How many bags of concrete do I need? (20 kg bags)

How it works

Choose the shape you are filling and enter its size. Enter lengths in metres and thicknesses, depths and diameters in millimetres. The result shows cubic metres.

The calculator works out the volume of concrete, adds any waste allowance you enter, then divides by the volume one 20 kg bag makes. It always rounds up to whole bags, because you cannot buy part of a bag.

Add a bag price and a ready-mix price if you want to compare costs. Ready-mix suppliers often charge a delivery or small-load fee, so enter that too. The calculator then shows the volume where ready-mix becomes the cheaper choice.

Formula

• Work out the concrete volume

  Slab or footing = length × width × thickness Post hole = (π × radius² − post area) × depth × hole count Round pier = π × radius² × height × pier count

  Use metres for the dimensions. The post is subtracted because it takes up space inside the hole.

• Convert volume into bags

  Bags = ceil(volume × (1 + waste %) ÷ bag yield) This product yield = 0.0084375 m³ per 20 kg bag

  The calculator rounds up because bags are sold whole.

• Compare bag mix with ready-mix

  Bag cost per m³ = bag price ÷ bag yield Switch volume = delivery fee ÷ ((1 + waste %) × (bag cost per m³ − ready-mix price per m³))

  A switch point is shown when ready-mix has the lower material rate. If bags cost less per m³, there is no switch point.

Worked example

1. 1

   Calculate the slab volume

   3 m × 3 m × 0.1 m = 0.9 m³

   A 3 m by 3 m slab that is 100 mm thick needs this much concrete before waste.

2. 2

   Round up to whole bags

   ceil(0.9 ÷ 0.0084375) = 107 bags

   Using the maker's yield, buy 107 × 20 kg bags with no waste allowance.

3. 3

   Add 10% waste

   0.9 m³ × 1.10 = 0.99 m³ → 118 bags

   Enter a waste allowance that suits your formwork and site conditions.

Assumptions and limits

The maker's sheet says "2 x 20kg bags cover a 450x500mm slab approximately (75mm thickness)." That is 0.45 x 0.5 x 0.075 = 0.016875 m³ for 2 bags, or 0.0084375 m³ per bag (about 118.5 bags per m³). The sheet gives this as an approximate figure.

Yields differ between products and with how much water you add. Check the yield printed on the bag you buy.

The waste allowance defaults to 0% because the maker sheets we checked do not publish one. Add your own allowance for uneven ground, spills and formwork that bows.

Post holes assume a square post standing on the bottom of the hole, so the post takes up space over the full depth. Enter 0 for the post width if you fill the hole before setting the post.

Costs use your prices. Bag cost uses whole bags. The switch point uses the cost per m³ without rounding, so the real change-over can be a bag or so either side. Ready-mix suppliers may also have minimum loads, which this calculator does not include.

Frequently asked questions