Concrete Volume & Quantity
Finds the volume of concrete for a rectangular slab or footing, or for a round column, pier or post hole. It gives the volume in cubic meters and cubic yards, the units concrete is ordered in, and the number of 80 lb bags of premixed concrete for a pour small enough to mix by hand. Contractors and homeowners use it to order ready-mix or to buy bags for a pad, a footing or fence posts.
Equations
Slab or footing: V = L·W·t
Round column: V = (π/4)·D²·h
Bags = ⌈V / 0.017 m³⌉
L and W are the length and width of the slab, t its thickness, D the column diameter and h its height; the page takes t and h from the same Thickness / Height field. Each 80 lb bag of premixed concrete yields about 0.6 ft³, or 0.017 m³, of placed concrete [2]. The bag count is rounded up to whole bags. One cubic yard is 27 ft³, or 0.7646 m³.
A rectangular column or wall is a slab stood on end: enter its two plan dimensions as Length and Width and its height as Thickness / Height. Each field has its own unit, so you can mix feet for the plan and inches for the thickness.
Limits and common mistakes
- The volume is the neat volume of the shape. Add about 5 to 10 % for spillage, uneven subgrade and formwork that bows out.
- Enter the thickness in its own unit. A 4 in slab entered as 4 ft orders twelve times too much concrete.
- For several identical footings or holes, multiply one result by their number; round the bags up after multiplying, not before.
- Bags suit pours of up to about a cubic yard. Beyond that, ready-mix is usually cheaper and gives a more uniform pour.
- The bag count is for 80 lb bags only. A 60 lb bag yields about three quarters as much.
Worked example
A garden shed needs a concrete pad 8 ft by 6 ft and 4 in thick. How many bags should you buy?
- Choose Shape Slab / Footing (rectangular).
- Set the unit of Length to ft and enter 8. Set the unit of Width to ft and enter 6.
- Set the unit of Thickness / Height to in, enter 4 and press Solve. The Solution shows:
Volume: 0.45307 m³
Volume: 0.592593 yd³
Estimated 80 lb Bags Required: 27
Check by hand. 8 × 6 × 4/12 = 16 ft³, and 16 / 27 = 0.593 yd³. At 0.6 ft³ a bag, 16 / 0.6 = 26.7, so 27 bags. With a 10 % allowance for waste, buy 30.