Hardscape
Concrete Calculator
Get cubic yards and exact bag counts for slabs, footings, cylindrical piers, and stairs, plus whether mixing bags or ordering ready-mix makes sense at your volume.
Updated
Dimensions
4 in for patios and walkways, 5–6 in where vehicles park.
10% is standard. Subgrade is never perfectly flat, and running short mid-pour creates a permanent cold joint.
Concrete needed
44 cu ft (1.25 m³), including 10% overage
- 80 lb (36.3 kg) bags
74
- 60 lb (27.2 kg) bags
98
- Ready-mix order
1.75cu yd
1.34 m³
Bag options
40 lb (18.1 kg) bag
50 lb (22.7 kg) bag
60 lb (27.2 kg) bag
80 lb (36.3 kg) bag
Ready-mix delivered
| Bag size | Bags needed | Approx cost |
|---|---|---|
| 40 lb (18.1 kg) bag | 147 | $735 |
| 50 lb (22.7 kg) bag | 118 | $708 |
| 60 lb (27.2 kg) bag | 98 | $637 |
| 80 lb (36.3 kg) bag | 74 | $555 |
| Ready-mix delivered | 1.75 cu yd (1.34 m³) | $245–$385 |
Under 3 cubic yards most ready-mix suppliers add a short-load fee, often $100 or more. Worth pricing against bags at this size.
Call 811 (or your local utility locate service) before digging footings or post holes. It is free, and required in most of the US.
Standard bag yields
Yields are consistent across brands: a 40 lb bag yields 0.30 cu ft, 50 lb yields 0.375, 60 lb yields 0.45, and 80 lb yields 0.60. That makes 45 eighty-pound bags per cubic yard.
Weight figures use 150 lb per cubic foot, the standard density for normal-weight concrete.
How this is calculated
Volume, then bags
Concrete volume is length × width × thickness, all in feet, then divided by 27 for cubic yards. The thickness is the part people convert wrong, a 4 inch slab is 0.333 ft, not 4.
Bag yields are printed on the bag and are consistent across brands: a 60 lb bag yields 0.45 cubic feet, and an 80 lb bag yields 0.60. Divide your volume in cubic feet by the yield to get the count.
cubic yards = (length ft × width ft × thickness in ÷ 12) ÷ 27
| Bag | Yield | Bags per cu yd | Bags per 4 in slab sq ft |
|---|---|---|---|
| 40 lb | 0.30 cu ft | 90 | 0.11 |
| 50 lb | 0.375 cu ft | 72 | 0.09 |
| 60 lb | 0.45 cu ft | 60 | 0.074 |
| 80 lb | 0.60 cu ft | 45 | 0.056 |
Bags or ready-mix?
The crossover is around one cubic yard. Below that, bags are simpler and cheaper once you account for short-load fees. Above two yards, bags become genuinely impractical, a single cubic yard is 45 eighty-pound bags, which is 3,600 lb to carry and mix.
Ready-mix suppliers typically charge a short-load fee below about 3 yards, which can add $100 or more. There is also a hard constraint bags do not have: a large slab must be placed in one continuous pour, and a mixer cannot keep up with a truck.
Always order over
Add 10% to any concrete order. Subgrade is never perfectly flat, forms bow slightly under load, and the difference between a 3.9 inch and 4.1 inch average slab is real volume.
Running short mid-pour is the one failure you cannot fix. The first batch starts curing while you mix more, and you end up with a cold joint, a permanent plane of weakness through the slab.
Worked example
A 4-inch shed slab
A 10 ft × 12 ft slab poured 4 inches thick, with a 10% overage allowance.
- Volume
- 10 × 12 × (4 ÷ 12) = 40 cu ft
- With 10% waste
- 44 cu ft
- Cubic yards
- 44 ÷ 27 = 1.63 cu yd
- 80 lb bags
- 44 ÷ 0.60 = 74 bags
- Weight in bags
- 74 × 80 = 5,920 lb
1.75 yards of ready-mix, or 74 eighty-pound bags. At nearly three tons of bags, ready-mix wins here even carrying a short-load fee.
Frequently asked questions
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