Thumbnail Aspect Ratio Calculator

The arithmetic, in one line

height = width × ratio-height ÷ ratio-width. At 16:9 that is width × 9 ÷ 16: 1,280 becomes 720 exactly, while 1,000 wants 562.5 — not a whole pixel, so the calculator reports 563 and says it rounded. The other direction is the same fraction turned over: width = height × 16 ÷ 9, so a height of 720 gives 1,280, and a height of 500 gives 888.89, reported as 889 with the rounding flagged. Every result on this page is computed at build time by the same tested engine that powers the calculator, from the example inputs named beside it, so the copy cannot drift from the code.

Why some widths divide cleanly and others do not is plain arithmetic. 16 and 9 share no common factor, so width × 9 ÷ 16 is a whole number only when the width itself is a multiple of 16; going the other way, the width comes out whole only when the height is a multiple of 9. That one fact explains the shape of every standard video size — 640→360, 1,280→720, 1,600→900, 1,920→1,080, 2,560→1,440, 3,840→2,160 — and why an arbitrary width such as 1,366 lands on 768.38 instead. The guide Aspect-ratio math for thumbnails walks through that reasoning at length; this page stays with what the tool does and how to read what it prints.

Worked examples: width to height at 16:9

Each row below is a real call to the engine's heightForWidth function, made when this page was built. Exact values are printed to two decimals, the precision the tool's result box uses; the last column is the flag the tool shows as "rounded".

Width	Exact height (to 2 decimals)	Reported	Whole pixel?
800	450	450	Yes
1,000	562.5	563	No — rounded
1,100	618.75	619	No — rounded
1,200	675	675	Yes
1,366	768.38	768	No — rounded
1,440	810	810	Yes
1,800	1,012.5	1,013	No — rounded
2,048	1,152	1,152	Yes

The engine keeps both numbers rather than choosing one. The exact value is the quotient as the engine holds it, shown here — as in the tool's result box — to two decimals: the table's 768.38 for a width of 1,366 is the engine's 768.375, which the guide prints to three decimals. The reported value is the nearest whole pixel, and an exact half goes upward: 562.5 becomes 563 and 1,012.5 becomes 1,013. 618.75 rises to 619 because its fraction sits above a half, while 768.38 drops to 768 because its fraction sits below one. The flag is set whenever the exact value was not already whole. That is the whole point of showing it: a result box that printed only the rounded pair would look clean while being a fraction of a pixel off, and you would only find out when an export came back one row short of the ratio you asked for.

Worked examples: height to width at 16:9

Leave the width field empty and the tool switches to widthForHeight: the same fraction inverted. Heights that are multiples of 9 come out whole; the rest do not, and the flag says so.

Height	Exact width (to 2 decimals)	Reported	Whole pixel?
450	800	800	Yes
500	888.89	889	No — rounded
540	960	960	Yes
600	1,066.67	1,067	No — rounded
720	1,280	1,280	Yes
900	1,600	1,600	Yes
1,000	1,777.78	1,778	No — rounded

When the height drives the calculation, each result line reads width × height with the computed width first and your height second, so the pair is still written in the usual order. A height of 500 is a good example of why the direction matters: 888.89 is not a width you can export, so the tool reports 889 and flags it — and if the whole-pixel version is what you need, the nearest multiple of 9 above or below your height is the number to type instead.

The ratios this calculator knows

The ratio list is fixed data inside the engine, video-first, and the notes come from the same table. The third column is what the result box prints for a width of 1,920 — a full-HD frame's width — top to bottom, with the 16:9 line set in bold on the page.

Ratio	Where it shows up	Height at 1,920 wide	Whole pixel?
16:9	YouTube thumbnails and players	1,080	Yes
4:3	Older video; sddefault storage frame	1,440	Yes
1:1	Podcast thumbnails, channel avatars	1,920	Yes
9:16	Shorts, Reels, vertical video	3,413	No — exact 3,413.33
21:9	Ultra-wide banners	823	No — exact 822.86
4:5	Portrait social feeds	2,400	Yes

Two rows deserve a second look. 9:16 is the 16:9 fraction turned on its side, so a 1,920-wide vertical frame wants 3,413.33 of height and is reported as 3,413; the width that makes 9:16 whole is a multiple of 9, not of 16. 21:9 reduces to 7:3, so a whole height needs a multiple of 7, not of 21; 1,920 is not one, so it lands on 822.86 and is reported as 823, while 1,750 — not a multiple of 21, but a multiple of 7 — gives 750 exactly. The calculator does not order these ratios by preference or recommend one; it lists them in the engine's order so that a single width gives you every companion size at once.

How to read the result

The result box shows one line per ratio, each as width × height in pixels. When the computed dimension is not whole, the line adds the exact value and the word "rounded" in brackets, so the honest number is never more than a glance away. The 16:9 line is emphasized because that is the thumbnail ratio, and it is also what the Copy button puts on the clipboard: a one-line summary of the pair you are most likely to type somewhere else. Reset clears both fields and returns the box to its placeholder.

