How to Calculate How Much Concrete You Need

The Volume Formula

The single most common error is depth. Slab thickness is quoted in millimetres or inches while length and width are in metres or feet, and multiplying 5 × 4 × 100 gives you a number two thousand times too large. Convert first: 100 mm is 0.1 m, and 4 inches is 0.333 ft.

Worked Example: A Slab

A patio slab 5 m long, 4 m wide, 100 mm thick.

• Convert the depth: 100 mm = 0.1 m
• Volume = 5 × 4 × 0.1 = 2.0 cubic metres
• Add 10% for waste: 2.0 × 1.1 = 2.2 cubic metres to order

If you are buying ready-mix, that is the number you give the supplier. If you are mixing on site, it is the number that feeds the materials calculation below.

Working in Cubic Yards

Ready-mix concrete is sold by the cubic yard across North America, and cubic yards are an awkward unit because a yard is three feet in each direction, making one cubic yard 27 cubic feet.

The same slab in imperial: 16.4 ft × 13.1 ft × 0.333 ft = 71.5 cubic feet, divided by 27 gives 2.65 cubic yards, plus 10% is about 2.9.

Converting directly between the two systems, one cubic metre is roughly 1.31 cubic yards. Our 2.2 cubic metres comes out at 2.88 cubic yards, which matches the imperial route closely enough for ordering.

Why You Add 10%

The theoretical volume assumes a perfectly level excavation, no spillage, and formwork that does not bulge. None of those hold on a real site.

Sub-base is rarely perfectly flat, so low spots quietly absorb extra volume. Some concrete is lost in the barrow, the chute, and on the ground. Formwork flexes slightly under the weight of wet concrete. And edges often need a little more than the nominal depth.

Ten percent covers ordinary conditions. Increase it toward 15% for a rough or uneven base, for small awkward pours where proportionally more gets wasted, or for footings dug by hand where the trench walls are irregular.

The asymmetry is what justifies the extra cost. Surplus concrete is a nuisance you dispose of. Running out mid-pour creates a cold joint, a plane of weakness where fresh concrete meets partly set concrete, and on a structural element that is a genuine defect rather than a cosmetic one.

What the Mix Ratios Mean

A mix ratio such as 1:2:4 gives the proportions of cement, sand, and coarse aggregate by volume. One part cement, two parts sand, four parts stone.

The M number is the characteristic compressive strength in newtons per square millimetre at 28 days. More cement in the ratio means more strength and more cost.

These nominal mixes are appropriate for domestic and light work. Anything structural, anything carrying significant load, and anything subject to building control should use a designed mix specified by an engineer rather than a rule of thumb from an article.

Turning Volume Into Bags

Dry ingredients occupy more space than the finished concrete, because water fills the voids between particles during mixing. The standard adjustment is a dry volume factor of about 1.54.

Method

1. Multiply your wet volume by 1.54 to get dry volume.
2. Add the parts of your ratio together to get the total number of parts.
3. Divide dry volume by total parts to get the volume of one part.
4. Multiply by each ratio number for cement, sand, and aggregate.
5. Convert cement volume to bags using a density of about 1,440 kg per cubic metre and a 50 kg bag.

Applied to our 2.2 cubic metre slab at M20 (1:1.5:3)

• Dry volume = 2.2 × 1.54 = 3.39 m³
• Total parts = 1 + 1.5 + 3 = 5.5
• One part = 3.39 ÷ 5.5 = 0.616 m³
• Cement = 0.616 m³ × 1,440 = 887 kg, which is about 18 bags of 50 kg
• Sand = 0.616 × 1.5 = 0.92 m³
• Aggregate = 0.616 × 3 = 1.85 m³

As a sanity check, an M20 mix works out at roughly 8 bags of cement per cubic metre, and 2.2 × 8 is 17.6, which agrees with the 18 above. A 1:2:4 mix needs closer to 6.5 bags per cubic metre.

Footings, Columns, and Odd Shapes

Strip footings are just long thin slabs: length × width × depth, calculated per run and added together. Remember to count the full perimeter, including internal walls.

Square or rectangular columns are the same formula with height as the third dimension. Circular columns and pier holes need the circle area first: π × radius² × height. A 300 mm diameter pier 1.2 m deep is π × 0.15² × 1.2 = 0.085 m³, so a dozen of them come to just over a cubic metre.

For anything L-shaped or irregular, split it into rectangles, calculate each separately, and add. Trying to average an odd shape into a single rectangle is where quantities go wrong.

Common Mistakes

1. Mixing units, most often leaving thickness in millimetres or inches while length and width are in metres or feet.
2. Forgetting the waste allowance, which is the difference between a completed pour and a cold joint.
3. Dividing cubic feet by 9 instead of 27 when converting to cubic yards.
4. Using wet volume in the materials calculation without applying the 1.54 dry factor, which underestimates every ingredient.
5. Ignoring the sub-base. If it is uneven or has settled, actual volume can exceed the calculation by more than your allowance.
6. Adding extra water on site to make the mix easier to place. It improves workability and reduces final strength, sometimes substantially.

Frequently Asked Questions

How much concrete do I need for a slab?

Multiply length by width by thickness with all three in the same unit, then add about 10%. A 5 m by 4 m slab at 100 mm thick is 2.0 cubic metres, so order 2.2. Convert thickness to metres before multiplying.

How many bags of cement are in a cubic metre?

It depends on the mix. An M20 mix at 1:1.5:3 needs roughly 8 bags of 50 kg per cubic metre, while a leaner 1:2:4 mix needs closer to 6.5. Multiply by your total volume including the waste allowance.

What does the 1.54 factor do?

It converts finished wet concrete volume into the larger volume of loose dry materials you need to buy. Dry cement, sand, and aggregate contain air gaps that fill with water and paste during mixing, so the ingredients always occupy more space than the result.

How thick should a concrete slab be?

Around 100 mm, or 4 inches, is typical for domestic patios, paths, and shed bases. Driveways carrying vehicles usually need 150 mm or more, often with reinforcement. Anything structural should follow an engineer’s specification rather than a general figure.

Is ready-mix cheaper than mixing on site?

For anything above roughly one cubic metre, ready-mix is usually cheaper once you count bag prices, hire, labour, and time, and it delivers far more consistent quality. Hand mixing makes sense for small pours where a delivery minimum charge would dominate the cost.

How much does a cubic metre of concrete weigh?

Roughly 2,400 kg for normal-weight concrete. This matters when planning access, since a small 2 cubic metre pour is close to five tonnes of material arriving, and formwork and any supporting structure need to carry it while wet.

Can I pour concrete in cold or hot weather?

Both create problems. Near freezing, hydration slows sharply and fresh concrete can be damaged by frost. In high heat, it stiffens quickly, becomes hard to finish, and can crack from rapid moisture loss. Suppliers offer admixtures for both, and shading, covering, and timing the pour for cooler hours all help.

The Bottom Line

Convert every dimension to the same unit, multiply length by width by depth, then add 10% before you order. For mixing on site, multiply the wet volume by 1.54, divide by the parts in your ratio, and convert the cement share into 50 kg bags at about 1,440 kg per cubic metre. Keep nominal mixes for domestic work and get a designed mix for anything structural. Run your dimensions through the concrete calculator, and when in doubt order the extra barrow, because a cold joint costs more than surplus concrete ever will.