Quick answer
To calculate concrete, multiply length × width × thickness (all in feet) for cubic feet, then divide by 27 for cubic yards. A 10 × 10 slab at 4 inches needs 33.3 cubic feet ÷ 27 = 1.23 cubic yards. Add 5–10% for waste. One cubic yard equals about 45 eighty-pound bags of concrete.
The concrete calculation formula
The formula for calculating concrete volume is:
Cubic yards = (Length × Width × Thickness in feet) ÷ 27
Convert inches to feet first by dividing by 12 — a 4-inch slab is 0.333 feet. The shortcut formula: cubic yards = square feet × thickness (inches) ÷ 324. For 100 sq ft at 4 inches: 100 × 4 ÷ 324 = 1.23 cubic yards.
How to calculate concrete for a slab
For a rectangular slab, measure length and width in feet, decide on thickness (typically 4 inches for patios, 6 inches for driveways), and calculate:
- Multiply length × width × thickness (in feet) for cubic feet.
- Divide by 27 for cubic yards.
- Add 5–10% for waste and uneven subgrade.
- Decide between ready-mix delivery and bagged concrete.
Example: A 12 × 12 patio slab at 4 inches: 12 × 12 × 0.333 = 47.9 cubic feet ÷ 27 = 1.78 cubic yards. With 10% waste, order 1.95 cubic yards.
Concrete bag calculator
For small projects, bagged concrete is convenient. One cubic yard (27 cubic feet) requires:
| Bag size | Yield per bag | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 cu ft | 90 bags |
| 60 lb | 0.45 cu ft | 60 bags |
| 80 lb | 0.60 cu ft | 45 bags |
Example: A 1.78 cubic yard patio needs about 80 eighty-pound bags (1.78 × 45). At $5 per bag, that’s $400 — ready-mix at $150/cu yd would cost about $300 delivered.
Ready-mix vs. bagged concrete
| Factor | Ready-mix | Bagged |
|---|---|---|
| Cost per cu yd | $100–$200 | $200–$300 |
| Minimum order | 1–3 cu yd | 1 bag |
| Mix consistency | Excellent (plant-mixed) | Varies (hand-mixed) |
| Setup time | Minutes (must pour fast) | Hours (mix as you go) |
| Best for | Slabs > 1.5 cu yd | Small jobs, repairs |
| Strength | Controlled (3,000–4,000 PSI) | Typically 4,000 PSI |
Ready-mix is almost always better for projects over 1.5 cubic yards. It’s cheaper, stronger, and produces a consistent pour. Bagged concrete wins for small repairs, fence posts, and projects where a delivery truck can’t reach the site.
Concrete cost per cubic yard
| Option | Cost | Notes |
|---|---|---|
| Ready-mix (delivered) | $100–$200/cu yd | Plus $150–$300 minimum order fee |
| Bagged (80 lb) | $5/bag ($225/cu yd) | 45 bags per cubic yard |
| Bagged (60 lb) | $4/bag ($240/cu yd) | 60 bags per cubic yard |
| Concrete delivery fee | $50–$150 | Distance-dependent |
| Short-load fee | $50–$100 | For orders under minimum |
Example: A 3-cubic-yard driveway approach at $130/cu yd = $390 + $100 delivery = $490 total. The same project bagged (135 eighty-pound bags at $5) would cost $675 — and you’d mix 135 bags by hand.
Concrete for footings and columns
For footings, calculate the trench volume: length × width × depth ÷ 27. A 20-foot footing at 18 inches wide × 8 inches deep: 20 × 1.5 × 0.667 = 20 cubic feet ÷ 27 = 0.74 cubic yards.
For round columns, use the cylinder formula: cubic yards = (π × radius² × height) ÷ 27. A 12-inch column (0.5 ft radius) 8 feet tall: π × 0.25 × 8 = 6.28 cubic feet ÷ 27 = 0.23 cubic yards.
Frequently asked questions
Related calculators
For the volume math behind concrete, use the cubic yard calculator. For the area of a slab, try the square footage calculator. For the gravel base under a slab, see the gravel calculator.