I don't understand the question of fair rented days versus personal use days. I rented a room in the house in which I live for 10 months of the year.

[AnnetteB6]

When answering this question, you must consider only the portion of your house that is being rented.  It is its own separate rental entity.  Therefore you will only have personal use days for the rented room if you used that space during the time it was not rented. 

Given your question saying it was rented for 10 months, your personal use time cannot exceed 2 months.  The personal time would be even less if the room was converted to rental use during the year.  Any time that the room was advertised as available to rent but remained vacant is not considered personal use unless you did actually use the room during the vacant period.

[tegan-nicole]

I owned (and lived in) the house January - December 2020.  I rented out one room of the house January-October 2020.  My tenant/roommate used all of the household as did I (kitchen, living room, bathrooms, etc.), so I'm still unsure of how to divide days rented versus personal use. 

[AnnetteB6]

If the bedroom that was rented from January to October reverted back to personal use after the tenant moved out, then your personal use days will be the days in November and December.  If you kept the bedroom available as a rental by leaving it vacant and advertising it for rent during November and December, then your personal use days would be zero.

When you enter the square feet of the portion of your house that was a rental property, you will consider the square feet of the bedroom and half of the common area.  If you and your tenant shared all areas of the house (including the bedrooms) then half of the square feet would be considered the rental property and your personal use of the rental space would be the two months that you did not have a tenant.

@tegan-nicole

[Carl]

Here's the simple explanation.

-Percentage of your home rented out  that is "exclusive to the renter". Whatever that percentage may be.

- Percentage of business use will be 100%, because it's the business use percentage of that area that was "exclusive to the renter."

- Number of days rented - the day count typically starts on the first day a renter "could" have moved in, under the assumption that you did not use that specified rental space it for "ANY" type of personal use starting from the date you converted that space to a rental. The day count ends on the date you converted the space back to personal use. Of course, this doesn't apply if it continued to be a rental into 2021 and you did not use that space for "any" type of personal use.

- Percentage of business use - Assuming you did not use that space for any type of personal use, *WHILE IT WAS CLASSIFIED AS A RENTAL* your business use percentage is ONE HUNDRED PERCENT business use starting from the time that space was converted to a rental, and ending when (if) you converted it back to personal use. What you used that space for while it was *not* classified as rental space, just flat out does not count for anything.

- Number of personal use days - Assuming that you did not use that space for any type of personal use during the period of time it was classified as a rental, this will be ZERO. What you used it for while it was not classified as a rental just flat out does not count for anything.

Please see my response in the thread at https://ttlc.intuit.com/community/investments-and-rental-properties/discussion/re-if-i-am-renting-on... so that you will avoid an issue that "could" cause problems for you. *THIS* *IS* *IMPORTANT*!!!!