Concrete Slab Calculator — Volume & Bag Count
Enter slab length, width and thickness to get the concrete volume in cubic metres and cubic yards, along with bag counts for 40, 60 and 80 lb pre-mix bags.
Units
m
m
cm
4.709 yd³ • 127.1 ft³
3.6 m³
4.71 yd³- 1
Thickness in metres
15 cm ÷ 100 = 0.15 - 2
Slab area
6 m × 4 m = 24 - 3
Volume
24 m² × 0.15 m = 3.600
How does this calculator work?
Multiply slab length × width × thickness (consistent units) to get the concrete volume. A 20 ft × 13 ft slab 6 in thick needs about 4.81 yd³ (3.68 m³). Divide by the bag yield (0.022 yd³ for an 80 lb bag) for bag count; round up and add 5–10% waste.
Formula
How this is calculated
A concrete slab is a flat rectangular box, so its volume is simply length × width × thickness. The only complication is keeping units consistent throughout the multiplication. In metric mode, length and width are entered in metres and thickness in centimetres; the thickness is divided by 100 before multiplying, giving a volume in cubic metres. In imperial mode, length and width are in feet and thickness is in inches; thickness is divided by 12 to convert to feet, the product gives cubic feet, and dividing by 27 converts to cubic yards (since 27 ft³ = 1 yd³). The exact conversion factor 1 yd³ = 0.7646 m³ is used to present both metric and imperial volumes.
Bag counts are based on published manufacturer yields — approximately 0.60 ft³ per 80 lb bag, 0.45 ft³ per 60 lb bag and 0.30 ft³ per 40 lb bag (Quikrete / Sakrete data sheets, 2025). The slab volume in cubic feet is divided by each bag yield and rounded up. Add roughly 5–10% extra when ordering to cover spillage, an uneven sub-base and over-excavation.
Frequently asked questions
Volume = 20 × 13 × (6/12) = 130 ft³ ÷ 27 ≈ 4.81 yd³ (≈ 3.68 m³). You would need roughly 217 × 80 lb bags. Always add 5–10% extra for waste.
Footpaths and patios are typically 75–100 mm (3–4 in) thick; driveways and garage floors 100–150 mm (4–6 in). Structural slabs follow engineer specifications. Always check local building codes.
Pre-mixed bags suit small slabs under about 1 yd³ where a truck delivery is not cost-effective. For larger slabs, ready-mix concrete delivered by truck is more economical and practical.
Also known as
TG we-Calculate Editorial Team. (2026). Concrete Slab Calculator — Volume & Bag Count [Online calculator]. TG we-Calculate. https://we-calculate.com/calculator/concrete-slab-calculator
TG we-Calculate Editorial Team. "Concrete Slab Calculator — Volume & Bag Count." TG we-Calculate. 2026. https://we-calculate.com/calculator/concrete-slab-calculator.
TG we-Calculate Editorial Team, "Concrete Slab Calculator — Volume & Bag Count," TG we-Calculate, 2026. [Online]. Available: https://we-calculate.com/calculator/concrete-slab-calculator
@misc{wecalculate_concrete_slab_calculator, title = {Concrete Slab Calculator — Volume & Bag Count}, author = {{TG we-Calculate Editorial Team}}, howpublished = {\url{https://we-calculate.com/calculator/concrete-slab-calculator}}, year = {2026}, note = {TG we-Calculate} }
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