Applying For Mortgage With RSU

My fiancé and I are looking at purchasing a new home and I’m trying to understand how RSU income can be counted for the mortgage process. I work for a company where I have a base salary, bonus and then a significant amount that comes as annual RSU grants.

The RSU grants vest 3 years after grant. I’ve been at this company for 3 years now so I have 3 grants already received. And my first grant vests in January.

Is there a certain formula used to include the the RSUs for qualification?