Abstract: For calculating microwave breakdown electric field in air simply and instantly, we give a new formula adopting characteristic diffusion length, which is combined with basic parameters in gas discharge and based on electron diffusion model. We fit the relationship between equivalent direct-current electric field and characteristic diffusion length from experiment recorded by A D Macdonald (1966), thus to avoid the influence of uncertainty of gas parameters on calculation accuracy. According to the definition of equivalent direct-current electric field, we give another formula to calculate breakdown electric field, which applies to a high air pressure. Also, in consideration of electric diffusion model and vacuum multipactor discharge based on secondary electron emission, we further give a formula applicable to a wide air pressure range by using a rational equivalent diffusion length. Compared with the formula by Yu Ming (2007), our results are more consistent with Macdonald's experimental value.