Resolution

Resolution has two meanings that are easy to mix up. Pixel resolution is the image's size in pixels, for example 3840 by 2160. Print resolution, measured in dots or pixels per inch (dpi or ppi), is how densely those pixels are laid down on paper. The same file prints small and sharp at high dpi, or large and soft at low dpi. To know whether an image is big enough for print, divide its pixel width by the target dpi to get the print width in inches.

In practice

• For screens and slides, roughly 1920 pixels wide (Full HD) is usually enough; 3840 (4K) leaves room for zooming and cropping.
• For close-viewed print, 300 dpi is the common target. Large posters and boards viewed from a distance are often printed at 150 dpi or less without visible loss.
• An A1 sheet at 300 dpi is about 7000 by 9900 pixels, much larger than most renders come out at.
• Changing the dpi number in a file's settings does not add detail. Only more pixels do.
• A print size checker and a file size estimator help plan before export.

In Rendair

When a render is too small for its use, Upscale & Enhance increases the pixel count and adds detail, which is the practical route from a screen-sized image to a print. See upscaling for how that differs from simply resizing, and aspect ratio for the separate question of image shape.