Concrete is ordered by volume, and volume is where DIY estimates most often go wrong — thicknesses in inches, dimensions in feet, and suppliers selling by the cubic yard all have to agree. Here is the full method, with the unit conversions done for you.
The two shapes that cover almost every project
Circle: volume = π × (diameter ÷ 2)² × thickness
Slabs, walkways, shed pads, and patio squares are rectangles. Pad footings, round patios, and circular hot-tub bases are circles. Both formulas need the same three ingredients: two dimensions and a thickness — and all three must be in the same unit before multiplying.
Step 1: Convert everything to feet
In US units, plan in feet: divide inches by 12. A slab "10 ft × 6 ft, 4 inches thick" is 10 × 6 × (4 ÷ 12) = 10 × 6 × 0.333 ft. Mixing feet and inches inside one multiplication is the most common concrete math error there is.
Step 2: Multiply for cubic feet
The rectangle above: 10 × 6 × 0.333 = 20 cubic feet. For a circle, a 10 ft diameter round pad 4 inches thick is π × 5² × 0.333 ≈ 26.18 cubic feet.
Step 3: Convert to the unit your supplier sells
| Conversion | Value |
|---|---|
| 1 cubic yard | 27 cubic feet |
| 1 cubic metre | ≈ 35.3 cubic feet |
| 1 cubic foot | ≈ 28.3 litres |
So the 20 cu ft slab is 20 ÷ 27 = 0.74 cubic yards. Ready-mix plants sell by the cubic yard; bagged concrete is labelled by the volume each bag yields — usually in cubic feet in the US. The Concrete Calculator shows all three units at once so either kind of quote can be checked directly.
Step 4: Add a waste allowance
Uneven subgrade, spillage, and the concrete that stays in the wheelbarrow all consume volume. A 5–10% allowance is typical for small pours on reasonably prepared ground; uneven sites want more. The slab becomes 20 × 1.10 = 22 cu ft (0.81 cu yd).
Step 5: Convert the requirement into a purchase
Two purchasing routes, never both at once:
- Bagged: 22 cu ft ÷ 0.6 cu ft per 60-lb bag = 36.7 → buy 37 bags. At a sample $6.50 per bag, about $240.50.
- Ready-mix: 0.81 cu yd × $155 per cubic yard ≈ $126 — before any short-load fees your plant may add for small orders.
Same pour, two supply methods. For a job this size the truck is often cheaper and far less labor; for a single footing, bags win. See Concrete Bags vs Ready-Mix for the full comparison.
Worked example: a round pad
A 10 ft diameter round pad, 4 inches thick, 10% waste:
| Step | Result |
|---|---|
| Volume: π × 5² × 0.333 | 26.18 cu ft |
| Required with waste: 26.18 × 1.10 | 28.8 cu ft |
| In yards: 28.8 ÷ 27 | 1.07 cu yd |
| Bags at 0.6 cu ft: 28.8 ÷ 0.6 | 48 bags |
Check it in the Concrete Calculator by switching the shape to Circle.
Common mistakes
- Mixing inches and feet in one multiplication. Convert thickness to feet first.
- Forgetting that thickness multiplies fast. Doubling thickness doubles the entire order — a 4-inch slab and a 6-inch slab are very different purchases.
- Using a generic bag yield. 60-lb and 80-lb bags yield different volumes; the yield printed on your bag is the only one that counts.
- Comparing bag cost to ready-mix cost but adding them in one order. They are alternatives; estimate both, order one.
- Skipping the waste allowance on ground that is anything less than perfectly level.
FAQ
How many 60-lb bags make a full cubic yard?
One cubic yard is 27 cu ft; a 60-lb bag yields roughly 0.45 cu ft, so 27 ÷ 0.45 ≈ 60 bags. That is the honest answer to "can I mix a yard by hand" — you probably do not want to.
Why does the calculator show cubic metres on a US job?
So nothing gets lost when a product, tool, or plan uses metric — many bagged products list metric yields, and the calculator keeps both systems equivalent rather than relabelled.
Does thickness depend on what the slab is for?
It depends on use, soil, and local codes — walkways and shed pads are commonly planned around 4 inches, but the right thickness for your project, especially anything structural or load-bearing, requires a qualified professional. This guide and the calculator handle quantity math only.