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Generated on: July 20, 2026 at 09:33 PM

Concrete Bags Calculator

Inputs

Length3 m
Width3 m
Thickness10 cm
Bag size80 lb (0.60 cu ft / 0.0170 m³)
Waste allowance10 %

Bag counts use the slab dimensions you enter and typical published yields, which vary by brand and by how wet the mix is. Treat the result as a planning estimate and check the yield printed on the bag before you buy.

Generated with OneCalc · https://onecalc.app/en/lifestyle/concrete-bags

Lifestyle

Concrete Bags Calculator

Work out how many bags of pre-mixed concrete a slab needs from its length, width, and thickness, with a waste allowance for spillage and setting loss. Supports 40, 60, and 80 lb bags with their standard wet-mix yields.

Inputs

≥ 0 m
≥ 0 m
≥ 0.01 cm
%
0 – 50 %

Results

Enter a value to see results.

Details

Bag Breakdown

\begin{aligned} V &= L \times W \times T \\ &= ? \end{aligned}
\begin{aligned} N &= \left\lceil \frac{V}{y} \times (1 + k) \right\rceil \\ &= ? \end{aligned}

A 3 m × 3 m slab at 10 cm thick holds ... of concrete and needs about ... bag(s).

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Concrete Bags

Pre-mixed concrete comes in bags that you add water to and mix on site. Estimating how many bags a slab needs comes down to two steps: work out the volume of the pour, then divide by how much wet concrete a single bag makes. A waste allowance on top keeps the pour from running short.

The bag-count formula

First find the volume of the slab by multiplying its three dimensions:

V=L×W×TV = L \times W \times T

Then divide the volume by the yield of one bag, add a waste allowance kk, and round up, because a partial bag still means buying a whole one:

N=⌈Vy×(1+k)⌉N = \left\lceil \frac{V}{y} \times (1 + k) \right\rceil

The yield yy depends on the bag weight. Typical published figures for wet-mix concrete are:

  • 40 lb bag: about 0.30 cu ft (0.0085 m³)
  • 60 lb bag: about 0.45 cu ft (0.0127 m³)
  • 80 lb bag: about 0.60 cu ft (0.0170 m³)

Worked example

Suppose you are pouring a patio slab 10 ft long, 10 ft wide, and 4 inches thick3 m long, 3 m wide, and 10 cm thick, using 80 lb bags and a 10% waste allowance.

Start with the volume. Four inches is one third of a foot:

V=10×10×412=33.33 cu ft\begin{aligned} V &= 10 \times 10 \times \tfrac{4}{12} = 33.33 \text{ cu ft} \end{aligned}

Each 80 lb bag yields about 0.60 cu ft, so:

N=⌈33.330.60×1.10⌉=⌈61.1⌉=62 bags\begin{aligned} N &= \left\lceil \frac{33.33}{0.60} \times 1.10 \right\rceil = \left\lceil 61.1 \right\rceil = 62 \text{ bags} \end{aligned}

Start with the volume in cubic metres:

V=3×3×0.10=0.90 m3\begin{aligned} V &= 3 \times 3 \times 0.10 = 0.90 \text{ m}^3 \end{aligned}

Each 80 lb bag yields about 0.017 m³, so:

N=⌈0.900.017×1.10⌉=⌈58.2⌉=59 bags\begin{aligned} N &= \left\lceil \frac{0.90}{0.017} \times 1.10 \right\rceil = \left\lceil 58.2 \right\rceil = 59 \text{ bags} \end{aligned}

Rounding up ensures there is enough mix to finish in one continuous pour.

Choosing a bag size

Larger bags cost less per unit of volume and mean fewer bags to lift and empty, so 80 lb bags suit larger pours such as a slab, footing, or driveway section. Smaller 40 lb bags are lighter and easier to handle, which makes them a better fit for post holes, small repairs, or mixing by hand without a mixer.

Whichever weight you choose, plan to mix the whole pour in one session. Concrete that sets between batches does not bond well, so buy a little extra rather than stopping partway through.

How much waste to allow

A 10% allowance covers the everyday losses: mix left in the wheelbarrow or mixer, spillage during placement, and a subgrade that ends up deeper than planned in places.

A small, well-formed pour on level ground may need less. A rough excavation, a sloping site, or a shape with many edges and corners can lose more, so allowing 15% or higher in those cases avoids a trip back to the store mid-pour.

Beyond the slab

The volume method here assumes a simple rectangular pour. For an irregular area, split it into rectangles, estimate each one, and add the results. For thicker structural work you may also need reinforcement and a compacted base, which the bag count does not include.

For other material-quantity estimates, see the related calculators below, including the Paver Patio Calculator for surfacing an area with pavers.

Frequently Asked Questions (FAQ)

How many bags of concrete do I need?

Start with the volume of the pour: length × width × thickness. Then divide that volume by the yield of a single bag and round up, because a partial bag still means buying a whole one.

A standard 80 lb bag makes roughly 0.6 cubic feet (0.017 m³) of set concrete, a 60 lb bag about 0.45 cubic feet, and a 40 lb bag about 0.3 cubic feet. Adding a waste allowance on top covers spillage and uneven ground so the pour does not run short.

Which bag size should I buy?

Larger bags cost less per unit of volume and mean fewer bags to carry and mix, so 80 lb bags suit bigger pours like a slab or footing. Smaller 40 lb bags are easier to lift and handle and make sense for small patch jobs, post holes, or when mixing by hand.

Whichever weight you pick, mixing a full pour in one continuous session gives the most consistent result, so buy a little extra rather than stopping partway to fetch more.

How much waste should I allow for?

A waste allowance of around 10% covers the usual losses: concrete stuck in the mixer or wheelbarrow, spillage, and subgrade that is deeper than planned in spots.

For a small, well-formed pour on level ground you might use less; for a rough excavation or an awkward shape with many edges, allowing 15% or more helps avoid a mid-pour trip back to the store.

Disclaimer

Bag counts use the slab dimensions you enter and typical published yields, which vary by brand and by how wet the mix is. Treat the result as a planning estimate and check the yield printed on the bag before you buy.

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