A salon or spa has three different businesses inside it and they have nothing in common economically.
Revenue
Service revenue — the treatments themselves. Retail revenue — products sold to clients. Booth or station rental income — where stylists rent space rather than being employed. Membership or package revenue where offered.
Rental income is not service revenue and should never be mixed with it. It carries no delivery cost, so blending it inflates the apparent margin on the service side and hides whether the treatments themselves pay.
Cost of delivery
Stylist or therapist pay, whether wage, commission or a mix. Professional supplies consumed in treatments. Cost of retail goods sold, which is a genuine cost of sale and quite separate from professional supplies.
Keeping retail cost apart from back-bar supplies is what makes product margin answerable — and retail margin in this trade is usually far better than service margin, which is worth knowing.
Overhead
Premises, equipment, booking software, insurance, marketing, laundry and cleaning.
Commission complicates the margin
Where stylists are paid a percentage, the cost moves with revenue, which makes the service side behave like a variable-cost business. Where they are salaried, it does not. Most salons are a mix, and the books should make clear which is which — see fixed and variable costs.
What this setup gives you
Service margin and retail margin separately, and rental income visible as the distinct thing it is.