A 10 by 10 foot slab poured 4 inches thick takes 33.3 cubic feet of concrete, which is 1.23 cubic yards. With a 10% allowance for spillage and uneven ground that becomes 1.36 cubic yards: 62 bags of 80 lb concrete mix, or 1.5 yards of ready-mix. Every shape works the same way: find the volume, add the allowance, then convert to bags or yards.
How the concrete volume is calculated
Concrete is ordered by volume, so every shape is reduced to length × width × depth in the same unit. In feet, a thickness in inches is divided by 12 first. Cubic feet divided by 27 gives cubic yards, the unit ready-mix is sold in.
Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 12 ÷ 27
Round shapes use the area of a circle, π × radius², times the depth. A sonotube or post hole is a cylinder standing on end; a circular slab is a very flat one. Steps are measured from the side: each step is a block as tall as all the risers below it, and the calculator adds them up and multiplies by the width. Curb and gutter is the cross-section of the curb plus the gutter flag, times the length.
In metric the same formula runs in meters and gives cubic meters directly, so there is no factor of 27. One cubic yard is 0.765 m³.
Worked example: a slab plus six post holes
Many jobs are more than one shape, which is why the calculator lets you add sections and sums them into one order. Take a 10 × 10 ft slab 4 in thick and six 12 in sonotubes 4 ft deep for a small deck next to it:
- Slab: 10 × 10 × 4 ÷ 12 = 33.33 cu ft
- Tubes: π × 0.5² × 4 = 3.14 cu ft each, 18.85 cu ft for six
- Together: 52.18 cu ft, plus 10% = 57.40 cu ft, or 2.13 cu yd
That is 96 bags of 80 lb mix, or 2.25 cubic yards of ready-mix once rounded up to the next quarter yard.
Concrete per 100 square feet
Use this to sanity-check a flat pour before you measure every section. The figures are plan volume, before any waste.
| Thickness | Cubic yards per 100 sq ft | Square feet one cubic yard covers |
|---|---|---|
| 3½ in | 1.08 | 92.6 |
| 4 in | 1.23 | 81 |
| 5 in | 1.54 | 64.8 |
| 6 in | 1.85 | 54 |
| 8 in | 2.47 | 40.5 |
The 2024 International Residential Code sets 3½ inches as the minimum for a concrete floor slab on ground (R506.1). For footings the minimum is 12 inches wide and 6 inches thick (R403.1.1).
Bags or ready-mix
Bag counts use the yields printed on the manufacturers' data sheets. Quikrete lists 0.30 cu ft for a 40 lb bag, 0.375 for 50 lb, 0.45 for 60 lb and 0.60 for 80 lb; Sakrete's High-Strength mix gives the same 0.30, 0.45 and 0.6 for its 40, 60 and 80 lb bags. Both are 4,000 psi mixes for pours at least 2 inches thick.
Past about a cubic yard, bags get heavy fast: the 62 bags in the opening example weigh 4,960 lb before water. Ready-mix trucks carry 8 to 12 cubic yards, according to the National Ready Mixed Concrete Association (NRMCA), so a small order arrives as a short load, which suppliers usually charge extra for. The calculator shows both so you can decide.
Why the 10% allowance
NRMCA advises ordering 4% to 10% more than the volume worked out from the plan dimensions, to cover spillage, forms that bulge, uneven ground and over-excavation. Its guidance on yield shows how quickly a small error adds up: a slab that ends up ⅛ inch thicker than the planned 4 inches is 3% short. On irregular subgrade, or for a job with several small placements, stay at 10%. For a clean, formed pour on a compacted base you can go lower.
Calculators for each part of the job
- Concrete slab calculator: patios, pads and floors by length and width or square feet, with a thickened edge and an installed price.
- Concrete bag calculator: 40, 50, 60 and 80 lb bags side by side, with the water to mix them.
- Concrete yard calculator: the ready-mix order, trucks, short loads and what to say when you call the plant.
- Concrete footing calculator: continuous footings, pads and foundation walls with the IRC minimums checked.
- Sonotube concrete calculator: bags per tube or post hole, with the post subtracted.
- Concrete steps calculator: steps with a landing, checked against the riser and tread limits.
- Rebar calculator: bars, laps, weight and chairs for the slab or footing you just measured.
- Concrete cost calculator: ready-mix, delivery, labor and reinforcement priced for your state.
For volumes of anything else sold by the yard, the cubic yard calculator converts any box or cylinder.
Mixing your own batch instead of buying bags? The concrete mix ratio calculator turns 1:2:3 and other ratios into cement, sand and gravel.