OTHER · CONCRETE
Concrete Calculator
Calculate cubic yards and how many bags of concrete to buy for any slab or footing. Bags are always rounded up; bag yield is flagged as manufacturer-specific.
About This Calculator
Enter your slab dimensions (length, width, thickness) and choose a bag size. The calculator converts inches to feet, computes the volume in cubic feet and cubic yards, and rounds the bag count up so you never run short.
How It Works
Volume (ft³) = length × width × (thickness ÷ 12). Volume (yd³) = ft³ ÷ 27 (exact: 1 yd³ = 27 ft³). Bags = ⌈ft³ ÷ bag yield⌉. The 80-lb bag default uses 0.60 ft³ yield (Quikrete spec) — this is a manufacturer figure, not a physical constant, and varies by brand and mix.
The Formula
vol (ft³) = L × W × (thick_in ÷ 12) bags = ⌈vol ÷ yield⌉
- L, W
- slab length and width in feet
- thick_in
- slab thickness in inches
- yield
- cu ft per bag (0.60 for 80-lb, 0.45 for 60-lb)
Frequently Asked Questions
- Should I add extra for waste?
- For simple slabs, the rounded-up bag count usually provides a small buffer. For complex pours with forms, footings, or irregular shapes, add 5–10% extra and confirm with your supplier.
- How many 80-lb bags make a cubic yard?
- At 0.60 ft³ per bag, you need ⌈27 ÷ 0.60⌉ = 45 bags per cubic yard. At that quantity, ordering ready-mix concrete is often more economical.
- Does the bag yield vary by manufacturer?
- Yes — the 0.60 ft³ yield is based on Quikrete's 80-lb bag specification. Other brands may yield slightly more or less. Check your bag's label before finalizing your order.
Estimates only. Coverage figures (sq ft/gallon, bag yield, BTU/sq ft) are industry rules of thumb with real-world variance. Verify quantities with your supplier before purchasing.