Concrete Slab Calculator
Use our free concrete slab calculator to instantly find the volume of concrete you need in cubic yards or cubic metres, plus the number of premix bags required. Simply enter the length, width and thickness of your slab — results update as you type.
Select Bag Size
Results will show bags needed for your selected size
Imperial Bags (US/Canada)
Default: 80 lb (imperial) · 20 kg (metric)
How to Use This Calculator
- Select your unit system — Imperial (ft/in) or Metric (m/cm).
- Enter the length and width of the slab.
- Enter the thickness in inches (imperial) or centimetres (metric).
- Results update instantly — no button needed.
- Use the +10% waste figure when placing your order.
Concrete Slab Formula
The calculation is straightforward:
Cubic Yards = Volume (ft³) ÷ 27
Cubic Metres = Volume (ft³) × 0.0283168
For premix bags, we use the manufacturer's yield per bag:
- 40 lb bag — yields approx. 0.30 cu ft (0.011 yd³)
- 60 lb bag — yields approx. 0.45 cu ft (0.017 yd³)
- 80 lb bag — yields approx. 0.60 cu ft (0.022 yd³)
Practical Tips for Pouring a Concrete Slab
- Prepare the subbase: Compact at least 4 inches of gravel before pouring to prevent cracking.
- Use control joints: Space control joints every 8–12 feet to control where cracks appear.
- Order ready-mix for large pours: Anything over 1 cubic yard is generally more economical as ready-mix concrete from a batch plant.
- Check the mix: A 4,000 psi mix is standard for driveways and garage floors; 3,000 psi works for patios.
- Allow proper curing: Keep the slab moist for at least 7 days after pouring to achieve full strength.
Frequently Asked Questions
How do I calculate how much concrete I need for a slab? +
Multiply the length by the width by the thickness (depth). Convert depth to feet if using inches (divide by 12), then divide the total cubic feet by 27 to get cubic yards. Our calculator handles all of this automatically.
How many bags of concrete do I need for a 10x10 slab at 4 inches thick? +
A 10 ft × 10 ft × 4 in slab is approximately 1.23 cubic yards. You would need roughly 56 × 60 lb bags, or 45 × 80 lb bags. Always add 10% for waste.
How thick should a concrete slab be? +
Standard residential slabs (patios, driveways, garage floors) are typically 4 inches thick. Heavy-duty slabs such as those supporting vehicles or equipment are 6 inches or more.
What is a cubic yard of concrete? +
A cubic yard is 3 ft × 3 ft × 3 ft = 27 cubic feet. Ready-mix concrete is ordered by the cubic yard. One cubic yard weighs approximately 4,000 lb (1.8 tonnes).
Should I add extra concrete for waste? +
Yes — always order at least 10% more than your calculated volume to account for spillage, uneven subgrade, and formwork gaps. Our calculator highlights the +10% figure for you.