Deflections and stresses in a circular elastic plate loaded by an air blast wave have been studied with analytical equations. In the derivation it is assumed that the blast wave is so weak that the deflection of the plate is of the same size as its thickness. The equations can possibly be valid for steel plates if no plastic deformation is allowed and for example for circular window glasses. It is possible to derive equations for rectangular plates also, which is in better agreement with the shape of window glasses. The calculated effective stress and deflection have been compared with a simulation in AUTODYN and the agreement is found to be acceptable. With the limited data at hand it is likely that the maximum effective stress can be determined within +/- 10%.