How Much Concrete Do I Need? Cubic Metres, Bags and Ready-Mix

The formula

Volume (m³) = length × width × depth, with every measurement in metres.

The most common mistake is entering depth in centimetres. A 10 cm slab is 0.10 in the formula, not 10.

Thai builders and ready-mix suppliers order concrete in cubic metres, and they usually call one cubic metre a "kew" (from "cubic"). One kew is 1,000 litres.

How far one cubic metre goes

These are net volumes with no allowance for waste. The table tells you how much concrete a given thickness needs, not what thickness your job needs. A slab poured 12 cm deep instead of 10 cm uses 20% more concrete, so measure the depth at several points after digging.

Example: a 4 × 3 m patio, 10 cm thick

Volume = 4 × 3 × 0.10 = 1.2 m³ (1,200 L)

Add 10% → 1,320 L

Bagged mix: if the bag label says one bag makes 17 L of mixed concrete, you need 1,320 ÷ 17 = 77.65, so 78 bags.

Ready-mix: order the 1.32 m³ figure and ask the plant about its minimum order, delivery charge and whether you'll need a pump truck.

The 17 L figure comes from one product’s data sheet. A bag’s dry weight does not tell you how many litres of mixed concrete it yields; use the wet yield on that product’s label.

Other shapes

Post holes and footings: width × length × depth. Ten fence-post holes of 0.3 × 0.3 × 0.6 m hold 54 L each, so 540 L in total. Adding 10% gives 594 L, which is 35 bags at 17 L per bag.

Round columns: 3.1416 × radius² × height.

Slabs with thickened edges or several depths: calculate each part separately and add them together.

Bagged mix or ready-mix?

Bagged mix suits small or awkward jobs: fence posts, equipment pads, repairs, and anywhere a truck can't reach. Once a job needs dozens of bags, these become as important as the price:

Continuous pour: plan the work and crew so a pause does not create an unplanned cold joint across the slab.

Mixing labour: you need enough people to mix as fast as the concrete is poured.

Water and tools: have enough on site before you start.

Carrying: each bag is heavy, and they all have to reach the pour site.

For patio-sized jobs, ready-mix is often the better deal once labour is counted. Get prices for both.

What volume can't tell you

The calculator gives you the amount of concrete only. It doesn't decide:

concrete strength (Thai orders usually state it in ksc)

a safe thickness for the load

reinforcement

joints

how the ground underneath is compacted

Slabs that carry vehicles, footings, and anything connected to the house structure need a design checked by an engineer.

FAQ

Is a "kew" the same as a cubic metre? Yes. 1 kew = 1 m³ = 1,000 litres.

How much extra should I order? The calculator defaults to 10%. Order more if the ground is uneven or the formwork isn't square.

Can I enter centimetres? No. The tool uses metres, so a 10 cm depth is 0.10.