Resize Image to YouTube Thumbnail Size

Why a Thumbnail Is 16:9

A thumbnail is judged on one measurement before any other: it has to be 16:9. YouTube's own help pages set that shape, ask for a width of at least 640 pixels, accept JPG, PNG and GIF, and currently recommend 3840 x 2160 - a figure that read 1280 x 720 for years and moved up as displays did. Both numbers draw the same rectangle; only the pixel count differs, and 1280 x 720 sits comfortably inside every stated limit. File weight is the one place the platform is not uniform. An upload made from the mobile app is capped at 2 MB, while the same thumbnail sent from a desktop browser may run to 50 MB, so the single figure repeated across older articles is now only half the rule. Neither ceiling is anywhere near you at this size: a 1280 x 720 JPEG of a photograph normally lands between 100 and 300 KB. Shape is where the real work happens, because very little arrives already 16:9. A 4032 x 3024 photo off a phone is 4:3; scaled until it covers the frame it becomes 1280 x 960, and 240 pixels of height - a quarter of the picture - are trimmed off, half from the top and half from the bottom. Padded instead, it comes back 960 x 720 with a 160 pixel band of background down each side, again a quarter of the finished thumbnail. A square export from a design app is harsher: 44 percent of its height goes to the crop, or 44 percent of the thumbnail becomes side bands. A 1080 x 1920 phone capture is the hard case, and worth doing the arithmetic on before you commit to it. Filled into the frame it keeps a horizontal strip worth 32 percent of its height and discards the rest; kept whole it shrinks to 405 pixels wide inside a 1280 pixel frame, leaving 875 pixels of flat colour, which is over two thirds of the thumbnail. Resolution is the other thing to watch, and 1280 x 720 is only 0.9 megapixels, which sounds unmissable until you notice where thumbnail sources come from. A still exported from a 1080p project has room to spare. A screenshot taken while the video played in a window on a laptop often does not - a 640 x 360 grab has to double in each direction, and enlargement invents nothing, so the tool reports the factor rather than applying it silently. That factor is the usual reason one thumbnail looks smeared beside its neighbours in a list of results. Decoding, cropping and encoding all run through the browser's own canvas on your machine, so nothing dropped here is transmitted anywhere - including frames of a video you have not published yet.