How concrete volume is calculated
Concrete is sold by volume, in cubic yards. Every estimate comes down to working out the volume of the space you're filling and converting it into that unit — and the conversion is where nearly every mistake happens.
The ÷ 27 catches people out because a yard is only 3 feet — so it feels like the divisor should be 3. But volume scales in three directions at once, so a cubic yard is 3 × 3 × 3 = 27 cubic feet. Skip that step and you'll order 27 times too much concrete.
The second trap is thickness in inches. Slabs are specified in inches while length and width are in feet, so the thickness has to be converted before you multiply. A 4-inch slab is 4 ÷ 12 = 0.333 feet. The calculator above handles this with the per-field unit dropdowns, but if you're doing it by hand, convert first.
Worked example — a 10 × 10 patio slab
A patio 10 ft × 10 ft poured at 4 inches thick.
Convert the thickness: 4 ÷ 12 = 0.333 ft. Volume: 10 × 10 × 0.333 = 33.3 cubic feet.
Convert to yards: 33.3 ÷ 27 = 1.23 cubic yards. Add 10% overage and order 1.36 cubic yards — in practice you'd ask the supplier for 1.5.
Footings and columns
The same principle applies, with the shape changed:
- Strip footing — total run length × width × depth, all in feet, ÷ 27. A 40 ft run at 16 in wide and 8 in deep is 40 × 1.333 × 0.667 = 35.6 cubic feet, or 1.32 cubic yards.
- Round column or Sonotube — π × radius² × height, ÷ 27. Radius is half the diameter, so a 10-inch tube has a 5-inch (0.417 ft) radius.
- Multiple identical pours — calculate one, then multiply by the count. Deck footings are usually a dozen or more identical tubes.
Round columns produce deceptively small numbers. A 10-inch tube 4 feet tall holds only about 0.08 cubic yards — but fifty of them is 4 cubic yards, firmly into ready-mix territory.
Bags or ready-mix?
This is the decision the volume figure is really for, and it's worth getting right because the failure mode is unpleasant: running out of concrete halfway through a pour.
A bag of concrete mix yields far less than people expect once water is added:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 80 lb | ≈ 0.60 cu ft | 45 |
| 60 lb | ≈ 0.45 cu ft | 60 |
| 40 lb | ≈ 0.30 cu ft | 90 |
Sit with that middle row for a second. One cubic yard is 45 eighty-pound bags — that's 3,600 lb of material, near enough 1.8 tons, that has to be lifted, mixed, and placed before the first batch starts setting. For a single person with a wheelbarrow and a mixer, that is not a realistic afternoon.
A workable rule of thumb:
- Under ~0.5 cubic yards — bags are fine. Around 23 bags of 80 lb, manageable for one or two people.
- 0.5 to 1 cubic yard — judgement call. Depends on how many hands you have and whether you own a mixer.
- Over ~1 cubic yard — order ready-mix. Most suppliers have a minimum load and a short-load fee, but it's usually still cheaper than a day of hand-mixing, and vastly more consistent.
There's a quality argument too, not just a labour one. Hand-mixed batches vary — slightly different water in each one — and those variations become visible colour differences and inconsistent strength across the finished slab. Ready-mix arrives as one uniform batch.
Why you always order extra
Order about 10% more than you calculated. This isn't padding; it's the difference between a finished pour and a permanent defect. Three things eat concrete that no formula accounts for:
- The subgrade isn't level. Nobody's gravel base is perfectly flat. Every low spot takes more concrete than the nominal thickness.
- Forms bow outward. Wet concrete is heavy — around 4,000 lb per cubic yard — and formwork flexes under it. A slight bulge across a long form adds real volume.
- Material is lost. Spillage from the chute, residue in the wheelbarrow, waste at the edges.
The reason this matters more than for other materials: if you run out mid-pour, the concrete already placed begins to set before the next batch arrives. The joint between them is a cold joint — a plane of weakness that doesn't bond properly and often cracks along that exact line later. You cannot fix it afterwards. Extra concrete is cheap; a cold joint through a new driveway is not.
If you do end up with surplus, have a use ready before the truck arrives — a spare paver, a post base, a small pad by the shed. Decide in advance, because you'll have about an hour.
How thick should the slab be?
Thickness drives both your volume and whether the slab survives. Standard guidance:
| Application | Typical thickness | Notes |
|---|---|---|
| Patio, walkway, shed base | 4 in | Foot traffic and light loads |
| Driveway, car parking | 5–6 in | Over a compacted base |
| Heavy vehicles, RV pad | 6 in+ | Reinforcement usually required |
| Garage floor | 4–6 in | Depends on vehicle weight |
Going from 4 to 6 inches raises your concrete volume by 50%, so it's a meaningful cost decision — but under-thickness is the more expensive mistake, because a cracked slab is replaced, not repaired. Local building codes take precedence over any rule of thumb, and anything structural should be specified by an engineer rather than a calculator.
One thing worth knowing: the base under the slab matters as much as the thickness. Four inches over a well-compacted gravel base outperforms six inches poured onto soft, uncompacted ground. Concrete fails when it's unevenly supported, not usually because it's thin.
Common mistakes when estimating concrete
Forgetting to divide by 27. The single most common error. Cubic feet and cubic yards differ by a factor of 27, not 3.
Leaving thickness in inches. Convert inches to feet by dividing by 12 before multiplying, or your volume will be twelve times too large.
Using the diameter instead of the radius for columns. The formula needs radius — half the diameter. Using diameter gives four times too much concrete.
Ordering the exact calculated amount. Always add overage. Running short mid-pour creates a cold joint that permanently weakens the slab.
Underestimating bag counts. An 80 lb bag makes only about 0.6 cubic feet. Multi-yard pours by bag are far more work than they appear on paper.