I've always found estimating hours for a project to be somewhat difficult, especially when the client wants a concrete number right at the beginning for the ent

I've always found estimating hours for a project to be somewhat difficult, especially when the client wants a concrete number right at the beginning for the entire project. I usually roughly estimate how long I think it will take, and then raise that by 1/3 to account for changes in specification by the client along the way.