# Laser Beam Spot Size Calculator

URL: https://speedycalculator.com/calculators/physics/laser-spot-size/
Last updated: 2026-10-30

> The laser spot size calculator finds the focused spot diameter and depth of focus of a laser beam from its wavelength, the beam diameter at the lens, the focal length and M².

## Inputs

- Wavelength λ (nm, µm, mm, m, pm, Å)
- Beam diameter at the lens D (mm, cm, m, in, µm)
- Focal length f (mm, cm, m, in, µm)
- Beam quality factor M²

## Results

- Spot diameter
- Depth of focus

## Formula

```
Spot diameter d = 4·M²·λ·f / (π·D),  depth of focus = 2·z_R = 2π·w₀²/(M²·λ)
```

## Explanation

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

FieldWhat to enterWavelength λUnits: nm, µm, mm, m, pm, ÅBeam diameter at the lens DUnits: mm, cm, m, in, µmFocal length fUnits: mm, cm, m, in, µmBeam quality factor M²A 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)53210.160579815.24071,06420.32091,33025.40111,59630.48142,12840.6418

## 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.

## FAQ

**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.


## Sources

- [OpenStax – University Physics Volume 3](https://openstax.org/details/books/university-physics-volume-3)
- [HyperPhysics (Georgia State University)](http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html)