Well read the regulation:
https://www.bradfordtaxinstitute.com/Endnotes/Prop_Reg_1_280A-1e6.pdf
Pub 527 also has an example:
Example.
Your beach cottage was available for rent from June 1 through August 31 (92 days). Except for the first week in August (7 days), when you were unable to find a renter, you rented the cottage at a fair rental price during that time. The person who rented the cottage for July allowed you to use it over the weekend (2 days) without any reduction in or refund of rent. Your family also used the cottage during the last 2 weeks of May (14 days). The cottage wasn’t used at all before May 17 or after August 31.
You figure the part of the cottage expenses to treat as rental expenses as follows.
The cottage was used for rental a total of 85 days (92 − 7). The days it was available for rent but not rented (7 days) aren’t days of rental use. The July weekend (2 days) you used it is rental use because you received a fair rental price for the weekend.
You used the cottage for personal purposes for 14 days (the last 2 weeks in May).
The total use of the cottage was 99 days (14 days personal use + 85 days rental use).
Your rental expenses are 85/99 (86%) of the cottage expenses.
Note.
When determining whether you used the cottage as a home, the July weekend (2 days) you used it is considered personal use even though you received a fair rental price for the weekend. Therefore, you had 16 days of personal use and 83 days of rental use for this purpose. Because you used the cottage for personal purposes more than 14 days and more than 10% of the days of rental use (8 days), you used it as a home. If you have a net loss, you may not be able to deduct all of the rental expenses.
