How the concrete calculator works
Concrete is ordered by volume. The calculator works out the volume of each element in cubic feet, adds your waste allowance and divides by 27 to give cubic yards, the unit ready-mix plants sell. It also divides the cubic feet by the yield of standard premixed bags so you can compare a truck delivery with mixing it yourself.
Formulas
- Slab, footing or wall: length × width × thickness (ft)
- Round column or post hole: π × (diameter ÷ 2)² × height
- Stairs: each step is treated as a solid block from the ground to its tread, which is how most contractors do a quick takeoff
Bag yields used: 40 lb ≈ 0.30 ft³, 60 lb ≈ 0.45 ft³, 80 lb ≈ 0.60 ft³. Check the bag label, because yields vary slightly between products.
Worked example: a 12 × 12 patio
A 12 ft square patio poured 4 inches thick is 12 × 12 × 0.333 = 48 cubic feet. That is 1.78 cubic yards, or 1.96 yards with 10% extra. In bags it would be 88 × 80 lb bags, close to 3½ tons of material to lift and mix, so a 2-yard ready-mix order is the practical choice.
Common slab thicknesses
| Project | Typical thickness |
|---|---|
| Walkway, patio, shed floor | 4 in |
| Garage floor (cars) | 4–5 in |
| Driveway | 4–6 in |
| Hot tub or heavy equipment pad | 6 in or more |
These are common practice, not engineering advice. Local building codes, soil conditions and loads decide the final design, so check with your building department for anything structural.
Before you pour
- Compact a gravel base under slabs; the gravel calculator sizes it in tons.
- Measure the formed area, not the plan, and use the square footage calculator for irregular shapes.
- Schedule the truck for a dry, mild day and have enough helpers: concrete starts to stiffen in about an hour and a half.
Frequently asked questions
How many 80 lb bags of concrete make a yard?
About 45. An 80 lb bag yields roughly 0.6 cubic feet, and a cubic yard is 27 cubic feet, so 27 ÷ 0.6 = 45 bags. You would need 60 bags of the 60 lb size or 90 of the 40 lb size.
How thick should a concrete slab be?
Four inches is standard for patios, walkways and garage floors for cars. Driveways that carry heavier vehicles are usually 5–6 inches.
When is ready-mix cheaper than bags?
Beyond about one cubic yard, a ready-mix truck is usually cheaper and much faster. Many suppliers charge a short-load fee for orders under a few yards, so compare both.
How much extra concrete should I order?
Add 5–10%. Forms bulge, subgrades are uneven and some concrete stays in the truck or wheelbarrow.
How much does a yard of concrete weigh?
Roughly 4,000 lb (about 2 tons) for normal-weight concrete, based on about 150 lb per cubic foot.
Sources and official references
- Concrete basics and resources – American Concrete Institute (ACI)
- Cement and concrete technical resources – Portland Cement Association (PCA)
Last reviewed October 10, 2026. Results are estimates for planning; see our disclaimer.
