Looking For Help

Hello everyone, I'm thinking about starting a carpet cleaning business.  I have a question though, I am not really good at math, so I was wondering what is the easiest way to measure a room to that you get what you'd charge per square foot and still make money without it being obvious to a customer that you struggle in this area?  I want to be successful and I know what's needed to be done, but measurements through me off and always get me confused.  Can I measure it a certain way or get some sort of calculator?  Any advice or help is appreciated.