Work out the volume of concrete for a slab or footing, add a waste allowance, and see the equivalent number of 20 kg bags — so you order the right amount.
For anything beyond about 0.5 m³, ordering pre-mixed concrete is usually cheaper and easier than bagging. Round up when ordering — you can’t add to a pour that’s short.
traqR keeps your material quantities on the job card, so what you calculate flows into the quote and the order.
Start free trialMultiply length by width by thickness (in metres) to get the concrete volume in cubic metres. A 4 × 3 m slab at 100 mm is 1.2 m³.
Real pours lose concrete to spillage, over-dig and uneven subgrade, so add 5–10% so you don’t come up short mid-pour.
For small jobs, roughly 108 × 20 kg bags make a cubic metre. For larger pours, ordering pre-mixed by the cubic metre is cheaper and faster.
Multiply the length, width and thickness (all in metres) to get the volume in cubic metres, then add a waste allowance. A 4 m × 3 m slab at 100 mm thick is 1.2 m³, or about 1.3 m³ with 8% waste.
Roughly 108 × 20 kg bags of pre-mixed concrete make up one cubic metre. So a 1.3 m³ job is about 140 bags — which is why anything more than a small footing is usually cheaper to order as pre-mixed concrete delivered by the truck.
Some concrete is always lost to spillage, over-dig, uneven ground and what stays in the barrow or mixer. Ordering exactly the calculated volume risks running short, and stopping a pour to get more concrete creates a cold joint. A 5–10% allowance avoids that.
No — it’s the concrete volume only. You’ll also need to allow for reinforcement, formwork and a compacted base, and structural slabs and footings should be designed to the relevant standards and any engineering requirements.
This calculator estimates concrete volume for a rectangular slab or footing and is a general aid only. It doesn’t cover reinforcement, formwork, base preparation or structural design, which must follow the relevant standards and engineering requirements. Always confirm quantities before ordering.
traqR keeps material quantities on the job card, so what you calculate flows into the quote and the order. Try it free for 14 days.