Concrete Calculator
Add every part of the pour (slabs, footings, round columns and steps) and get the total cubic yards, bags or ready-mix trucks, rebar or wire mesh, gravel base and which option is cheaper.
Cubic yards = length (ft) × width (ft) × thickness (in) ÷ 324, plus 5–10% extra. A 10 × 10 ft slab 4 in thick is 1.23 cu yd, or about 1.36 cu yd with 10% extra: 62 bags of 80 lb or 1½ yards of ready-mix. Above roughly 1 cubic yard, ready-mix is usually easier and often cheaper than bags.
Concrete for common slabs (+10%)
| Slab | Cubic yards | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 10 × 10 ft × 4 in | 1.36 | 62 | 82 |
| 10 × 12 ft × 4 in | 1.63 | 74 | 98 |
| 12 × 12 ft × 4 in | 1.96 | 89 | 118 |
| 20 × 20 ft × 4 in | 5.43 | 245 | 326 |
| 12 × 24 ft × 5 in | 4.89 | 220 | 294 |
| 20 × 24 ft × 6 in | 9.78 | 440 | 587 |
How thick should concrete be?
| Use | Thickness |
|---|---|
| Walkways, patios, shed floors | 4 in |
| Driveways (cars) | 5–6 in |
| Hot tub pads, RV parking | 6 in |
| Garage floors | 4–6 in, per local code |
Frequently asked questions
How do I calculate how much concrete I need?
Multiply length × width × thickness in feet for cubic feet and divide by 27 for cubic yards. With thickness in inches, multiply length × width in feet by the thickness in inches and divide by 324. Add 5–10% for spillage and uneven ground.
How many bags of concrete are in a yard?
About 45 bags of 80 lb (0.6 cu ft each), 60 bags of 60 lb or 90 bags of 40 lb make one cubic yard.
When should I order ready-mix instead of bags?
Above about 1 cubic yard, mixing 45+ bags by hand is hard work and ready-mix is often cheaper. Below 2–4 yards, many suppliers add a short-load fee; enter it to compare.
How much concrete does a truck hold?
Most ready-mix trucks carry up to about 9–11 cubic yards. Bigger pours need several trucks scheduled back to back.
Do I need rebar in a concrete slab?
Many 4 in patio slabs use welded wire mesh or fiber-reinforced mix; driveways and structural slabs often use #3 or #4 rebar at 12–18 in on center. Follow your local code and engineer for structural work.
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Volumes: slabs and footings length × width × depth; columns π r² h; steps width × run × rise × n(n+1) ÷ 2. Bag yields from premixed concrete labels (80 lb 0.6 cu ft, 60 lb 0.45, 40 lb 0.30). Fresh concrete weighs about 4,050 lb per cu yd. Typical thickness: 4 in for patios and walks, 5–6 in for driveways and heavier loads. Rebar laps of about 18 in; welded wire 5 × 10 ft sheets. Structural work must follow your local code.