I rented a room from my second home for few months. The other room is occupied by my brother in law for which there is no rent. How to add up days rented vs personal use?

[DianeC958]

The number of days rented are based on when you made it available to rent, even if it was not rented, plus the days it was rented.

Since you are allowing your brother-in-law to live there and pay no rent, that is considered personal use of the property.

Topic No. 415 Renting Residential and Vacation Property

[Carl]

Rental Property Dates & Numbers That Matter.

Date of Conversion - If this was your primary residence or personal use property before, then this date is the day AFTER you moved out. In your case, then is the first day a renter "could" have moved in.
In Service Date - This is the date a renter "could" have moved in. Usually, this date is the day you put the FOR RENT sign in the front yard.
Number of days Rented - the day count for this starts from the first day a renter "could" have moved in. That should be your "in service" date if you were asked for that. Vacant periods between renters count also PROVIDED you did not live in the house for one single day during said period of vacancy.
Days of Personal Use - This number will be a big fat ZERO. Read the screen. It's asking for the number of days you lived in the property AFTER you converted it to a rental. I seriously doubt (though it is possible) that you lived in the house (or space, if renting a part of your home) as your primary residence or 2nd home, after you converted it to a rental.

Percentage of Property that was business use - In your case, this will be the percentage of square footage of the structure that was "EXCLUSIVE" to the renter.
Business Use Percentage. 100%. I'll put that in words so there's no doubt I didn't make a typo here. One Hundred Percent. After you converted this property or space to rental use, that percentage of space  was one hundred percent business use. What you used it for prior to the date of conversion doesn't count.