Canvas Size Calculator

Sizing a canvas means converting between three descriptions of the same rectangle: physical dimensions, sample count, and data volume. Pixels equal inches multiplied by resolution, so a 24 by 36 inch poster at 300 samples per inch needs 7,200 by 10,800 pixels, which is 77.76 megapixels. Uncompressed data follows from bit depth: at 24 bits per pixel that file occupies about 222 MB, and at 32 bits with an alpha channel roughly 296 MB, before any layers, masks, or history are counted. Working backwards gives the maximum print size an existing file supports, since dividing pixel dimensions by an acceptable resolution yields inches, which is how a 4,000 by 6,000 capture becomes a 13.3 by 20 inch print at 300 ppi or a 26.7 by 40 inch print at 150. The resolution figure is a viewing-distance judgement, not a fixed rule. At a typical reading distance of about 12 inches, the eye resolves detail near one arcminute, which corresponds to roughly 300 samples per inch, so that number became the default for prints held in the hand. Posters read from two metres are perfectly sharp at 150 ppi, and billboards are routinely output at 10 to 20 ppi. Two clarifications prevent expensive mistakes. Samples per inch in an image file are not the same as the printer dot count, since an inkjet quoting 1440 dpi lays many tiny ink dots to render each image sample. And resampling upward adds pixels without adding information, so the honest maximum print size is set at capture, not in the resize dialogue.

Both halves of this calculation inherited their conventions from printing. Standard paper sizes trace to Georg Christoph Lichtenberg, who noted in a letter of 1786 that a sheet with sides in the ratio of one to the square root of two halves into sheets of the same proportion. Walter Porstmann built the DIN 476 series on that property in 1922, anchoring A0 at one square metre, and the system became ISO 216 in 1975. Painters had their own conventions: nineteenth-century French colourmen sold stretchers in numbered figure, paysage, and marine formats, while the American trade settled on the inch sizes still stocked today, among them 8 by 10, 11 by 14, 16 by 20, 18 by 24, and 24 by 36. The 300 figure so often quoted for print resolution comes from halftone reproduction. Frederic Ives and contemporaries developed practical halftone screens in the 1880s, breaking continuous tone into dots of varying size so that photographs could be printed on a press. Screen frequency is measured in lines per inch, and the working rule became roughly two image samples per halftone line, so the 133 to 175 lpi screens typical of quality offset work implied image files near 300 ppi. Desktop publishing carried the convention into software after PostScript arrived in 1984 and Photoshop shipped in 1990, at which point artists began specifying canvases in pixels first and inches second, and file size joined dimensions as a practical design constraint.