Article
How much concrete to order
Before you order concrete, calculate the cubic meters. Three dimensions, waste, and what you are not filling give a figure you can compare with a quote.
The concrete calculator does this. Next door: mortar.
The idea in brief
Concrete volume is length times width times thickness, in cubic meters.
A 10 cm thickness is 0.10 when the other sides are in meters. A 5% waste covers pumping losses and gaps in the forms, not a rounded figure chosen for looks.
On this page: m³ means cubic meters, the unit used to order concrete. cm means centimeters: with sides in meters, 10 cm is entered as 0.10 m. A 5% waste means you add 5 per 100 to the clean volume.
When this helps
| Situation | What the card shows |
|---|---|
| You are ordering concrete for a slab or a floor pour. | The calculator multiplies length, width, and thickness, then adds waste. |
| You are ordering concrete for a footing. | You use the same three sides. On a footing, waste is often closer to 10%. |
| You have several identical posts. | You enter one post and the count. The card multiplies that volume by the number of pieces. |
| The slab has a step or a thick trench that you are not filling. | You subtract what you are not filling, and only then add waste. |
A box, then waste
Volume is length times width times thickness, times how many identical pieces, times (1 + waste/100), minus cutouts. A 10 cm slab thickness is 0.10 m, not 10, or the result jumps two orders of magnitude.
A 5 by 4 m slab at 10 cm with 5% waste is 2.1 m³. A 10 by 0.4 by 0.5 m footing at 10% is 2.2 m³. Six 0.3×0.3×1 m posts at 5% are 0.567 m³. On a house slab 5% is often the floor, because formwork is not a lab.
What the card does not promise
This is not a mix class, not slump, and not a delivery window. If a slab has a thick service trench or a step, enter a deduction instead of pretending it is a full box. Residue in the pump and chute lives in the waste percent, not in a mystery extra meter.
A worked example
A 5 by 4 m slab
Thickness 10 cm, waste 5%.
- 5 × 4 × 0.10 = 2.0 m³ of slab.
- Plus 5%: 2.1 m³ to order.
Typical numbers from the card
| Inputs | Result |
|---|---|
| The slab is 5 by 4 m, 10 cm thick, and waste is 5%. | The order is 2.1 m³. |
| The footing is 10 by 0.4 by 0.5 m, and waste is 10%. | The order is 2.2 m³. |
| You have six 0.3 by 0.3 by 1 m posts, and waste is 5%. | The order is 0.567 m³. |
Typical mistakes
| Mistake | What goes wrong | How to avoid it |
|---|---|---|
| You type a 10 cm thickness as 10, even though the other sides are in meters. | The result comes out two orders of magnitude too large, as if the slab were 10 m thick. | When the sides are in meters, type the thickness as 0.10. |
| You order the exact volume and skip waste. | You run short of mix for the forms and for residue in the pump. | On this card, 5 to 10% waste is typical. |
| You count a full slab and do not subtract a void. | The volume comes out too high. | Enter what you are not filling as a deduction. |
| You try to count masonry mortar on this card. | Mortar is a different material from ready-mix concrete. | Count mortar on the separate mortar page. |
Before you order concrete
- Length, width, and thickness use the same units.
- If the sides are in meters, a 10 cm thickness is 0.10, not 10.
- Waste is entered. On this card it is usually 5 to 10%.
- You have deducted the voids you are not filling.
- For several identical pieces, you have entered the count.
- The card does not give a mix class and does not set a delivery time.
Frequently asked questions
How much concrete for a 5×4 m slab at 10 cm with 5%?
2.1 m³. 5 × 4 × 0.10 × 1.05.
Do I type 10 cm as 10?
Not if the other sides are in meters. 10 cm is 0.10 m. Entering 10 makes the result two orders of magnitude too large.
Why add waste?
Forms retain some mix, the base settles, and the pump leaves residue. 5 to 10% is a typical range on this card, not a rulebook.
How do I count several posts?
Enter one post and the count. Six 0.3×0.3×1 m pieces at 5% are 0.567 m³.
Does this replace a drawing?
No. The card multiplies the sizes you give. Rebar and mix class sit outside this formula.
Where is mortar?
On a separate page. Masonry mortar is not the same cubic meter as ready-mix concrete.