$$ \theta_{\text{m}}=arcsin(sin(\theta_{\text{i}})+m\lambda\mu) $$

Where:

• $\theta_{\text{m}}$ [°]: Angle at which diffracted light exists grating, measured from the surface normal.
• $\theta_{\text{i}}$ [°]: Angle at which incident light hits grating at, measured from the surface normal.
• $m$: Order of diffraction.
• $\lambda$ [nm]: Wavelength of light.
• $\mu$ [1/mm]: Fringe frequency.

Source:

Diffraction Gratings Tutorial