Laser Spot Size Calculator
Calculate focused Gaussian laser spot diameter from wavelength, focal length, and input beam diameter. Get quick results in your browser, free with no signup.
Focused Laser Spot Size
When a laser beam is focused through a lens, the minimum spot diameter determines power density at the workpiece. This is critical for laser cutting, welding, engraving, and microscopy.
Formula
$$d = \frac{4 \lambda f}{\pi D}$$\(d\) is the focused spot diameter (at the 1/e² intensity contour for a Gaussian beam), \(\lambda\) is wavelength, \(f\) is focal length, and \(D\) is the input beam diameter at the lens.
Practical Notes
Shorter wavelengths and longer focal lengths produce larger spots. A larger input beam (relative to the lens) yields a smaller focused spot. Real systems may differ slightly due to aberrations and beam quality factor M².
Frequently Asked Questions
What beam profile does this assume?
This calculator uses the standard Gaussian beam focusing formula for the 1/e² intensity diameter.
How do I reduce spot size?
Use a shorter wavelength, a shorter focal length, or expand the beam before the focusing lens so \(D\) is larger.
Are the units consistent?
Enter wavelength in nanometers and focal length and beam diameter in millimeters. The result is given in millimeters or micrometers.
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