Every quote for a composite floor slab hinges on one number: how much concrete is going into it. Get that number wrong and you’re either paying for a truck that never gets used, or stopping the pour halfway through because the site ran short. Neither is a good day. The good news is that the calculation itself isn’t complicated — it just depends on a detail that’s easy to overlook: the shape of the deck underneath.

Why You Can’t Just Multiply Area by Depth

With a flat slab poured on the ground, working out concrete volume is straightforward: length times width times depth. A composite floor slab isn’t flat on the underside — it’s sitting on a corrugated steel deck, and those corrugations (the ribs) take up space that would otherwise be concrete. So the real volume is the slab depth across the full area, minus whatever the ribs are displacing underneath.

The Two Numbers That Actually Matter

Two measurements drive the whole calculation. The first is the deck’s profile depth — how deep the ribs run, which for the profiles we stock usually falls somewhere between 40mm and 80mm. The second is the total slab depth your engineer has specified, measured from the underside of the deck’s ribs to the top of the finished floor. The gap between those two numbers is the “flat” layer of concrete sitting on top of the ribs, and that’s where most of the volume comes from.

As an illustration — not a spec, just to show how the maths works — a deck with a 55mm profile topped up to a 130mm total slab depth typically works out to somewhere around 0.09 cubic metres of concrete per square metre of floor. Push the total depth to 150mm on the same profile and you’re closer to 0.11 m³/m². Small changes in slab depth move that number more than people expect, because you’re paying for the extra concrete across the entire floor area, not just in one spot.

Why You Should Ask for the Table, Not Estimate It

Every deck profile has its own rib geometry, so the exact m³/m² figure is different for every product — a 60mm profile and an 80mm profile at the same slab depth won’t use the same amount of concrete, even though they look similar from a distance. Manufacturers publish volume tables for exactly this reason. Before you order concrete for a project using our deck, ask our team for the volume table matched to the profile and total slab depth your engineer has specified. It takes a couple of minutes, and it’s the difference between an accurate order and a guess.

A Quick Way to Sanity-Check a Supplier’s Number

• Confirm the deck profile depth being quoted — not just “our standard deck.”
• Confirm the total slab depth from the structural drawings, not the architectural finish level.
• Ask whether the figure allows for deck deflection under wet concrete — on longer spans, the slab can end up a few millimetres thicker at midspan than at the supports.
• Round up, not down. A slightly conservative order costs less than a second truck and a delayed pour.

None of this replaces your structural engineer’s specification — the total slab depth is their call, not a rule of thumb. But once that number is fixed, getting the concrete volume right is just a matter of using the correct table for the correct profile. It’s a five-minute conversation that saves a lot of headaches on pour day.