Here is one approach:
# Hair Salons = (# hours required for men's haircuts per year + # hours required for women's haircuts per year) / average service capacity of 1 hair salon (which would be: opening hours x average # hair dressers x utilization rate).
I usually like to estimate these from the demand side - the logic being that a sizing of demand is actually an estimate of the maximum efficient supply of hair salons that can be supported in the market. Estimating it from the supply side is also a great approach, but you leave yourself open to the risk that there is undersupply in the market.
High level illustration (you can add more segments to this)
Total service hours required for haircuts p.a.:
Total service capacity per hair salon
Therefore, total number of hair salons = 40m hours / 3600 hours = 11,000 hair salons
Sense check: 11,000 salons implies 1 salon every ~700 people in NYC (350 men, 350 women). Given men cut their hair 12 times p.a. and women cut their hair 4 times p.a., that implies 5600 visits per year, or around 15-20 clients per day, which feels reasonable.
Note again that demand side estimation assumes there is a maximum efficient supply in the market, which may not be case.
Hope that is helpful!
1. I would first clarify if its a unisex salon or not ? ( i have assumed its a men's salon here )
1. Calculate the average daily capacity of each salon : ( based on my experience men take 30 min for a haircut , i.e 2 haircuts by one barber in an hour , Midtown salons have 2-4 barbers /salon . average: 3 ; hence 3*2= 6 haircuts/hour)
2. Assume utilization rate / day : U can divide in weekday and weekend ( men usually go for haircuts on weekend 70 % utilized on weekend , 40% on weekday)
3. opening hours : 8 hours
4. Per day utilized capacity : Number of operating hours * utilization rate * capacity / hour
5 . per month = per day * 30
6. Assume frequency is once / month ( based on experience , but ill check with interviewer )
7. Per month capacity of each * number of salons = men population in NYC .