US Letter at 300 DPI: 2550 by 3300 pixels

Unlike the metric A series, this size is defined in inches, so the conversion at 300 dots per inch is exact with no rounding at all: 8.5 times 300 is 2550, and 11 times 300 is 3300. The greatest common divisor is 150, giving a ratio of 17:22, which is 1:1.2941 in portrait.

That shape is meaningfully different from A4. Letter is wider and shorter, 1:1.294 against 1:1.414, so a page laid out to fill one leaves an uneven band on the other. Text reflows, but anything positioned absolutely, a letterhead rule, a footer block, a background image, lands in the wrong place. Documents intended for both should be laid out inside the intersection of the two.

Scaling between them does not fix it either. Fitting Letter content onto A4 by scaling to the width leaves extra height at the bottom, and fitting to the height overflows the width. The intersection approach, keeping content inside 8.27 inches wide and 11 inches tall, is the only version that prints correctly on both without adjustment.

As with any print size, the pixel dimensions are meaningless without the resolution beside them. Halving to 150 dots per inch gives 1275 by 1650, which is the same physical page at half the detail, and doubling to 600 gives 5100 by 6600.