Quick answer: multiply length × width × thickness (all in feet) to get cubic feet, and divide by 27 for cubic yards. Add 5–10% extra for spillage and uneven ground. A 10 × 10 ft slab, 4” thick, needs about 1.23 cubic yards: 56 bags of 80 lb premix before waste, or about 62 bags with 10% extra. The concrete calculator works it out for slabs and post holes.
The formulas
Slab, patio, path or footing:
volume = length × width × thickness
Convert inches to feet first: 4” = 0.333 ft, 6” = 0.5 ft. In metric, use metres, so 100 mm = 0.1 m.
Round post hole:
volume = π × radius² × depth
For fence posts, subtract the post’s own volume, as our fence calculator does automatically.
Then convert to what you’ll buy:
- Cubic yards (for ready-mix in the US) = cubic feet ÷ 27
- Bags = volume ÷ bag yield, rounded up
How much does a bag of concrete make?
| Bag | Typical yield | Bags per cubic yard |
|---|---|---|
| 80 lb | about 0.60 cu ft | 45 |
| 60 lb | about 0.45 cu ft | 60 |
| 40 lb | about 0.30 cu ft | 90 |
| 25 kg | about 0.0115 m³ | about 87 per m³ |
| 20 kg | about 0.0092 m³ | about 109 per m³ |
Yields vary a little by brand, and the exact figure is printed on the bag.
Worked examples
| Job | Volume | Bags (with 10% extra) |
|---|---|---|
| 10 × 10 ft slab, 4” thick | 33.3 cu ft (1.23 yd³) | 62 × 80 lb, or 82 × 60 lb |
| 12 × 12 ft patio, 4” thick | 52.8 cu ft with extra (1.96 yd³) | 88 × 80 lb |
| 20 × 20 ft garage pad, 6” thick | 220 cu ft with extra (8.1 yd³) | Order ready-mix |
| 3 × 3 m slab, 100 mm thick | 0.99 m³ with extra | 87 × 25 kg |
| One 10” × 36” post hole | 1.6 cu ft | 3 × 80 lb (minus the post) |
How thick should a slab be?
| Use | Typical thickness |
|---|---|
| Garden path, shed base, AC pad | 4” (100 mm) |
| Patio | 4” (100 mm) |
| Driveway for cars | 4–6” (100–150 mm), with reinforcement |
| Heavy vehicles, RVs | 6” (150 mm) or more |
Local building codes and soil conditions can require more, so check before you pour a driveway or garage slab. Most slabs also go on a compacted gravel base of about 4” (100 mm), which you don’t count in the concrete volume.
Bags or ready-mix?
- Under about 1 cubic yard (0.75 m³): bags are practical. That’s up to about 45 × 80 lb bags.
- 1–2 cubic yards: possible with bags and a rented mixer, but it’s hard work, and you need to pour before the first batch starts setting.
- Over about 2 cubic yards: order ready-mix. Ask about short-load fees for small orders. Some suppliers also offer mini-mix trailers or volumetric trucks for small jobs.
Mixing 60+ bags by hand is a long, heavy day. Recruit help, or rent a mixer.
Tips for a good pour
- Order or buy 5–10% extra. Running short mid-pour leaves a weak cold joint.
- Check the forms are level and braced, and measure the depth in several places. A slab that’s ½” thicker than planned uses noticeably more concrete.
- Add reinforcement (wire mesh or rebar) for driveways and larger slabs, plus control joints every 8–10 ft or so for 4” slabs, so cracks form where you want them.
- Keep it moist while it cures, ideally for about a week. Concrete keeps gaining strength for weeks; most mixes reach their design strength at about 28 days.
- Wear gloves and eye protection. Wet concrete is caustic and can burn skin.
FAQ
How many bags of concrete for a 10×10 slab?
At 4” thick, about 56 bags of 80 lb (or 75 × 60 lb), plus 10% extra, so 62 × 80 lb bags.
How many 80 lb bags are in a yard of concrete?
About 45, because each makes about 0.6 cubic feet and a cubic yard is 27 cubic feet.
How much concrete do I need for a fence post?
About 1–2 bags of 80 lb mix per post for a typical 4×4 in a 10” hole, or 3–4 bags of 50 lb fast-setting mix. It depends on hole size and depth; use the fence calculator.
Should I add extra concrete?
Yes, 5–10%. Ground is rarely perfectly level, forms bow, and some always spills.
How this guide was written: bag yields are typical manufacturer figures. Building codes and the product instructions take priority. See how I test and review.