Laser Beam Spot Size Calculator
Calculate the diffraction-limited focused spot size of a laser beam from wavelength, focal length, and input beam diameter.
What this calculator does
This tool calculates the smallest theoretically achievable focused spot size for a laser beam passing through a lens, limited by diffraction — an important figure of merit for laser cutting, engraving, and precision optics applications.
The formula
Diffraction-limited focused spot diameter d = 4λf / (πD), where λ is the laser wavelength, f is the lens focal length, and D is the input beam diameter at the lens. This is the standard Gaussian beam focusing formula, showing that shorter wavelength, shorter focal length, or larger input beam diameter all produce a smaller (tighter) focused spot.
This represents the ideal diffraction-limited case; real-world spot size can be larger due to lens aberrations, beam quality (M²), and alignment imperfections.