First, I consider all my customers. I don’t rush a project if it will cause problems with other customers.
Usually rushing a project causes me to reschedule my workload, and to keep all my projects on schedule, I may have to work overtime. I charge extra for rush projects to make those projects pay for the overtime, even if the rush work isn’t done during overtime hours. If it’s a reasonable rush, say accelerating my work by two hours to meet a FedEx pickup time, and I can get all the other work done that day by staying an hour or two late, then I’d charge one and a half times my normal rate for the work to be rushed. If say I only rushed 2 hours of work on an 80 hour project, I’d only bill those two hours at the rush rate.
If the rush is rather unreasonable, say the customer wants a 4 week project done in 2, and I had to work extra hours each day, and all weekend to get the work done, I’d charge double time for the whole project.
Other considerations may also come into play. I may be trying to compete with other companies in my area, so I may be willing to do that 4 week job in 2 weeks at the regular rate. Or, instead of doing the 4 week job in 2 weeks, the customer and I agree to cut lots of corners, and make the 4 week job a 2 week job done in 2 weeks. Then I should just charge the regular rate.
Or maybe I’m willing to risk losing the customer, and just answer that the 4 week project will take 4 weeks.
Eric Berna