Spot size and depth of focus calculator
Focused laser spot diameter, depth of focus and power density from wavelength, lens focal length, beam diameter and beam quality, for CO₂, fiber and diode lasers.
How small your laser really focuses, and how much height you have before it goes soft, from the lens and the beam. The spot sets the finest line you can cut, the line interval worth engraving at, and the power density that does the work; the depth of focus tells you how flat the material has to be.
The formula
A Gaussian beam of diameter D at a lens of focal length f focuses to a spot of 4 M² λ f ÷ (π D). Depth of focus here is twice the Rayleigh range, π (spot ÷ 2)² ÷ (λ M²): the span over which the spot stays within about 41 % of its smallest size. Real machines land a little larger than this (lens quality, alignment, a beam that is not quite Gaussian), so treat it as the floor.
What the numbers mean at the laser
- CO₂ with a 2" lens and an 8 mm beam: about 0.1 mm spot, 1.5 mm depth; a 1.5" lens for engraving detail, a 2.5" or 4" lens for 10 mm acrylic.
- Fiber at 1064 nm: ten times smaller spot for the same lens, which is why it marks metal with sharp detail and cuts with a tiny kerf.
- Diode at 455 nm: wavelength helps, but the beam is a rectangle with M² well above 1 along one axis, so spots are typically 0.08 × 0.15 mm, not round.
- The kerf is close to the spot plus whatever the material melts or chars around it: expect 0.1–0.3 mm on wood and acrylic.
Related tools
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- Inlay kerf offset: How far to grow the insert and shrink the pocket so an inlay fits without a gap.
- Sheet gauge ⇄ mm: Gauge to millimetres and back for steel, stainless, aluminum and SWG, with the tables.
FAQ
Why is my kerf bigger than the spot?
Heat spreads. The spot is where the energy lands; the kerf is what the material loses, which on wood and acrylic is the spot plus the melt and char zone on each side. Measure it with the kerf test gauge.
Where do I put the focus on thick material?
A third of the way into the sheet is the usual compromise: the spot is small enough at the top to start cleanly and has not grown too much at the bottom. Beyond the depth of focus you need a longer lens or several passes.
What is M²?
Beam quality: how far from a perfect Gaussian the beam is. Sealed CO₂ tubes run 1.1–1.5, glass tubes 1.2–2, fiber lasers near 1.1, diode lasers 1.5–30 depending on axis. A higher M² focuses to a proportionally larger spot.