or FFmpeg form?: Yes. The CSS declaration names the requested ratio; a rounded pixel result may differ and its actual ratio is shown separately. The FFmpeg argument scales to the output pixel dimensions. It does not correct sample/pixel aspect ratio, desqueeze footage, crop or encode a file.

Exact reduction of positive decimal/scientific measurements within normalized decimal scales -324 to +324. Pixel output is rounded to positive whole or even dimensions up to 9,007,199,254,740,991 per side, with final ratio equality checked exactly and exact alternatives when they fit. Decimal displays are rounded to four places. Numeric planning only; no file resizing, pixel-aspect correction or provider certification.