Work out concrete bags required instantly with clear inputs, formula shown and shareable results.
Slab volume is length times width times thickness, with thickness converted from inches to feet. Bagged concrete states a yield in cubic feet per bag, so dividing the volume by that yield gives the bag count. A five percent allowance covers spillage, uneven subgrade and over-excavation.
Slab volume
ft³ = length x width x thickness in / 12
Bag count
Bags = volume x 1.05 / yield per bag
ft³ = length x width x thickness in / 12. Slab volume is length times width times thickness, with thickness converted from inches to feet.
Above about one cubic yard, roughly 45 eighty-pound bags, mixing on site becomes slower and less consistent than ordering a delivery.
This calculator takes 4 inputs: Slab length, Slab width, Slab thickness, Yield per bag. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.