What are the parameters for exhaust fan sizing? When you have a walk-in closet with no door, that adjoins the bath, is it necessary to consider it’s volume also?
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Code-check states that the minimum vent requirement for a BR is a 1.5 sq. ft. operable window, or a 50 cfm fan to outside. It also states that you should consider two rooms one if the opening between them is is a minimum of 50% of the common wall between the two rooms AND more than 10% floor area AND at least 25 sq. ft.
In english, I think they're trying to say that if the opening on the common wall is at least half of the sq footage of the wall, and if the adjoining room floor area is 10% of the BR and at least 25 sq ft, then the two rooms are one. So you need to measure the door opening and common wall, as well as the floor area of both spaces.
Personally, if the walk-in closet is not a walk-through closet, but just dead air space, I wouldn't worry about it. You're not likely to get much air exchange from a dead air space. But that's just my 2 cents.
If you want, buy a larger fan. Check the fan packaging, it usually tells you what size rooms they are good for.