Calculate how much concrete you need for a slab, patio, shed base or pad. Enter the size and thickness to get the volume in cubic yards and the number of premix bags.
Last updated: October 2, 2026
Bag counts use typical yields of about 0.60, 0.45 and 0.30 cubic feet per 80, 60 and 40 lb bag. Check the yield printed on the bag you buy.
How the calculation works
cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
cubic yards = cubic feet ÷ 27Ready-mix concrete is ordered by the cubic yard. For small jobs, premixed bags are easier: you mix them with water on site.
Bags or a truck?
Bags make sense for small pads, post footings and repairs. Once a job needs dozens of 80 lb bags, mixing by hand becomes slow and tiring, and it’s hard to pour the whole slab before the first batch starts to set. For larger slabs, compare the cost of a ready-mix delivery, and ask about minimum order sizes.
Typical slab thickness
- Walkways and patios: about 4 inches.
- Shed bases: often 4 inches, more for heavy equipment.
- Driveways: commonly 4 to 6 inches, depending on vehicles and soil.
Local codes, soil conditions and loads can require different thicknesses, reinforcement or a compacted gravel base. Check local requirements for structural slabs and footings.