Spot Size Calculator
Enter the laser wavelength, the focal length of the lens, the beam diameter at the lens and the beam quality M². The calculator gives the focused spot diameter, the depth of focus and the focusing angle.
- Spot diameter
- Depth of focus
- Any wavelength
Beam diameter is at the 1/e² points where it reaches the lens. M² is 1 for a perfect beam and 1.05 to 1.3 for good fibre lasers.
A lens can only focus a beam so small. The smallest spot depends on wavelength, focal length and how wide the beam is at the lens: d = 4λfM² ÷ (πD). A 1064 nm laser with an 8 mm beam through a 100 mm lens focuses to about 19 µm.
Spot size chart
8 mm beam, M² 1.1. Search for a focal length.
| Focal length (mm) | 532 nm | 1064 nm | 10,600 nm (CO₂) |
|---|---|---|---|
| 50 | 4.7 | 9.3 | 93 |
| 100 | 9.3 | 18.6 | 186 |
| 163 | 15.2 | 30.4 | 302 |
| 254 | 23.7 | 47.3 | 471 |
| 420 | 39.1 | 78.2 | 779 |
Spot diameters in µm. Longer wavelengths and longer lenses give larger spots.
Focal length and spot
1064 nm, 8 mm beam. Slide the focal length.
The beam is drawn much wider than real so you can see it narrow to a waist.
How to work out spot size
Four numbers. Step through it.
Wavelength
In mm.
Expanding the beam before the lens with a beam expander makes D larger and the spot smaller.
Short, medium or long lens
Focal length. Compare them.
Smallest spot.
Balanced.
Large working area.
Galvo scanners use f-theta lenses, usually 100 to 420 mm, chosen for field size as well as spot.
Before choosing optics
Optics checks. Tick each item.
Tick each item as you go.
Laser light can blind instantly. Use eyewear rated for the wavelength and power.
What you get
Focus figures. Each tile explains one result.
In µm.
In mm.
One side.
Full angle.
µm².
Power density rises with the square of the spot size shrinking: half the spot gives four times the intensity.
Frequently Asked Questions
No question matches that search.
How do I calculate laser spot size?
d = 4 × λ × f × M² ÷ (π × D).
What is M²?
A beam quality number. A perfect Gaussian beam is 1.
How do I get a smaller spot?
Use a shorter lens, a wider beam or a shorter wavelength.
What is depth of focus?
Twice the Rayleigh range, the distance where the spot stays near its smallest.
Why is the CO₂ spot so large?
Its wavelength is 10 times longer than a fibre laser.
What is a 1/e² diameter?
Where intensity falls to 13.5% of the peak.
Does a smaller spot mean less depth?
Yes, depth of focus shrinks with the spot squared.
Know your focus before you fire
Enter the laser and lens to get spot size and depth of focus. Choose optics that suit the detail and thickness of your work.
More size calculators
Each one has its own calculator, worked examples and answers to common questions.