Full Width at Half Maximum

  The full width at half maximum or FWHM is a simple measure of the width of a distribution, and is easily obtained from empirical distributions, histograms. As one of the two parameters describing a Breit-Wigner distribution whose standard deviation is infinite, it is most frequently used in connection with distributions describing resonant states.

For a distribution described by the probability density f(x) the FWHM is defined by |x₂-x₁| where x₁,x₂ are points to the left and right of the mode x_m (defined by f(x_m) = max), with f(x₁)=f(x₂)=f(x_m)/2. For the normal distribution one has the relation

between FWHM and , standard deviation.

The FWHM can only be defined for unimodal distributions.