Planning a patio, shed base or driveway? Enter the slab’s length, width and thickness to see how much concrete to order in cubic yards, or how many premix bags to buy.
The concrete formula
Cubic yards = Length (ft) × Width (ft) × Thickness (in) ÷ 12 ÷ 27
Dividing thickness by 12 converts inches to feet, giving cubic feet. Dividing by 27 converts cubic feet to cubic yards.
Example
A 10 × 10 ft slab, 4 inches thick: 10 × 10 × 4 ÷ 12 = 33.3 cubic feet. Add 10% for spillage and uneven ground and you need 36.7 cu ft, or 1.36 cubic yards – about 62 bags of 80 lb premix.
Bag yields
| Bag size | Yield | Bags per cubic yard |
|---|---|---|
| 80 lb | 0.60 cu ft | 45 |
| 60 lb | 0.45 cu ft | 60 |
| 40 lb | 0.30 cu ft | 90 |
Typical slab thickness
- 4 inches – patios, walkways, shed floors.
- 5–6 inches – driveways for cars and light trucks.
- 6–8 inches – heavier vehicles or garage floors for RVs.
Bags or ready-mix?
For under about a cubic yard, bags are practical. Above that, ready-mix delivered by truck is usually cheaper per yard and far less work – though many suppliers have a minimum order or short-load fee.
Concrete needed for common slabs (4 inches thick)
| Slab size | Cubic yards | 80 lb bags |
|---|---|---|
| 4 × 4 ft (AC pad) | 0.20 | 9 |
| 8 × 10 ft (shed) | 0.99 | 45 |
| 10 × 10 ft (patio) | 1.23 | 56 |
| 12 × 12 ft | 1.78 | 80 |
| 20 × 20 ft (2-car pad) | 4.94 | 223 |
(No waste allowance – add 5–10% when ordering.)
Footings, posts and columns
- Round post holes: volume = π × radius² × depth. A 10-inch-wide, 3-foot-deep hole holds about 1.6 cu ft – roughly three 60 lb bags with the post in place.
- Continuous footings: length × width × depth. A 20 ft footing 12 inches wide and 12 inches deep is 20 cu ft (0.74 cu yd).
- Stairs: calculate each step as a block and add them together.
Metric quick guide
Volume in cubic metres = length (m) × width (m) × thickness (m). A 3 m × 3 m slab, 100 mm thick, needs 0.9 m³ – about 1 m³ with waste. One cubic metre of concrete weighs about 2,400 kg.
Mixing ratios for small jobs
If you’re mixing from scratch rather than using premix, a common general-purpose ratio by volume is 1 part cement : 2 parts sand : 3 parts gravel, with just enough water for a workable mix. Too much water weakens concrete, so add it gradually.
Tips for a strong slab
- Compact the sub-base (gravel) well and keep it level.
- Use reinforcement – wire mesh or rebar – for driveways and larger slabs.
- Cut control joints about every 8–12 ft on a 4-inch slab to manage cracking.
- Keep the slab moist for about a week while it cures; avoid pouring in freezing weather.
Real-life scenarios
- Hot tub pad: An 8 × 8 ft pad, 4 inches thick, needs 0.79 cu yd – about 40 bags of 80 lb mix with 10% extra.
- Fence posts: Twelve posts in 10-inch holes, 3 ft deep, need roughly 19 cu ft of concrete – about 33 bags of 80 lb mix (a little less once the posts are in).
- Driveway: A 12 × 40 ft driveway at 5 inches is 7.4 cu yd – order 8 yards of ready-mix with fibre or mesh reinforcement.
- Shed base: A 10 × 12 ft shed on a 4-inch slab needs 1.48 cu yd; with waste, 1.6 yards. Many suppliers deliver “short loads” for a fee.
Ordering ready-mix concrete
Tell the supplier the volume (rounded up to the nearest quarter or half yard), the strength you need (commonly 3,000–4,000 psi for residential slabs), and whether you need fibre, air entrainment for freeze–thaw climates, or a pump. Have forms, reinforcement and helpers ready before the truck arrives – trucks charge for waiting time.
Related calculators
- Square footage calculator – measure the slab area
- Tile calculator – tile the finished slab
- Paint calculator – finish the shed or garage
FAQs
How many bags of concrete are in a yard?
About 45 bags of 80 lb premix, 60 bags of 60 lb or 90 bags of 40 lb.
How long before I can walk on new concrete?
Light foot traffic after about 24–48 hours; vehicles after around 7 days. Concrete reaches most of its design strength in about 28 days.
How much does a cubic yard of concrete weigh?
About 4,000 lb (roughly 1.8 tonnes).
How many cubic metres is a cubic yard?
1 cubic yard = 0.7646 m³. The calculator shows both.