Concrete slab calculator

The formula, in plain words

1. 1Slab area = length times width, in square metres.
2. 2Slab volume = area times thickness in millimetres, divided by 1000. That gives cubic metres.
3. 3Edge beam volume = (slab area minus the inner rectangle inside the beam) times the beam depth. The inner rectangle is (length minus twice the beam width) by (width minus twice the beam width). That counts each corner once.
4. 4Internal beams frame inside the edge beam ring. Each rib runs the full inside dimension by the rib width by the beam depth. Number of internal beams at spacing s across a span S is ceiling(S over s) minus 1. In grid mode, subtract the intersections so a crossing is counted once, not twice.
5. 5Total concrete = slab volume plus edge beam plus internal beams.
6. 6For bags: total volume times bags per cubic metre (about 110 for 20 kg premixed concrete), plus wastage, round up. Other bag sizes scale by weight.
7. 7For ready-mix: total volume times (1 plus wastage), rounded up to the next 0.2 m³ the truck delivers in.

Worked example

A 12 m × 8 m house slab at 100 mm with 300 × 300 mm perimeter edge beams and a two-way internal grid of 300 mm ribs at 4 m centres, poured as ready-mix with a 10% wastage allowance.

1. 1.Slab area: 12 × 8 = 96 m². Slab volume: 96 × 100 / 1000 = 9.6 m³.
2. 2.Edge beam ring: 96 − (11.4 × 7.4) = 96 − 84.36 = 11.64 m². Edge volume: 11.64 × 0.3 = 3.492 m³.
3. 3.Inner region: 11.4 × 7.4 m. Along-length ribs: ceil(7.4/4) − 1 = 1 rib × 11.4 × 0.3 × 0.3 = 1.026 m³.
4. 4.Along-width ribs: ceil(11.4/4) − 1 = 2 ribs × 7.4 × 0.3 × 0.3 = 1.332 m³.
5. 5.Intersections: 1 × 2 × 0.3 × 0.3 × 0.3 = 0.054 m³ (counted once, not twice).
6. 6.Internal beams total: 1.026 + 1.332 − 0.054 = 2.304 m³.
7. 7.Total concrete: 9.6 + 3.492 + 2.304 = 15.396 m³.
8. 8.With 10% wastage: 16.936 m³. Round up to 17.0 m³ ready-mix.

Answer: About 17 m³ of ready-mix. Same slab as bags would be roughly 1,863 × 20 kg bags, so bags are not the right tool for this pour.

How to measure it

• Measure the slab in metres. Length times width gives the area.
• Get the thickness from your engineer's or council's spec. Typical: 75 to 100 mm non-load-bearing, 100 to 125 mm shed and driveway.
• If the slab has a thickened perimeter edge, turn on 'Add edge beam' and enter its width and depth off the plans. Domestic edge beams are commonly 300 × 300 mm; waffle raft and stiffened raft slabs go deeper.
• If the slab has internal ribs (waffle raft, stiffened raft) turn on 'Add internal beams', pick the direction (grid or one-way) and enter the spacing and rib width from the engineer's plan. Flip the drawing to 'Underside' to see the whole beam network from below.
• Choose bags or ready-mix. Small slabs (under about 0.5 m³) are usually bags; anything bigger is cheaper and quicker as ready-mix.
• Ask your supplier about the minimum load and short-load fee. A truck under-loaded still costs the same.

Where the numbers come from

Every default in the calculator is a number a supplier or manufacturer publishes. We list the source next to it so you can check.

Frequently asked questions