How much wastage should I add?

5% is standard for a straightforward slab on level ground. Use 10% for irregular forms, post holes or sloping sites where spillage and over-excavation are more likely. The calculator adds 5% by default.

How many bags does a cubic metre equal?

About 60 bags of 40 kg premix, since each bag yields roughly 0.0166 m³ of finished concrete. For a cubic yard, roughly 45 bags. Order a few extra rather than running short mid-pour.

Should I mix by hand or order ready-mix?

For anything over about 1–1.5 m³, ready-mix is cheaper and far less labour. Bagged premix suits small repairs, post holes and slabs under a cubic metre. The calculator shows both so you can judge.

How many bags is one cubic metre?

About 60 bags of 40 kg premix, since each yields roughly 0.0166 m³. For a cubic yard, about 45 bags. For a slab over a cubic metre, ready-mix is usually cheaper and far less effort than mixing dozens of bags by hand.

How do I calculate post-hole concrete?

A post hole is a cylinder: volume = π × radius² × depth. For a 300 mm diameter hole 600 mm deep, volume = 3.1416 × 0.15² × 0.6 = 0.042 m³ per hole. Multiply by the number of holes and use this calculator’s bag count for the total.

Cooking & DIY/Home Calculators

Concrete Volume Calculator

Ordering or mixing concrete means knowing the volume of the void you are filling, plus a margin for spillage and an uneven base. This calculator takes the length, width and depth of a slab, footing or post hole and returns the volume in cubic metres, cubic yards and the number of premix bags you would need to mix it yourself.

Enter the dimensions and pick the unit (metres or feet). The calculator works out the volume, adds a 5% wastage margin, and converts it to cubic yards for US suppliers and to standard 40 kg premix bags (each yielding about 0.0166 m³) for hand-mixing. For large pours, order ready-mix by the cubic metre or yard instead.

It is the calculation that gets a pour right the first time — the volume of a slab, footing or post hole in cubic metres and yards, with a wastage margin and a bag count, so you order ready-mix or buy premix with confidence.

Length

Width

Depth

Unit

Premix bag (kg)

Enter your values to see the result.

How this is calculated

The volume is length × width × depth, with all three dimensions converted to metres first (feet × 0.3048). A 5% wastage margin is added for spillage, over-excavation and an uneven base — standard for a straightforward slab on level ground. The buffered volume is shown in cubic metres and converted to cubic yards by dividing by 0.764555, the cubic-metres-per-cubic-yard factor, for US ready-mix suppliers who sell by the yard.

For hand-mixing, the volume is converted to premix bags. A 40 kg bag of premix yields about 0.0166 m³ of finished concrete, and the bag yield scales with bag size (a 30 kg bag yields about 0.0125 m³, an 80 lb bag about 0.0166 m³). The bag count is the buffered volume ÷ bag yield, rounded up to a whole bag. For anything over about 1–1.5 m³, ready-mix delivered by truck is cheaper and far less labour than bag-mixing.

The model assumes a simple rectangular prism (a slab, footing or trench). Cylindrical post holes need a different area (π × radius² × depth); for those, calculate the hole volume separately and sum. The wastage margin is editable in effect through the inputs — for irregular forms or sloping sites, increase the depth or add a margin. Concrete strength and slump also affect yield slightly, so order a few extra bags or half a cubic metre rather than running short mid-pour.

Worked example

A 3 m × 2 m slab, 100 mm (0.1 m) thick, 40 kg bags.

1Volume = 3 × 2 × 0.1 = 0.6 m³.
2With 5% waste = 0.63 m³.
3In cubic yards = 0.63 ÷ 0.764555 = 0.82 yd³.
4Bag yield = 0.0166 m³; bags = ceil(0.63 ÷ 0.0166) = 38 bags.

Frequently asked questions

How much wastage should I add?: 5% is standard for a straightforward slab on level ground. Use 10% for irregular forms, post holes or sloping sites where spillage and over-excavation are more likely. The calculator adds 5% by default.
How many bags does a cubic metre equal?: About 60 bags of 40 kg premix, since each bag yields roughly 0.0166 m³ of finished concrete. For a cubic yard, roughly 45 bags. Order a few extra rather than running short mid-pour.
Should I mix by hand or order ready-mix?: For anything over about 1–1.5 m³, ready-mix is cheaper and far less labour. Bagged premix suits small repairs, post holes and slabs under a cubic metre. The calculator shows both so you can judge.
How many bags is one cubic metre?: About 60 bags of 40 kg premix, since each yields roughly 0.0166 m³. For a cubic yard, about 45 bags. For a slab over a cubic metre, ready-mix is usually cheaper and far less effort than mixing dozens of bags by hand.
How do I calculate post-hole concrete?: A post hole is a cylinder: volume = π × radius² × depth. For a 300 mm diameter hole 600 mm deep, volume = 3.1416 × 0.15² × 0.6 = 0.042 m³ per hole. Multiply by the number of holes and use this calculator’s bag count for the total.

Related calculators

Method: volume = length × width × depth (converted to metres), +5% wastage; cubic yards = m³ ÷ 0.764555; 40 kg bag yield ≈ 0.0166 m³. Last updated: September 2026.

These results are general estimates from established formulas and conversion factors. Paint coverage, tile yields, concrete volumes, fuel economy, oven behaviour and clothing and shoe sizes vary by product, surface, vehicle, oven and brand. Always confirm with your supplier, the product label or the brand’s own size chart before buying or acting on a calculation.