The fields are read leniently: grouping commas, stray spaces and currency or percent signs are stripped before parsing, so a pasted "1,280" works; text that still is not a plain number is refused in the field with a message before the engine is called. If both fields are filled, the width wins — the hint above the form says so — because a width and a height together already describe a ratio, and the tool would have to guess which one you meant to keep. The engine itself has its own guards, quoted here from its output:

Input	What the engine says
Width 0	The width must be greater than 0.
Width -640	The width must be greater than 0.
Width 100,001	The width is limited to 100,000 pixels.
Height 0	The height must be greater than 0.

Zero and negative dimensions are refused because a dimension of zero or less is not a size — the engine's only lower bound is "greater than 0", exactly as its message says. The upper bound of 100,000 pixels is a deliberate cap inside the engine, not a platform rule; nothing this site knows about needs a larger number, and the cap keeps obviously mistyped values from producing plausible-looking output.

Checking the ratio of an image you already have

The tool on this page goes from one dimension to the other. The reverse question — what ratio is this image already? — is answered by the same engine's describeRatio function, which the thumbnail tester runs on every upload. It reduces the pair by their greatest common divisor to get a simplified ratio, reports the plain quotient width ÷ height, and answers "exactly 16:9?" with an integer test: width × 9 must equal height × 16. The rows below are computed from it at build time.

Width × height	Simplified	Width ÷ height	Exactly 16:9?
1,280×720	16:9	1.7778	Yes
1,600×900	16:9	1.7778	Yes
640×480	4:3	1.3333	No
1,366×768	683:384	1.7786	No
1,280×800	8:5	1.6000	No
1,000×563	1000:563	1.7762	No
100.5×60	none (not whole pixels)	1.6750	No

The common misreading is the decimal. 1,366×768 simplifies to 683:384 and divides to 1.7786; true 16:9 divides to 1.7778. Print both to two decimals and they read 1.78 and 1.78 — identical — which is why the engine decides 16:9 with the integer test and never by comparing decimals. Fractional dimensions such as 100.5×60 get a quotient (1.6750) but no simplified form, because a ratio of whole numbers needs whole numbers to reduce.

The 1,000×563 row is this calculator's own rounded answer for 1,000 wide, fed back in. It simplifies to 1000:563, and the 16:9 test says no: rounding the height to a whole pixel already broke the equality, so the reported pair is a pixel size, not the ratio. That is why the tool prints the exact value beside the pixel instead of storing the rounded pair as though it were 16:9, and why the tester and the size guide's checker report such a pair as not 16:9, with the crop it implies, rather than letting it pass as close enough.

What a 16:9 slot does to an image that is not 16:9

When an image of another shape is shown in a 16:9 frame that fills the slot, one axis has to lose pixels: an image wider than 16:9 loses width, a taller one loses height. The tester's coverCropReport function computes which axis and how much, and the same geometry applies whichever surface is doing the cropping.

Upload	Fits 16:9?	Axis cropped	Share lost
1,280×720	Yes	—	—
640×480	No	height	25%
1,280×800	No	height	10%
1,000×563	No	height	0.1%
1,000×1,000	No	height	43.8%
3,440×1,440	No	width	25.6%

A 4:3 frame (640×480) loses 25% of its height in a 16:9 slot; a 1,000×1,000 square loses 43.8%; a 3,440×1,440 ultra-wide frame keeps its full height and loses 25.6% of its width instead. The 1,000×563 row is the calculator's rounded answer again: the report says it does not fit, and the loss is 0.1% of its height — the rounding flag and the crop report describing the same half pixel from two sides. This is the practical reason to size at 16:9 before you compose rather than after: the calculator tells you the target pair, and the tester shows you what survives if you skip that step.

What the calculator does not decide

It does arithmetic on the numbers you type, and nothing else. It does not know YouTube's rules: the 16:9 recommendation and the 640-pixel minimum width come from Google's help article "Add video thumbnails", cited on the size guide and re-checked on July 25, 2026. At that minimum width, 16:9 gives a height of 360 — computed here, like everything else — but the calculator will just as readily return a height for a width of 100, which is below the minimum: 56.25, reported as 56 and flagged as rounded, with no word about the minimum because the engine has no concept of one. Upload size limits and accepted formats live on the same size guide with the same citation. Whether interface chrome will cover part of the frame is a question for the size guide's safe-area checker, whose zones model a convention rather than a published specification, and whether the picture still reads when it is shown small is what the tester is for. The calculator also does not pick a ratio for you: 16:9 comes first because this site is about thumbnails, and the other 5 are the neighbouring shapes listed in the engine's own notes.

Go deeper

The guide Aspect-ratio math for thumbnails covers the reasoning behind these numbers — why ratios are fractions, what rounding does to them, and how to choose dimensions that avoid it. The guides index lists the rest of the site's explanations, and the methodology page documents how the engine behind this calculator is tested.

Frequently asked questions

Values are processed locally in your browser and never transmitted. See the methodology page for how the engines are tested.