Calculate all your totals by multiplying the L x W x H then divide by (231) to get the total Gallons or divide by (1.805) to get the total Ounces. No matter how flat your slab is, factor in about 1/8th of an inch for epoxy seaping under your slab.
Pretty simple right? It really is after you get the hang of the process. This will get you pretty close to estimating how much epoxy will be needed. Remember to stir well and get down into the corners of the bucket. I personally like to let the epoxy settle in the bucket for 30 mins then go over it with my heat gun to minimize any bubbles before the pour.
Stay creative and enjoy the process!