I have read in several places that a "professional" carpet cleaners will always charge by the square foot and not by the room. That is WRONG, and here is why. After searching out common pricing structures when I first started my own business, I decided to go with a per sq. ft. charge. I ran into several jobs where the customer could not measure their house properly. this created a couple of problems, one being that I didn't provision enough time on my calendar to clean the jobs, and the other is that people thought that the bill was going to be much higher or lower then it really was. Even after getting the measurements right I had customers who were bad at math, instead of taking the total sq. ft. (lets say 500 sq ft) and multiplying it by 0.18 (what I was charging as a start-up company) that would give you 500 * .18 = $90 they wanted to take the sq ft of carpet and divide it by 18??? 500 / 18 = $27.77 This let to confusion and upset customers. The real way to make pricing by the sq ft work it to make an extra trip just to do the quote. This works great for large commercial jobs because they usually need multiple quotes, and the larger area makes up for the extra cost of driving just to quote, but not for your everyday residential job.
Pricing by the room! Wow what a difference this has made for my business. Easy to explain to customers and employees. I am sure there is more then one way to do this, but for me a room includes a closet and any size room, I don't count hallways, and I price the stairs separately. A few simple questions and I can tell you exactly how much your bill will be right over the phone. Maybe a few times rooms were bigger then I expected, but I have more then made up for it from the savings on not wasting time or gas on quotes.
I am a professional carpet cleaner, and I choose to price by room.