How to use this calculator
- Choose the shape: slab, footing, column or steps.
- Enter the length, width and depth (thickness) in the units shown.
- Set a waste allowance; 5–10% is typical.
- Read the volume in cubic yards and the number of 40, 50, 60 or 80 lb bags.
How it works
Volume (cu ft) = length × width × thickness ÷ 12. Divide by 27 for cubic yards, or by the bag yield for bags. Add 5–10% for uneven subgrade and spillage.
Assumes standard concrete mix bag yields: 80 lb ≈ 0.60 cu ft, 60 lb ≈ 0.45 cu ft. Check the yield on your bag. 1 cubic yard = 27 cubic feet.
Common slab sizes
- 4×4 slab0.20 yd³ at 4″
- 4×6 slab0.30 yd³ at 4″
- 4×8 slab0.40 yd³ at 4″
- 5×5 slab0.31 yd³ at 4″
- 6×6 slab0.44 yd³ at 4″
- 6×8 slab0.59 yd³ at 4″
- 8×8 slab0.79 yd³ at 4″
- 8×10 slab0.99 yd³ at 4″
- 10×10 slab1.23 yd³ at 4″
- 10×12 slab1.48 yd³ at 4″
- 10×16 slab1.98 yd³ at 4″
- 10×20 slab2.47 yd³ at 4″
- 12×12 slab1.78 yd³ at 4″
- 12×16 slab2.37 yd³ at 4″
- 12×20 slab2.96 yd³ at 4″
- 12×24 slab3.56 yd³ at 4″
- 14×14 slab2.42 yd³ at 4″
- 16×16 slab3.16 yd³ at 4″
- 16×20 slab3.95 yd³ at 4″
- 18×18 slab4.00 yd³ at 4″
- 20×20 slab4.94 yd³ at 4″
- 20×24 slab5.93 yd³ at 4″
- 20×30 slab7.41 yd³ at 4″
- 24×24 slab7.11 yd³ at 4″
- 24×30 slab8.89 yd³ at 4″
- 30×30 slab11.11 yd³ at 4″
- 30×40 slab14.81 yd³ at 4″
Frequently asked questions
How many 80 lb bags make a yard?
About 45 (27 ÷ 0.60).
When should I order ready-mix?
Many contractors switch from bags to a truck at around 1 cubic yard or more; check minimum-order fees locally.