Concrete Slab Calculator
Built for rectangular patio, shed-pad and sidewalk slabs. Enter length, width and thickness to get cubic yards to order plus an 80-lb bag count so you can compare ready-mix vs bagged mix for the same pour.
Enter your measurements to see what to buy.
How to calculate it
Built for rectangular patio, shed-pad and sidewalk slabs. Enter length, width and thickness to get cubic yards to order plus an 80-lb bag count so you can compare ready-mix vs bagged mix for the same pour.
Source: Volume = length × width × thickness; 1 cubic yard = 27 cubic feet. An 80-lb bag of mix yields about 0.60 ft³.
A 12×12 ft shed pad at 4 in thick: 12 × 12 × (4÷12) = 48 ft³ ÷ 27 = 1.78 yd³. With 10% waste: 1.96 yd³, or about 88 eighty-pound bags.
Want the full step-by-step walkthrough? Read our How to Plan and Pour a Concrete Patio Slab.
How to use this calculator
- Enter the slab length and width in feet and inches, then the thickness in inches.
- 4 in is standard for patios, shed pads and sidewalks; choose 5–6 in if vehicles will drive on it.
- The result shows cubic yards to order plus an 80-lb bag count for smaller hand-mixed pours.
- Add your per-yard ready-mix price for an optional material cost estimate.
Purchasing & waste guidance
- 4 in is standard for patio, sidewalk and shed slabs; go 5–6 in for anything carrying vehicle weight.
- Add 10% extra for spillage, uneven subgrade and over-excavation.
- Under a yard, bagged mix is practical; over a yard, ready-mix delivery is usually cheaper.
- Always prep a level gravel base and use forms plus rebar or mesh for slabs over 4 in.
Waste factor guidance
- 10% covers spillage and minor subgrade variation on a typical formed slab.
- Use 15% when the ground is uneven or the slab has many corners and cut-outs.
- For a large, simple shed pad on graded ground, 5% is often enough.
Common mistakes to avoid
- Pouring a 4 in driveway slab — vehicle loads need 5–6 in or the slab will crack.
- Ordering bags for a pour over a yard — ready-mix delivery is cheaper and far less labor.
- Skipping the gravel base and reinforcement, which leads to cracking even with the right volume.
Assumptions & what’s included
- Slab is a single rectangular pour with uniform thickness.
- Waste allowance covers spillage, uneven subgrade and minor over-excavation.
- Bag count uses 80-lb bags at 0.60 ft³ each. Price is per cubic yard and optional.
Frequently asked questions
How much concrete do I need for a 10×10 slab?
A 10×10 ft slab at 4 in thick is 33.33 ft³ = 1.23 yd³ before waste (about 1.36 yd³ with 10%). At 0.60 ft³ per 80-lb bag that is 56 bags before waste, or 62 bags with 10% — usually cheaper as ready-mix delivery.
How thick should a concrete slab be?
4 in for patios, walkways and shed pads. Driveways and slabs that support vehicles should be 5–6 in. Structural slabs should follow an engineer’s spec.
Should I use bags or order ready-mix for a slab?
Bagged mix makes sense under about 1 cubic yard. Above that, delivered ready-mix is usually cheaper and far less labor — use the Concrete Bag Calculator to compare.
How many 80 lb bags of concrete do I need for a 10×10 slab?
33.33 ft³ ÷ 0.60 ft³ per 80-lb bag = 55.6 → 56 bags before waste. With 10% waste: 36.7 ft³ ÷ 0.60 = 62 bags. Confirm the yield printed on your bag label — it can vary by brand.
Related guides
Step-by-step project guides that use this calculator.