Concrete Calculator: Slab on Grade (Rectangular) — Volume & Bags

Estimate concrete for a rectangular slab on grade. Enter length, width, slab thickness, and an optional perimeter thickened edge. Add a waste allowance and get base slab volume, thickening volume, and totals in m³, yd³, and ft³—plus approximate bag counts. Notes, formulas, and FAQs included.

Units Metric (m, mm) US Customary (ft, in)

Length

Width

Slab thickness

Typical residential slabs are ~100–125 mm (4–5 in); always follow your engineer and local code.

Waste allowance

Covers spillage and variation. Many jobs add 4–10%.

Add perimeter thickened edge (optional)

Enable thickened edge Models a rectangular “ring” around the slab (not triangular). Good for quick estimates.

Which edges

Edge band width

Horizontal width of the thickened portion from the slab edge inward.

Total edge thickness

Concrete thickness at the outer edge. The calculator adds only the extra depth beyond the base slab.

Results

Geometry

Area: 0.000 m²

Perimeter: 0.000 m

Base slab volume

0.000 m³

0.000 yd³

0.00 ft³

Thickened edge volume

0.000 m³

0.000 yd³

0.00 ft³

Total with waste

0.000 m³

0.000 yd³

0.00 ft³

Approximate bag counts (total with waste)

US bag sizes (QUIKRETE® yield)

40 lb bag00.30 ft³ per bag

50 lb bag00.375 ft³ per bag

60 lb bag00.45 ft³ per bag

80 lb bag00.60 ft³ per bag

90 lb bag00.675 ft³ per bag

Yields per bag from QUIKRETE data sheet. Actual yield varies with placement and compaction. Source

Metric bag sizes

Edit yields to match your local supplier data.

20 kg bag yield m³ 25 kg bag yield m³ 40 kg bag yield m³

20 kg bag0

25 kg bag0

40 kg bag0

How to use this slab calculator

Select metric or US units.
Enter slab length, width, and thickness.
Optional: enable a perimeter thickened edge, choose which sides, set the band width and edge thickness.
Add a waste allowance (many projects use 4–10%).
Read base, thickening, and total volumes, then review bag counts for ordering. Round up.

Formulas and assumptions

Base slab: area A = L × W, volume V_base = A × T, where T is slab thickness (converted to meters).
Perimeter thickening (rectangular band, extra depth Δ = max(T_edge − T, 0)):
- All edges: band area A_band = 2E(L+W) + 4E²
- Long sides only: A_band = 2EL
- Short sides only: A_band = 2EW
Thickening volume V_edge = A_band × Δ, where E is the band width.
Total: V_total = V_base + V_edge, with waste V_w = V_total × (1 + waste%).
Bag counts: bags = Required volume ÷ bag yield. US yields use ft³ per bag. Metric yields are editable in m³ per bag.
Scope: Volume only. This does not size reinforcement, joints, subbase, vapor retarder, or finish class.

References

Concrete waste allowance ~4–10%: NRMCA Concrete In Practice CIP 8, CIP 31.
Slab-on-ground practices: ACI 302.1R (floor and slab construction) and ACI 360R (slab-on-ground design) — see ACI resources: ACI Slab-on-Ground.
US bag yields: QUIKRETE® Concrete Mix 1101 data sheet.

FAQs

How thick should my slab be?

Common residential slabs are about 100–125 mm (4–5 in), but thickness depends on loads, subbase quality, and local code. Follow your engineer’s drawings.

Does the thickened edge model the triangular “haunch” exactly?

No. This option treats the thickening as a rectangular band for a conservative volume estimate. For a triangular or trapezoidal haunch, ask for the frustum-style variant.

How much extra should I order?

Many contractors add 4–10% for waste and placement variability (per NRMCA guidance). For difficult access or complex forms, consider the higher end.