This calculator will help you to calculate the Pulse Width at Fourier Transform Limit (in fs),with input value of Central wavelength (in nm) and Half Width of Laser Spectrum (in nm).
Pulse Width = $\frac{A\lambda_0^2}{c \triangle \lambda}$
Pulse Shape Constant, A:
Gaussian: 0.441
Sech: 0.315
Lorentz: 0.142
Center wavelength, $\lambda_0$; Speed of Light, C; Half Width of Laser Spectrum, $\triangle \lambda$;