Rule of thumb: add " to the thickness of your slab for your slab concrete budget. This assumes you have the job evenly graded to the right depth, and the grade is well compacted. If you check your grade and one spot is 4", some spots are 4.5" to 5"-the best solution for both the job quality and your concrete budget is to fix the grade. If you don't fix it, you have to come up with the average depth-them add your " to that thickness. Odd shapes: Transform odd shapes into rectangles and odd shapes are suddenly easy to figure.