Calculateur de Béton

Combien de mètres cubes et de sacs il vous faut : dalles, semelles, poteaux et escaliers

How much concrete do you need?

Volume, bags and ready-mix loads - add as many sections as your pour has

Dernière mise à jour: septembre 7, 2026

Getting the Volume Right

Concrete is sold in cubic yards, poured in cubic feet, and bought in bags rated by weight. Three different units for the same substance is where most ordering mistakes start.

1 cubic yard

= 27 cubic feet, and weighs about 4,050 lb — just over two tons. This is the unit ready-mix is quoted and delivered in.

Bag yields

An 80 lb bag makes 0.60 cu ft, 60 lb makes 0.45, 40 lb makes 0.30. One cubic yard is about 45 bags of 80 lb.

A ready-mix truck

Carries roughly 8 to 10 cubic yards. Below about one yard you will usually pay a short-load fee on top.

The calculator does all three conversions at once, so you can compare "45 bags to mix by hand" against "one short load delivered" before deciding.

Why We Round Bag Counts Up

Take Quikrete's own example: a 10ft by 6ft slab at 4 inches thick. That is 10 × 6 × ⅓ = 20 cubic feet. Divide by the 0.60 cu ft an 80 lb bag yields and you get 33.33 bags.

Supplier calculators typically round that to 33. But 33 bags yield 19.8 cubic feet — you would be a fifth of a cubic foot short. This calculator says 34.

The reason is not pedantry. If you run out partway through a pour, the concrete already placed begins to set before the rest arrives, and the boundary between them becomes a cold joint: a permanent plane of weakness that cracks first and lets water in. It cannot be fixed afterwards without cutting the slab out.

So expect our bag count to be one higher than a supplier's tool. That is the intent, not a discrepancy. A spare bag costs a few dollars; a cold joint costs the slab.

The Shapes, and How Each Is Measured

Slab and footing

Length × width × thickness. Enter length and width in feet but thickness in inches, because that is how slabs are specified — nobody asks for a 0.333-foot slab. A footing is the same arithmetic; it is listed separately only because you will usually want both in one job.

Round slab and round column

π × radius² × depth. Enter the diameter, not the radius — measuring across a circle is easy, finding its centre is not. Sonotube columns are the common case here, and the tube's stated size is its diameter.

Stairs

Each step is a block as wide as the stair, as deep as the run, and as tall as all the steps beneath it stacked up. Summing that gives width × run × rise × n(n+1)/2. Four steps therefore take ten step-blocks of concrete, not four — which is why stairs consume far more than people expect.

How Thick, and Why It Matters So Much

Thickness scales cost almost linearly, so it is worth choosing deliberately rather than by habit:

  • 4 inches — patios, shed bases, footpaths, garden slabs. The usual residential minimum.
  • 5–6 inches — driveways and anything a car sits on.
  • 6 inches and up — RVs, trucks, workshop floors with machinery.

Going from 4 to 6 inches is 50% more concrete. On a 20ft × 20ft driveway that is the difference between about 5 and 7.4 cubic yards — a whole extra short load. Footing depth is a different question again, governed by local frost depth and building code rather than by load.

What Is Not in This Number

The figure above is concrete volume and what it weighs. A real pour also needs:

  • Sub-base — compacted gravel, typically 4 inches under a slab
  • Forms — timber, stakes and bracing, plus the labour to set them true
  • Reinforcement — rebar or mesh, plus chairs to hold it at mid-depth
  • Delivery and access — short-load fees, and pumping if the truck cannot reach
  • Finishing — screeding, floating, edging, jointing and curing, which is skilled and time-critical work

On a typical residential slab these frequently total more than the concrete itself. Treat the material cost here as one line in the budget, not the budget.

Comment nous calculons cela

Méthode
Volume by shape, converted to bags and cubic yards
Formule
Slab/footing = L x W x thickness. Round slab = pi x (d/2)^2 x thickness. Column = pi x (d/2)^2 x height. Stairs = width x run x rise x n(n+1)/2, since each step sits on those beneath it. Everything is computed in cubic feet, then cubic yards = cu ft / 27, cubic metres = cu ft x 0.0283168466, weight = cu ft x 150 lb. Bags = ceil(cu ft / bag yield).
Source
Bag yields are the published Quikrete and Sakrete figures: 80 lb yields 0.60 cu ft, 60 lb yields 0.45, 40 lb yields 0.30. Density of 150 lb per cubic foot is the standard figure for normal-weight concrete, giving about 4,050 lb per cubic yard.
Limites
Bag counts are rounded UP on purpose. A 10ft x 6ft x 4in slab is 20 cu ft, or 33.33 bags of 80 lb; supplier calculators round to nearest and quote 33, but 33 bags yield only 19.8 cu ft and running short mid-pour leaves a cold joint - a permanent weak seam. Expect our count to be one higher than a supplier tool. Excluded entirely: forms, rebar or mesh, sub-base gravel, delivery, short-load fees, pumping, finishing and labour, which on a residential slab often cost more than the concrete. Weight assumes normal-weight concrete; lightweight mixes differ.
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Foire Aux Questions

Divide the volume in cubic feet by the bag yield: an 80 lb bag of concrete mix yields about 0.60 cubic feet, a 60 lb bag 0.45, and a 40 lb bag 0.30. A 10ft by 6ft slab at 4 inches thick is 20 cubic feet, so 34 bags of 80 lb. This calculator always rounds up, because being one bag short in the middle of a pour leaves a cold joint - a permanent weak seam - and you cannot buy part of a bag. Supplier calculators often round to nearest and will quote one bag fewer.

Roughly one cubic yard is the crossover. Below that, bags are practical: one cubic yard is about 45 bags of 80 lb, which is already over 1.5 tons to carry and mix. Above it, ready-mix is cheaper per yard and vastly less work, and it arrives already consistent. Be aware that suppliers charge a short-load fee below about one cubic yard, so a small pour can cost close to what a bigger one would.

A cubic yard is 27 cubic feet, and normal-weight concrete runs about 150 pounds per cubic foot, so one cubic yard weighs roughly 4,050 pounds - just over two tons. A standard ready-mix truck carries about 8 to 10 cubic yards, which is why this calculator reports your total in truck loads as well.

Four inches is the usual minimum for a patio, shed base or footpath. Go to five or six inches for a driveway, and thicker still where heavy vehicles will park. Thickness drives cost almost linearly, so the difference between four and six inches is 50% more concrete - worth deciding deliberately rather than by default. Local building codes and frost depth govern footings, which are a separate question from slab thickness.

Excavation is never perfectly level, sub-base settles under the weight of the pour, and forms bow slightly. Between them these routinely swallow 5 to 10 percent more concrete than the arithmetic says. The allowance is not padding - it is the difference between ordering once and having a truck leave you short.

No. This is the volume of concrete and what it weighs. Forms, rebar or mesh, sub-base gravel, delivery, pumping, finishing and labour are all separate, and on a typical residential slab they frequently add up to more than the concrete itself.

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