Cubic feet
36.7
Concrete Calculator
How much concrete you need for a slab, footing or round column — in cubic yards, cubic feet and bags.
A driveway or patio is usually 4 in. A slab for heavy vehicles, 6 in.
The width of the hole or tube, not the radius.
Same size each.
1.36 cubic yards
Including 10% waste. Order 1.5 yards if the plant sells in half yards.
Bags needed
62
Estimated cost
—
Without waste
1.23 yd³
Check the bag before you buy. Bag counts here use the yields printed
on standard Quikrete and Sakrete bags: 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. Other brands differ. The yield is printed on the
bag — if yours says something else, use that number.
How this is worked out
Concrete is sold by the cubic yard. Everything else is a conversion.
Slab, footing or wall
length (ft) × width (ft) × thickness (ft) = cubic feet
Then cubic feet ÷ 27 = cubic yards, because a cubic yard is 3 ft × 3 ft
× 3 ft, which is 27 cubic feet.
Round column or pad
3.14159 × radius (ft)² × height (ft) = cubic feet
The radius is half the diameter. Measure the hole across, then halve it.
Worked example — a 10 ft × 10 ft patio, 4 in thick
- Thickness in feet:
4 ÷ 12 = 0.3333 ft - Volume:
10 × 10 × 0.3333 = 33.33 cubic feet - In yards:
33.33 ÷ 27 = 1.23 cubic yards - Add 10% waste:
1.23 × 1.10 = 1.36 cubic yards - In 80 lb bags:
36.67 ÷ 0.60 = 61.1, so 62 bags
Bags per cubic yard
If you are buying bags rather than ordering a truck, this is the number that matters.
| Bag | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.60 cu ft | 45 |
Past about half a cubic yard, bags stop making sense. That is roughly 23 bags of 80 lb, mixed by hand, one at a time. Most people order ready-mix at that point.
Questions people ask
Why add a waste allowance?
Ground is never perfectly flat, forms bulge, and some concrete stays in the barrow. Running short mid-pour is far worse than having a little left over, because the joint between old and new concrete is a weak point. 10% is the normal allowance. Use 5% on flat, well-formed ground and 15% on rough or uneven ground.
How thick should a slab be?
4 inches for a patio, shed base, footpath or a normal car driveway. 5 to 6 inches where trucks, trailers or heavy machinery will sit on it. Thicker than that is a question for an engineer, not a calculator.
Can I order half a yard?
Most ready-mix plants sell in quarter or half yard steps and charge a short-load fee below about 1 yard. Ring the plant and ask before you round. If you are only a fraction over a whole yard, it is often cheaper to order the whole yard than to pay the short-load fee on a second delivery.
How much does a cubic yard of concrete weigh?
About 4,000 lb — two tons. That matters for what the ground and the barrow can take, and for how many trips mixing by hand actually means.
Does this include rebar, mesh or gravel base?
No. This is the concrete volume only. A slab normally sits on a compacted gravel base, and you work that out separately — the gravel calculator does it.