How to calculate concrete
Calculating concrete comes down to one thing: volume. Here is the exact method — the same math our calculators use — for slabs, footings, and round holes, plus how to turn that volume into cubic yards or bags.
The concrete formula
What is the concrete formula?
Concrete is sold by volume, so every estimate starts with volume = length × width × thickness, with every dimension in the same unit (feet). A 10 ft × 12 ft slab poured 4 inches thick is 10 × 12 × 0.333 = 40 cubic feet. Because a cubic yard is 27 cubic feet, you then divide by 27 to get the figure ready-mix suppliers actually quote: 40 ÷ 27 ≈ 1.48 cubic yards. For a round hole — a post footing or pier — swap the rectangle for a cylinder: volume = π × radius² × depth. A 12-inch-diameter hole 3 feet deep is 3.14159 × 0.5² × 3 ≈ 2.36 cubic feet.
Step by step
How do I calculate concrete step by step?
Work in one unit system, convert thickness before multiplying, and only round at the end. Rounding every intermediate step makes small pours look cleaner on paper but can undercount bags or yards on the final order.
- Measure the length, width, and thickness of your pour. Use the same unit for length and width (feet), and inches for thickness.
- Convert thickness to feet by dividing inches by 12 (for example, 4 inches = 0.333 feet).
- Multiply length × width × thickness to get the volume in cubic feet.
- Divide cubic feet by 27 to get cubic yards, the unit ready-mix suppliers use.
- Add a waste factor of about 10% so you do not run short.
- To buy bags instead, divide the waste-adjusted cubic feet by the bag yield (0.60 for 80 lb) and round up.
A worked example you can check
How many cubic yards are in a 12 by 12 slab?
Say you are pouring a 12 ft × 12 ft patio at 4 inches. Volume is 12 × 12 × (4 ÷ 12) = 48 cubic feet. Add a 10% waste factor and you have 52.8 cubic feet, or 52.8 ÷ 27 ≈ 1.96 cubic yards. If you would rather mix bags, divide the 52.8 cubic feet by the 80 lb bag yield of 0.60 cubic feet: that is 88 bags, rounded up — clearly the point where ready-mix delivery becomes the sensible choice.
Imperial vs metric
The arithmetic is identical; only the units change. In imperial, keep length and width in feet and thickness in inches, then divide cubic feet by 27 for cubic yards. In metric, work in meters and divide thickness in centimeters by 100 to get meters, which gives cubic meters directly. If you need to convert, 1 cubic meter ≈ 35.31 cubic feet ≈ 1.31 cubic yards. Mixing units mid-calculation — feet with centimeters, say — is the single most common source of a wildly wrong answer.
What waste factor should you use?
A waste factor covers spillage, uneven sub-grade, over-excavation, and the concrete that clings to the mixer and wheelbarrow. For most jobs 10% is a safe default. Tightly formed pours on a flat, compacted base can use 5%. Rough or sloping ground, hand-dug footings, and collapsing soil can justify 15–20%. Ready-mix is hard to top up once the truck leaves, so it is far cheaper to round up than to order a second small load and pay a short-load fee.
Check form depth before ordering
Measure the actual formed depth in several places, not just the plan thickness. A slab specified as 4 inches can become 4.5 or 5 inches if the base is low, the gravel is uneven, or the forms settle. That extra half inch adds about 12.5% more concrete across the whole slab, which is larger than many default waste factors. If the depth varies, use the deepest practical average rather than the thinnest edge.
How many bags of concrete are in a yard?
It depends on the bag, because each weight yields a different volume of mixed concrete. These are common planning figures — the authoritative number is printed on the bag and varies by brand and mix.
| Bag size | Approx. 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 |
Estimates only · not official manufacturer figures · always round bag counts up.
When to switch from bags to ready-mix
Bagged concrete wins on convenience for small jobs — a few post holes, a small pad, a repair. Once you pass roughly one cubic yard (about 45 × 80 lb bags), the mixing labor, the cost, and the risk of cold joints between batches all point to ready-mix delivery. Remember that suppliers usually have a minimum order and charge a short-load fee below about one yard, so price both routes before deciding.
Common mistakes to avoid
- Forgetting the waste factor entirely, then running short mid-pour.
- Measuring thickness in inches but treating it as feet (a 12× error).
- Ignoring a thickened edge or footing — those add real volume on a slab.
- Under-measuring hole depth; frost-depth footings are deeper than they look.
- Rounding bag counts down — you cannot buy part of a bag.
Let the calculator do it
Skip the arithmetic with the concrete calculator, or jump straight to the slab calculator, the bag calculator, or the post-hole calculator. For bag sizes and yields in detail, see the concrete bag sizes guide.