DPI and line interval converter

Convert DPI or LPI to a line interval in millimetres and back, and find the right interval for your laser's spot size.

Laser software asks for a line interval in mm; image software talks in DPI; forums say LPI. They are the same spacing. Convert between them here, and get the interval that matches your focused spot so you engrave no finer than the laser can draw.

The one formula

interval (mm) = 25.4 ÷ DPI, and DPI = 25.4 ÷ interval. 254 DPI is exactly 0.1 mm; 300 DPI is 0.0847 mm; 500 DPI is 0.0508 mm.

Matching the spot

A diode laser focuses to roughly 0.08 × 0.15 mm, a CO₂ with a 2" lens to about 0.1–0.2 mm. Lines closer than the spot overlap heavily: the same spot is hit several times, the engraving darkens and deepens, and the job takes proportionally longer. Set the interval to the spot size less a little overlap, then control darkness with power and speed. Run an interval test on the material if your software has one.

Image resolution

Scale the image so its pixel pitch equals the interval: a 100 mm wide engraving at 0.1 mm needs 1000 pixels across. More pixels are thrown away; fewer are interpolated.

Related tools

• Spot size & depth of focus: How small your lens focuses and how much height you have, from wavelength and focal length.
• Rotary attachment calculator: Steps and mm per rotation for roller and chuck rotaries, and the wrap-around width.
• Inlay kerf offset: How far to grow the insert and shrink the pocket so an inlay fits without a gap.

FAQ

Are DPI and LPI the same?

For a raster engraving, yes: both describe how many scan lines (or dots) fit in one inch. LPI is the engraver's word, DPI the image editor's.

What interval for photos on wood?

0.08–0.1 mm (254–318 DPI) on most diodes and CO₂ machines. Finer only helps with a very small spot, such as a fibre laser.

Does the interval change engraving time?

Directly: halving the interval doubles the lines and the time. See the engraving time calculator.