The Volume Formula
Concrete is ordered by volume, so every calculation reduces to finding the volume of a shape and converting it into the units your supplier uses.
Slab or footing: Volume = length × width × thickness
Round column: Volume = π × (diameter ÷ 2)2 × height
Stairs: sum of each step's rectangular block, since each step sits on the ones below it
The unit trap is thickness. Slabs are specified in inches while length and width are in feet, so the thickness must be divided by 12 before multiplying. Forgetting this overstates the order by a factor of twelve.
Volume Units
| Unit | Cubic feet | Cubic yards | Cubic metres |
|---|---|---|---|
| 1 cubic foot | 1 | 0.037 | 0.0283 |
| 1 cubic yard | 27 | 1 | 0.7646 |
| 1 cubic metre | 35.31 | 1.308 | 1 |
North American ready-mix is sold by the cubic yard; most of the rest of the world uses cubic metres. One cubic yard of concrete weighs roughly 4,000 lb (1,815 kg), which is why a full truck carrying 8–10 yards weighs over 30 tonnes.
Cubic Yards for a Slab
By area and thickness, before any waste allowance:
| Thickness | 50 ft² | 100 ft² | 200 ft² | 400 ft² | 800 ft² |
|---|---|---|---|---|---|
| 3″ | 0.46 | 0.93 | 1.85 | 3.70 | 7.41 |
| 4″ | 0.62 | 1.23 | 2.47 | 4.94 | 9.88 |
| 5″ | 0.77 | 1.54 | 3.09 | 6.17 | 12.35 |
| 6″ | 0.93 | 1.85 | 3.70 | 7.41 | 14.81 |
| 8″ | 1.23 | 2.47 | 4.94 | 9.88 | 19.75 |
Bag Yields
Pre-mixed bags are practical for small jobs. Yields are approximate and vary slightly by manufacturer.
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
The crossover point is roughly one cubic yard. Below that, bags usually win on cost and convenience; above it, ready-mix is cheaper and vastly less work — 45 eighty-pound bags is 3,600 lb of material to carry and mix by hand.
Why Add a Waste Allowance
Ordering the exact calculated volume reliably leaves you short. Sources of loss include:
- Uneven subgrade. A slab poured over ground that dips even half an inch consumes noticeably more than the nominal thickness.
- Spillage and residue in the truck, chute, wheelbarrows and tools.
- Over-excavated footings, which are almost always wider than drawn.
- Form deflection. Wet concrete is heavy and pushes forms outward.
Ten percent is the standard allowance for slabs and 10–15% for footings on rough ground. Running short mid-pour is far worse than a small surplus: a cold joint forms where fresh concrete meets partly set concrete, and it is a permanent weakness.
Concrete Strength
| Strength | Metric | Typical use |
|---|---|---|
| 2,500 psi | ~17 MPa | Non-structural fill, footings in mild climates |
| 3,000 psi | ~21 MPa | Residential slabs, patios, driveways |
| 3,500–4,000 psi | ~24–28 MPa | Driveways in freeze-thaw climates, garage floors |
| 4,500–5,000 psi | ~31–35 MPa | Structural work, commercial floors |
| 6,000 psi and above | 41+ MPa | High-rise columns, specialist applications |
Strength is measured at 28 days, by which point concrete has reached about 99% of its design strength. It gains roughly 70% in the first week.
Practical Notes
- Base preparation matters more than thickness. A well-compacted 4-inch gravel base under a 4-inch slab outperforms a 6-inch slab on soft ground.
- Control joints should be cut within 6–18 hours of the pour, to a depth of a quarter of the slab thickness, at spacings of about 24–30 times the thickness in feet. Concrete will crack; joints decide where.
- Do not add water on site. Extra water improves workability and permanently reduces strength — roughly 5% lost per extra gallon per cubic yard.
- Cure it. Keeping the surface damp for seven days can add 50% to the final strength compared with letting it dry out.
- Temperature. Below 5°C hydration effectively stops; above 32°C the mix sets too quickly to finish properly.
Frequently Asked Questions
How thick should a slab be?
Four inches for patios, walkways and most residential floors; five to six inches for driveways carrying vehicles; more for heavy equipment. Local codes take precedence.
How many bags in a cubic yard?
About 90 forty-pound bags, 60 sixty-pound bags or 45 eighty-pound bags. The table above shows the exact figures.
Can I order a partial cubic yard?
Most suppliers sell in quarter-yard increments with a minimum charge of one to two yards, plus a short-load fee below the minimum.
How long before I can use it?
Walk on it after 24–48 hours, drive a car on it after 7 days, and allow full loads after 28 days. Cold weather extends all of these.