How concrete volume is calculated
Concrete is ordered by volume — cubic yards in the United States and cubic meters almost everywhere else. The basic formula is straightforward: length × width × depth, all in the same units, then converted to your supplier's unit.
volume = length × width × depthFor a rectangular slab measured in feet, the result is cubic feet; divide by 27 to get cubic yards. For metric measurements in meters, the result is already cubic meters.
Conversion factors you will need
1 yd³ = 27 ft³1 yd³ ≈ 0.7646 m³1 m³ ≈ 1.308 yd³1 ft = 12 in = 0.3048 m
Slabs — the common case
A 12 ft × 12 ft patio slab at 4 in thick:
- Depth:
4 in / 12 = 0.333 ft - Volume:
12 × 12 × 0.333 = 48 ft³ - Cubic yards:
48 / 27 ≈ 1.78 yd³
Order in 0.25 yd³ increments at most ready-mix plants. Adding 10% for waste, spillage, and form give-out (concrete bulging beyond formwork) lands at about 2.0 yd³.
Footings, posts, and columns
For circular post holes, use the cylinder formula:
volume = π × radius² × depthA 12 in diameter (6 in radius = 0.5 ft) post hole 36 in deep:
- Volume:
π × 0.5² × 3 ≈ 2.36 ft³ per hole - 20 such holes:
47.1 ft³ ≈ 1.75 yd³, plus 10% waste = 2 yd³.
Strip footings under a foundation wall are rectangular and use the slab formula. Standard residential footings are 16–24 in wide and 8–12 in thick.
Stairs
A staircase is a sum of rectangular blocks — one for each step. Calculate each step as length × tread × rise. Many builders simplify by treating the staircase as a triangular prism plus the final block. For 5 steps with 36 in tread × 11 in run × 7 in rise:
- Each step:
3 ft × (0.917 ft × 0.583 ft / 2 + step block above) - Easier: model the stair envelope (length × tread + run × height + rise) and multiply by the width.
For a quick estimate: a 4-foot-wide, 5-step set of stairs takes roughly 1.0–1.2 yd³.
Concrete strength and mix selection
| Application | Typical strength (psi) | Notes |
|---|---|---|
| Foundation walls, footings | 2,500–3,000 | Most residential, code-compliant |
| Driveways, garage slabs | 3,500–4,000 | Resists vehicle loads and freeze-thaw |
| Sidewalks, patios | 3,000–3,500 | Lower load, but consider freeze-thaw |
| Commercial slabs | 4,000–5,000 | Higher load, longer service life |
| High-performance / piers | 5,000–6,000+ | Bridges, high-rise columns |
Cold-region pours need air-entrained concrete (4–7% air content) to resist freeze-thaw damage. Cure time matters: concrete reaches 70% of design strength at 7 days and ~95% at 28 days. Light foot traffic is usually OK at 24 hours, vehicles at 7 days.
Bag mixes vs ready-mix
For volumes under about 0.5 yd³ (~13.5 ft³), bagged mix is usually more economical:
- 40 lb bag ≈ 0.30 ft³ → about 90 bags per yard.
- 60 lb bag ≈ 0.45 ft³ → about 60 bags per yard.
- 80 lb bag ≈ 0.60 ft³ → about 45 bags per yard.
Above ~1 yd³, ready-mix delivery is cheaper per yard and avoids the mixing labor. Most plants charge a "short load" fee below 3–4 yd³ that can substantially raise the per-yard cost.
Add-ons and overage
- Waste factor — add 5–10% over the calculated volume. Spilled mix, uneven forms, and pump losses add up.
- Reinforcement — rebar or mesh adds no concrete volume but is required for almost every structural pour.
- Vapor barrier — under interior slabs (basement floors, garages) to block ground moisture.
- Sub-base — 4–6 in of compacted gravel under slabs in most climates.
- Curing — keep moist for at least 3 days; cover with plastic or use curing compound in hot or windy conditions.