How to calculate cubic feet of concrete
Concrete volume is the geometry of the form you'll fill: length × width × depth for a slab, π × radius² × height for a column, and so on. The trick is keeping units consistent — most slab thicknesses are in inches but the formula expects feet.
Quick depth conversions:
- 2 inches = 0.167 ft (uncommon for slabs, used for overlays)
- 3 inches = 0.25 ft (sidewalks, light-duty)
- 4 inches = 0.333 ft (driveways, patios — most common)
- 6 inches = 0.5 ft (heavy-duty driveways, garage floors)
- 8 inches = 0.667 ft (foundations, footings)
Common slab sizes
| Slab | Conversion factor | Worked example |
|---|---|---|
| 4 × 4 ft (small pad) | 4 × 4 × 0.333 | 5.33 ft³ (0.20 yd³) |
| 6 × 8 ft (utility shed) | 6 × 8 × 0.333 | 16 ft³ (0.59 yd³) |
| 10 × 10 ft (small patio) | 10 × 10 × 0.333 | 33.3 ft³ (1.23 yd³) |
| 12 × 12 ft (single-car parking) | 12 × 12 × 0.333 | 48 ft³ (1.78 yd³) |
| 20 × 20 ft (driveway) | 20 × 20 × 0.333 | 133 ft³ (4.94 yd³) |
| 24 × 24 ft (two-car garage floor) | 24 × 24 × 0.333 | 192 ft³ (7.11 yd³) |
Bagged versus pre-mixed delivery
For projects under ~30 ft³ (~1 cubic yard), bagged concrete from a hardware store is usually the cheapest path. Above that, pre-mixed truck delivery is faster, more consistent, and less labor.
- 80 lb bag — yields ~0.6 ft³ (covers 1.8 ft² at 4 inches thick)
- 60 lb bag — yields ~0.45 ft³ (covers 1.4 ft² at 4 inches thick)
- 40 lb bag — yields ~0.30 ft³ (covers 0.9 ft² at 4 inches thick)
- 1 cubic yard truckload — 27 ft³ = 45 bags of 80 lb or 60 bags of 60 lb concrete
Worked examples
Example 1: 12 × 12 ft patio at 4 inches thick
12 × 12 × 0.333 ≈ 48 ft³ (1.78 yd³). That's 80 bags of 80 lb concrete — at this size, ordering a 2-yard short load is usually faster and not much more expensive.
Example 2: 100 ft of footings, 12 in × 8 in
Cross-section: 1 ft × 0.667 ft. Volume: 100 × 1 × 0.667 = 66.7 ft³ (2.47 yd³). Order 2.5 yd³ to include 2% waste.
Example 3: A 12 inch diameter post hole, 4 ft deep
Cylinder: π × 0.5² × 4 ≈ 3.14 ft³. Six bags of 80 lb concrete per post.
Common mistakes to avoid
- Forgetting to convert thickness to feet. A 4-inch slab is 0.333 ft thick, not 4. Multiplying by 4 gives 12 times too much concrete.
- Skipping the waste factor. Order 5–10% extra for spills, uneven ground, and form bulge.
- Mixing bag yields with bag weight. A 60 lb bag is not three-fourths the volume of an 80 lb bag — yield depends on cement-to-water ratio. Use the bag's labeled cubic-foot yield.
- Calculating piers as cubes. Most piers are cylinders. Use π × r² × h, not L × W × H.
Related concepts and calculators
Concrete planning crosses several units. These pages cover the surrounding math:
- Cubic feet to cubic yards — the unit ready-mix trucks deliver in.
- Cubic feet to pounds — concrete is 150 lb/ft³, so weight matters for foundation and floor checks.
- Gravel calculator — most slabs need a compacted gravel base.
- Sand calculator — bedding sand for paver bases and underslab fill.
- Volume formulas — for footings, piers, or curved pours that go beyond a flat slab.
Frequently asked questions
Related calculators
Calculate cubic feet of gravel and convert to tons for ordering by weight.
OpenCalculate cubic feet of sand for sandboxes, paver bases, and projects.
OpenCalculate the cubic feet of any rectangular box, room, or container using length, width, and height.
OpenCalculate the cubic feet of a cylinder from radius or diameter and height. Mixed-unit support.
OpenConvert cubic feet to cubic yards instantly. 27 ft³ = 1 yd³.
Open