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Concrete Slab Calculator
Pouring a patio, shed base, driveway or hot-tub pad? Enter the slab's size and thickness to get the concrete you need — cubic yards if a ready-mix truck is coming, or pre-mix bags if you are mixing it yourself. Round slabs (fire-pit pads, tank and grain-bin bases) are covered too. A waste allowance is included, the ready-mix cost is optional, and the formula and worked examples below show exactly how the numbers are reached.
Results update as you type. Estimates only — confirm final quantities before ordering.
The formula
Round: Area (ft²) = π × (Diameter ÷ 2)², then Volume = Area × Thickness (in) ÷ 12
Cubic yards = ft³ ÷ 27 • Bags = ft³ ÷ yield per bag
The volume is increased by your waste allowance before anything else is calculated. Bag counts use the yields published in the QUIKRETE Concrete Mix data sheet: an 80 lb bag yields about 0.60 ft³, a 60 lb bag 0.45 ft³ and a 40 lb bag 0.30 ft³ of mixed concrete. This is the same verified math as our general concrete calculator, specialised for the two slab shapes — a rectangle entered here gives exactly the same answer there.
Worked example
Example 1 — rectangle: a 10 ft × 10 ft patio slab, 4 inches thick, 10% waste.
- Raw volume: 10 × 10 × (4 ÷ 12) = 33.33 ft³
- With 10% waste: 36.67 ft³ = 1.36 yd³
- Bags: 36.67 ÷ 0.60 = 62 × 80 lb | ÷ 0.45 = 82 × 60 lb | ÷ 0.30 = 123 × 40 lb
Example 2 — round: a hot-tub pad 12 ft across, 4 inches thick, 10% waste.
- Area: π × (12 ÷ 2)² = 113.1 ft²
- Volume: 113.1 × (4 ÷ 12) × 1.10 = 41.47 ft³ = 1.54 yd³
- Bags: 41.47 ÷ 0.60 = 70 × 80 lb | ÷ 0.45 = 93 × 60 lb | ÷ 0.30 = 139 × 40 lb
Reference chart
| Slab size | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 5 × 5 ft | 0.34 | 16 | 21 |
| 8 × 8 ft | 0.87 | 40 | 53 |
| 10 × 10 ft | 1.36 | 62 | 82 |
| 12 × 12 ft | 1.96 | 88 | 118 |
| 10 × 20 ft | 2.72 | 123 | 163 |
| 20 × 20 ft | 5.43 | 245 | 326 |
| Round, 10 ft across | 1.07 | 48 | 64 |
| Round, 12 ft across | 1.54 | 70 | 93 |
| Round, 16 ft across | 2.73 | 123 | 164 |
Frequently asked questions
How many yards of concrete are in a 10x10 slab?
At the standard 4 inch thickness, a 10 x 10 ft slab holds 1.23 cubic yards before waste — 1.36 yd³ with the usual 10% waste allowance. In bags that is 62 x 80 lb bags. Thickness is the lever: at 6 inches the same slab needs 2.04 yd³ with waste.
How thick should a concrete slab be?
Four inches is standard for patios, walkways, shed bases and hot-tub pads. Driveways and anything a car parks on are usually poured 5–6 inches thick, often with rebar or mesh. Check local code — frost-depth and soil rules vary, and the compacted gravel base under the slab (not counted here) is typically another 4 inches.
How do I work out concrete for a round slab?
Measure straight across the widest part — that is the diameter — and pick Round above. The area is π × (diameter ÷ 2)². Round slabs show up as fire-pit and seating pads, hot-tub bases, and pads under water tanks, silos and grain bins.
Bags or ready-mix for a slab?
Below about one cubic yard, bags are usually simpler and cheaper — no delivery or short-load fees. Above that, ready-mix wins quickly: a 10 x 10 slab is already 62 heavy bags to haul, mix and place before the concrete starts to set. Enter your local ready-mix price per yard above to compare the truck against bag prices.
Does this calculator include the gravel base under the slab?
No — it calculates the concrete slab only. Most slabs sit on a compacted gravel base, often about 4 inches deep. Size that base with our gravel calculator, using the same footprint and your base depth.
Related calculators
- Concrete Calculator — Cubic yards and bag counts for slabs, patios and pads.
- Concrete Yard Calculator — How many cubic yards of concrete to order — yards first.
- Gravel Calculator — Gravel in cubic yards and tons, by depth and density.
Sources & method: Bag yields: QUIKRETE Concrete Mix No. 1101 technical data sheet (80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 40 lb = 0.30 ft³ of placed concrete); 27 ft³ = 1 yd³; round-slab area A = π × (diameter ÷ 2)²; rectangle results verified identical to this site's Concrete Calculator.