Calculators / Concrete
Bags or ready-mix
Concrete calculator
Slabs, footings and pads. Enter the dimensions and thickness to get cubic yards and bag counts for every common bag size.
How deep, by project
| Project | Depth | Notes |
|---|---|---|
| Patio or walkway slab | 4 in | Over 4 inches of compacted gravel. Reinforce with mesh or fibre. |
| Driveway slab | 5–6 in | Six inches if anything heavier than a car will park on it. |
| Shed or workshop pad | 4–6 in | Thickened to 8–12 inches at the edges where walls land. |
| Footing under a wall | 8–12 in | Width and depth per local code and frost line. Check before you dig. |
| Fence post hole | Hole depth | A third of the post length in the ground, minimum. Use the circle shape. |
The bag-to-truck crossover
One cubic yard is 45 bags of 80-pound mix — 3,600 pounds of material, hand-mixed. That is the point where nearly everyone should be calling for ready-mix instead, and many people should call well before it.
The practical limit for a single person with a mixer and a wheelbarrow is around 15 to 20 bags in one session, because everything has to be placed and finished before the first batch sets. Past that, either get help or get a truck.
Order in quarter-yard steps
Ready-mix is sold in quarter-cubic-yard increments with a minimum load, usually 1 cubic yard, and a short-load surcharge below 3 or 4 yards. Always round up — being half a wheelbarrow short of finishing a slab is a far worse outcome than paying for a quarter yard you dump in a spare form.
The calculator adds a waste allowance by default. Ten percent is standard for a slab on a prepared base; go higher if the subgrade is uneven, because every dip swallows concrete.
Thickness, base and reinforcement
A 4-inch slab on 4 inches of compacted gravel outperforms a 6-inch slab poured on soft ground. The base does more work than the extra concrete.
This calculator gives you volume only. Reinforcement, control joints, and footing dimensions are structural questions governed by local code — for anything a building sits on, check with your local building department before you pour.
Common questions
How many bags of concrete are in a cubic yard?
Forty-five 80-pound bags, 60 of the 60-pound bags, 72 of the 50-pound, or 90 of the 40-pound. An 80-pound bag yields about 0.6 cubic feet.
How much concrete do I need for a 10x10 slab?
At 4 inches thick, 33.3 cubic feet, or 1.23 cubic yards — about 56 bags of 80-pound mix. That is firmly in ready-mix territory.
How thick should a concrete patio be?
Four inches over a compacted gravel base is standard for foot traffic. Go to 5 or 6 inches if vehicles will ever be on it.
How much extra concrete should I order?
Ten percent over the calculated volume for a slab on a well-prepared base. Running short mid-pour leaves a cold joint that cannot be repaired, only hidden.
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