Concrete Slab Calculator
Get cubic yards and 40, 60, or 80 lb bag counts for any slab size, with adjustable thickness, shape, and waste factor.
Your Concrete Estimate
- Pick a shape - rectangular for slabs and footings, or round for circular slabs, columns, and post holes (enter the diameter).
- Choose units - Imperial (feet, inches, cubic yards) or Metric (meters, mm, cubic metres).
- Enter dimensions - length and width of the slab. Use preset chips for common thicknesses.
- Check thickness - 4 inches is standard for patios. Use 5-6 inches for driveways and heavy loads.
- Set waste factor - 10% is standard. Go higher for uneven subgrade or irregular shapes.
- Review estimate - compare bag count vs. ready-mix pricing with your local supplier.
For a slab, multiply length x width x thickness (all in feet) to get cubic feet, then divide by 27 for cubic yards. A 10 x 10 ft slab at 4 inches thick needs about 1.24 cubic yards - roughly 56 bags of 80 lb mix, or one ready-mix order. The shortcut for any 4-inch slab: divide the square footage by 81 to get cubic yards. Enter your dimensions above for the exact count in yards and 40, 60, or 80 lb bags.
How Much Concrete Do I Need for a Slab?
Two numbers decide it: the area of the slab and how thick you pour it. Concrete is ordered by the cubic yard for ready-mix or counted in 40, 60, and 80 lb bags for smaller DIY pours, and the calculator gives you both so you can compare before you buy.
Take that same 10 x 10 patio: at 4 inches it is about 1.24 cubic yards, but bump it to 6 inches and it jumps to 1.85 - thickness moves the total as much as area does, so set it to match the load (more on that below). For a fast 4-inch estimate without the calculator, divide the square footage by 81: a 200 sq ft patio is roughly 200 / 81 = 2.5 yards. That 81 is just how many square feet one cubic yard covers at 4 inches, so it only holds at that thickness.
How a Concrete Slab Is Built
Bags or Ready-Mix? Where the Line Falls
Concrete by Slab Size (4-Inch Thickness)
| Slab Size (ft) | Square Feet | Cubic Yards | 80 lb Bags |
|---|---|---|---|
| 4 x 4 | 16 | 0.20 | 9 |
| 4 x 8 | 32 | 0.40 | 18 |
| 8 x 8 | 64 | 0.79 | 36 |
| 10 x 10 | 100 | 1.24 | 56 |
| 10 x 12 | 120 | 1.48 | 67 |
| 12 x 12 | 144 | 1.78 | 80 |
| 20 x 20 | 400 | 4.94 | 222 |
| 20 x 30 | 600 | 7.41 | 334 |
| 30 x 40 | 1,200 | 14.81 | 667 |
At 4-inch thickness, before waste. One 80 lb bag yields about 0.6 cu ft (roughly 45 bags per cubic yard). Add 5-10% for waste. For a 6-inch pour, multiply cubic yards by 1.5. Bag counts that high are impractical - order ready-mix above about 2 cubic yards.
How Many Bags of Concrete Are in a Yard?
It takes about 45 bags of 80 lb mix, 60 bags of 60 lb mix, or 90 bags of 40 lb mix to make one cubic yard. Each 80 lb bag yields roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet (27 / 0.6 = 45). That math is why bags stop making sense on bigger pours: a single cubic yard already means hauling and mixing 45 bags by hand.
As a quick rule, bagged mix is the economical choice under about 1 cubic yard, gets compared against a short-load delivery fee between 1 and 2 yards, and gives way to ready-mix above 2 yards - both on cost and on the sheer labor of mixing. The calculator returns the exact bag count for all three sizes so you can price it out.
Finish the Slab Estimate
A slab is more than the concrete. The usual order from here:
Recommended Slab Thickness by Project
Thickness follows load. Too thin and it cracks under weight; too thick wastes concrete. What contractors typically pour:
| Project Type | Recommended Thickness | Notes |
|---|---|---|
| Sidewalks & garden paths | 3.5-4 in | Foot traffic only |
| Patios & pool decks | 4 in | Standard residential pour |
| Garage floors (cars) | 4-5 in | 4 in minimum; 5 in for heavier vehicles |
| Driveways | 5-6 in | 6 in at the apron where it meets the street |
| Workshops / RV pads | 6 in | Heavier point loads need more thickness |
| Commercial / heavy equip. | 6-8 in | Engineer-specified, usually with rebar |
Is 4 inches enough? For a residential patio, walkway, or standard garage floor, yes. What matters more is what sits under it: a 4-inch slab on a well-compacted 4-6 inch gravel base outperforms a 6-inch slab poured on soft ground. Get the base right before you reach for more thickness.
Formulas Used
The math behind every result:
Round slab (ft3) = 3.1416 x radius (ft)2 x Thickness (ft)
Cubic Yards = Volume (ft3) / 27
Quick 4-in slab (yd3) = Area (ft2) / 81
Bags (80 lb) = Volume (ft3) / 0.6
With Waste = Cubic Yards x (1 + Waste %)
Metric inputs are converted to imperial for the core calculation, then results convert back when metric mode is on.
Code & Engineering Notes
- Residential slabs-on-ground - footings, thickness, and reinforcement - are governed by ACI 332, the residential concrete code referenced by the IRC. Your local building department adopts and enforces it, so confirm requirements before a structural pour.ACI 332 - Residential Concrete Code
- Reinforcement (welded wire mesh or rebar) belongs near mid-depth of the slab, not on the subgrade - it only controls cracking when the concrete surrounds it. This calculator plans concrete volume only; follow your project drawings for reinforcement.ACI 332 - Slabs-on-Ground
Related Calculators
Working out the budget? The price field above gives a quick material estimate, but for installed pricing by slab size and region, labor vs. materials, and ready-mix vs. bag cost, see the 2026 Concrete Slab Cost Guide.