A slab is just a flat box of concrete, so its volume is length times width times thickness — the only trick is keeping the thickness in the same units as the rest (100 mm is 0.1 m). From that volume two practical numbers follow: how many premix bags it would take, using each bag's small yield, and how much it weighs as ready-mix, at concrete's density of about 2.4 tonnes per cubic metre. A modest waste margin covers the uneven sub-base and bulging formwork that always eat a bit more than the theory.
Reviewed: June 19, 2026 · Author: Naveen P N, Founder — AI Calculator · Verified against: volume = L×W×t with standard concrete density.
The slab equations
Convert thickness to metres before multiplying, or the volume comes out a thousand times too big. Bag yield is small and brand-specific — a 20 kg bag makes only about nine litres of concrete — which is why bag counts climb fast and ready-mix wins on anything but tiny jobs. The 2.4 tonne-per-cubic-metre figure turns the volume straight into a delivery weight for the ready-mix truck.
Worked example — a garage slab
Scenario: A 5 m × 4 m garage slab, 100 mm thick, 5% waste, priced both as 20 kg bags and as ready-mix.
The slab is 2 m³, or 2.1 m³ with waste. As ready-mix that is about 4.8 tonnes — one small truck. As bags it would be a back-breaking 234 × 20 kg, which is why nobody bags a slab this size; bags are for a post pad or a path repair. Increase the thickness to 150 mm and the volume rises straight to 3 m³ (7.2 t), since volume tracks thickness one-for-one.
Frequently Asked Questions
volume = length × width × thickness(m). A 5×4 m slab at 100 mm = 2 m³; order ~2.1 m³ with 5% waste.
volume ÷ bag yield. 20 kg ≈ 0.009 m³, so 2 m³ ≈ 234 bags — use ready-mix above ~0.5 m³.
Garden/shed ~100 mm, patios 75–100 mm, car driveways 100–150 mm. Thickness drives volume directly.
~2.4 t/m³, so a 2 m³ slab ≈ 4.8 t. Matters for delivery and ground/structure loading.
Yes — 5–10% for uneven base and formwork. Running short means a cold joint or emergency load.