Home › Guides › How Much Concrete Do I Need
Home & ConstructionConcrete is sold by volume — cubic yards for truck delivery, bags for DIY jobs — and the conversion from "a 10×10 patio" to "how many yards" trips up almost every first-timer. Get it wrong and the truck leaves you short mid-pour, which is a genuine disaster: concrete doesn't wait.
Here's the exact math for slabs and footings, the bag conversion, and the rule for when to stop mixing bags and call a truck.
Try it yourself: Enter length, width, and thickness for cubic yards and bag counts. Concrete Calculator →
The cubic-yard formula
Concrete volume in cubic yards (there are 27 cubic feet in a cubic yard):
Cubic yards = length (ft) × width (ft) × (thickness (in) ÷ 12) ÷ 27
Worked example: 10 × 10 ft patio, 4 in thick
10 × 10 × (4 ÷ 12) = 33.33 cubic feet. Divide by 27 → 1.24 cubic yards. Order 5–10% extra for spillage and uneven subgrade: 1.24 × 1.1 ≈ 1.36 yards — order 1.5. Running short mid-pour creates a cold joint (a weak seam); a little extra is far cheaper than a second delivery.
Bag math for small jobs
Bag yields (mixed concrete volume):
| Bag size | Yield | Bags per cubic foot |
|---|---|---|
| 80 lb | ~0.60 cu ft | ~1.7 |
| 60 lb | ~0.45 cu ft | ~2.2 |
| 40 lb | ~0.30 cu ft | ~3.3 |
Our 33.33 cu ft patio would need 33.33 ÷ 0.60 ≈ 56 bags of 80-lb mix. That's the practical limit talking: past about 30–40 bags, a ready-mix truck is cheaper and far less backbreaking.
How thick should it be?
- 4 inches — walkways, patios, shed floors.
- 5–6 inches — driveways and anything carrying vehicles.
- Footings — depth below the frost line per local code; width typically twice the wall width.
Timing a pour
Concrete is workable for roughly 1–2 hours after mixing (less in heat). Have forms built, rebar or mesh placed, and help on site before the truck arrives. Full strength takes about 28 days; it's walkable in 24–48 hours and drivable in about a week.
Footings and piers: the cylinder formula
Round footings and sonotubes need the cylinder volume: π × r² × height, all in feet, divided by 27 for cubic yards. A 12-inch-diameter pier (r = 0.5 ft) going 3 ft deep: 3.1416 × 0.25 × 3 = 2.36 cu ft = 0.087 cubic yards. Ten such piers = 0.87 yd³ — about 39 bags of 80-lb mix, which is why deck builders often just order a small ready-mix load (many suppliers sell by the half-yard with a short-load fee).
DIY mixing tips
- Water discipline. Too much water is the #1 DIY mistake — it weakens concrete dramatically. Add water gradually; the mix should hold its shape, not flow like soup.
- Mix in batches you can place in 30–45 minutes. A standard wheelbarrow batch is 2 bags of 80-lb mix.
- Work in shade or cool hours when possible — heat accelerates setting and shortens your working window.
- Cure it, don't just let it dry. Keep concrete damp for several days (mist it, cover with plastic) — proper curing is what builds full strength.
Ordering checklist
- Compute cubic yards with L × W × (thickness ÷ 12) ÷ 27.
- Add 5–10% for waste and uneven subgrade.
- Under ~1 cubic yard: bags are practical. Over that: call for ready-mix.
- Confirm access: the truck needs to get within chute range (~15–20 ft) of the forms.
- Have help, tools, and forms ready before the truck arrives — the clock starts at batching.
Small-load reality check: most ready-mix suppliers have a minimum (often 1–2 yards) and charge a short-load fee below it. For a 1.5-yard patio, the fee still usually beats mixing 40+ bags by hand — get quotes both ways before deciding.
Frequently asked questions
About 0.60 cubic feet of mixed concrete. A 60 lb bag yields ~0.45 cu ft and a 40 lb bag ~0.30 cu ft. Divide your total cubic feet by the yield to get the bag count.
4 inches for patios, walkways, and shed floors; 5–6 inches for driveways. Footings must extend below the local frost line — check your building code.
Once you're past roughly 30–40 bags (about a cubic yard), a truck is usually cheaper and dramatically less labor. Bags make sense for fence posts, small pads, and repairs.
You get a cold joint — a weak seam where fresh concrete meets partially set concrete. Always order 5–10% extra; the small overage cost beats a structural weak point.
Disclaimer: Structural work, footings, and anything load-bearing should follow local building codes and, for large pours, an engineer's specification.