Laser cutters, engravers, optical drives and microscopes all need to know how small a beam can be focused. Shorter wavelengths, bigger input beams and shorter focal lengths give smaller spots – at the cost of a shorter depth of focus.
How to use the Laser Beam Spot Size Calculator
- Wavelength λ
- Beam diameter at the lens D
- Focal length f
- Beam quality factor M²
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Enter the wavelength, the beam diameter at the lens, the focal length and the beam quality factor.
Inputs explained
| Field | What to enter |
|---|---|
| Wavelength λ | Units: nm, µm, mm, m, pm, Å |
| Beam diameter at the lens D | Units: mm, cm, m, in, µm |
| Focal length f | Units: mm, cm, m, in, µm |
| Beam quality factor M² | A number |
What each result means
- Spot diameter = 4×[Beam quality factor M²]×[Wavelength λ]×[Focal length f]÷(pi×[Beam diameter at the lens D])
- Depth of focus = 2×pi×([Spot diameter]÷2)^2÷([Beam quality factor M²]×[Wavelength λ])
Example values
How the main result changes when Wavelength λ changes and everything else stays at the example values above:
| Wavelength λ (nm) | Spot diameter (µm) |
|---|---|
| 532 | 10.1605 |
| 798 | 15.2407 |
| 1,064 | 20.3209 |
| 1,330 | 25.4011 |
| 1,596 | 30.4814 |
| 2,128 | 40.6418 |
Formula
Spot diameter d = 4·M²·λ·f / (π·D), depth of focus = 2·z_R = 2π·w₀²/(M²·λ)
Worked example
Inputs — Wavelength λ: 1,064 nm; Beam diameter at the lens D: 8 mm; Focal length f: 100 mm; Beam quality factor M²: 1.2.
Results — Spot diameter = 20.3209 µm; Depth of focus = 0.50802 mm.
Key terms
- Beam waist
- The narrowest point of a focused beam.
Frequently asked questions
How can I make the spot smaller?
Use a shorter focal length, a larger input beam or a shorter wavelength.
What is the Rayleigh range?
The distance over which the beam stays within √2 of its waist size.
Why does a small spot have a short depth of focus?
Tightly focused beams diverge quickly.
What is M²?
Beam quality; 1 is a perfect Gaussian beam.