Concrete
Slab, footing or post holes — the volume, and the bag count that actually fills it.
Your buy list
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80 lb bags · 0 yd³- Volume
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- Cubic yards
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- Weight mixed
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- Water needed
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Waste included — subgrade is never flat.
Bags or a truck
Concrete is sold two ways and the crossover is about one cubic yard. Under that, bags win: you mix what you need, when you need it, and an unopened bag goes back to the store. Over it, bags stop being a saving and start being a problem — 45 of them is a full day of mixing, and concrete placed over concrete that has already stiffened makes a cold joint that will crack along its line.
The volume itself is simple: length times width times thickness, all in the same units, then divided by 27 to get cubic yards. What people get wrong is the thickness — a "4 inch" slab poured over ground that dips an inch is a 5 inch slab in places, which is where the 10 % waste allowance goes. Order the truck a quarter yard over, never under.
What a slab needs underneath
| Pour | Thickness | Base | Reinforcement |
|---|---|---|---|
| Walkway | 4 in | 4 in compacted gravel | Fibre mesh or none |
| Patio | 4 in | 4 in compacted gravel | Wire mesh |
| Driveway | 5–6 in | 6 in compacted gravel | #3 rebar at 18 in |
| Garage floor | 5 in | 4–6 in gravel + vapour barrier | Wire mesh or rebar |
| Shed pad | 4 in | 4 in compacted gravel | Wire mesh |
The gravel under a slab is its own calculation — the gravel calculator takes the same footprint and a depth and gives you the tonnage the quarry will invoice.
Post holes
Switch the shape above to round and the form counts holes instead of slabs. A 10 inch hole 3 feet deep holds about 1.6 cubic feet of concrete, 1.8 with waste — three 80 pound bags — and a six-post fence line is most of a pallet. Dig below the frost line where you have one; a post set above it will heave itself loose within a few winters.
On the counter
The assumptions this calculator ships with — every one is adjustable in the form.
- Bag yield
- 80 lb = 0.6 cu ft mixed, 60 lb = 0.45, 50 lb = 0.375, 40 lb = 0.30
- Waste
- 10 % by default — subgrade is never as flat as the plan
- Ready-mix
- Suggested past one cubic yard; suppliers sell in quarter-yard steps
- Not included
- Rebar, mesh, gravel base and forms — those are separate buys
Questions from the aisle
How many bags of concrete for a 10x10 slab?
At 4 inches thick that is 33.3 cubic feet, or 36.7 with 10% for uneven subgrade — 62 eighty-pound bags, about 1.36 cubic yards. That is well past the point where a ready-mix truck is cheaper and vastly easier: a yard and a third of concrete is not a wheelbarrow job.
How many 80 lb bags in a cubic yard?
Forty-five. An 80 pound bag yields about 0.6 cubic feet mixed, and a cubic yard is 27 cubic feet. Sixty-pound bags yield 0.45, so it takes 60 of those.
When should I order ready-mix instead?
Around a cubic yard — roughly 45 bags. Past that you are mixing for hours while the first pour sets, which is how cold joints happen. Most suppliers have a one-yard minimum and a short-load fee that is still cheaper than a failed slab.
How deep should a post hole be?
A third of the post length or below the frost line, whichever is deeper, and three times the post width across. A 4x4 usually gets a 10 or 12 inch hole; switch the form above to round pours and it will count the bags per hole.