Looking for a concrete bag calculator for slab how many bags? This free tool gives you cubic yards needed and the number of 40, 60 or 80 lb bags to buy. Just enter your numbers and press Calculate — results are instant.
Concrete is ordered by volume: length times width times depth gives cubic feet, and dividing by 27 gives cubic yards, which is how ready-mix is sold. If you are mixing bags yourself, divide the cubic feet by what one bag yields — about 0.6 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag and 0.3 for a 40 lb bag — and round up, because you cannot buy part of a bag. The calculator also adds a 10% extra figure, which is the usual allowance for spillage and uneven ground.
Example: Length 10 ft · Width 10 ft · Depth 4 inches · 80 lb bags
Bags Needed (80 lb): 56
About 0.6 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag and 0.3 for a 40 lb bag. The calculator divides your volume by the bag you choose and rounds up.
For spillage, uneven ground, and formwork that is not perfectly square. Running short in the middle of a pour is far worse than having one bag left over.
For small jobs, bags are usually simpler. For anything over about a cubic yard, ready-mix delivered by truck is usually cheaper and gives a more consistent mix.