Halftone LPI, Image DPI & Grey Levels Calculator

How it works

Grey levels = (device DPI ÷ LPI)² + 1 (max 256); image PPI required = LPI × quality factor (1.5–2.5); max print width = pixels ÷ required PPI; max LPI for 256 levels = DPI ÷ 16

Printing presses cannot lay down lighter or darker ink, only dots of different sizes, so continuous-tone images are broken into a halftone screen measured in lines per inch: coarse for absorbent newsprint and textiles, 150–200 for coated commercial work. Each halftone cell is built from the output device’s much finer spots, and the number of spots per cell sets how many grey levels it can render — a 2,400 dpi platesetter at 150 lpi manages 257, but a 600 dpi laser at 150 lpi only 17, which is why office printers band. Image resolution follows a separate rule: pixels should be about twice the screen ruling at final size, so 300 ppi for 150 lpi; more is wasted, much less looks soft. Dots also spread on paper (dot gain), darkening midtones by 15–30%, which press profiles compensate for.

Worked example

A 150 lpi screen on a 2,400 dpi platesetter gives the full 256 grey levels. Images should be 300 ppi at final size, so a 3,000 × 2,000 px photo prints cleanly up to 10 × 6.7 in; at the intended 8 in width it is 375 ppi — fine. On a 600 dpi laser the same 150 lpi would give only 17 greys; that printer should run at ≤ 85 lpi (37 levels) or use error diffusion.

Frequently asked questions