Concrete Calculator: Rectangular Slab Volume
Use this free concrete calculator to estimate slab volume fast. Enter slab length, width, and thickness, choose units, and include a waste allowance. The tool returns volume in cubic meters, cubic yards, and cubic feet, plus approximate bag counts. Notes and formulas are included below for clarity.
Results
Net area
0.000 m²
Net volume
0.000 m³
0.000 yd³
0.00 ft³
With waste
0.000 m³
0.000 yd³
0.00 ft³
Approximate bag counts
US bag sizes (QUIKRETE® yield)
Yields per bag from QUIKRETE data sheet. Actual yield varies by placement and compaction. Source
Metric bag sizes
Default yields are common published figures. Edit to match your local product.
Typical 20–25 kg bag yield ≈ 0.009–0.011 m³, 40 kg ≈ 0.018–0.019 m³. Check your supplier’s sheet. Examples: Drymix NZ and Express Concrete.
How to use this concrete calculator
- Select metric or US units.
- Enter slab length, width, and thickness.
- Add a waste allowance. Industry guidance is to add 4% to 10% for waste and over-excavation.
- Optionally subtract an internal opening.
- Read the volume and approximate bag counts. Round up when ordering.
Formulas and assumptions
- Volume:
V = L × W × T. Units are converted to meters internally, soVis in m³. - Opening: Net area = slab area minus opening area.
- Waste:
Vwaste = V × (1 + waste%). - Bag counts: bags =
Required volume ÷ bag yield. US yields use ft³ per bag. Metric lets you set m³ per bag from your supplier. - Typical slab thickness: many residential slabs are about 4 in (≈100 mm). Always follow local code and your engineer.
- Joint spacing rule of thumb for slabs on ground: 24 to 36 times slab thickness.
References
- Waste allowance 4–10%: NRMCA CIP 8 & CIP 31 (CIP 8, CIP 31)
- Minimum slab thickness on ground: 2021 IRC R506, 3.5 in (89 mm) (ICC)
- Joint spacing: 24–36 × thickness (USBR summary of ACI guidance, NRMCA CIP 6)
- US bag yields: QUIKRETE® Concrete Mix 1101 (data sheet)
FAQs
How much extra concrete should I order?
Industry guidance suggests adding 4% to 10% to cover waste, spillage, over-excavation, and thickness variation. For small slabs or hand placement, lean toward the higher end.
What thickness should I use for a house slab or driveway?
Many residential slabs are around 4 inches, but code minimums and loading can differ. Follow your local building code and your engineer’s specification.
Does concrete density matter for volume?
No. Volume is purely geometric. Density is useful if you also need weight for logistics. Normal concrete is roughly 2400 kg/m³.
When should I saw cut control joints?
Typically once the slab gains enough strength to avoid raveling, often within a few hours after placement. Spacing is commonly 24–36 times slab thickness. Confirm with your engineer and local practice.