Pouring a patio, a shed slab, footings or fence posts? This guide shows exactly how much concrete you need in cubic yards and how many 60 lb or 80 lb bags to buy, with a free calculator, a ready slab chart and worked examples in feet and inches.
| 1 cubic yard | 27 cubic feet |
|---|---|
| 80 lb bags per yd³ | 45 |
| 60 lb bags per yd³ | 60 |
| 1 yd³ at 4 in thick covers | 81 sq ft |
| 1 yd³ at 6 in thick covers | 54 sq ft |
Concrete volume formulas
Work in feet. Convert inches to feet by dividing by 12, then convert cubic feet to cubic yards by dividing by 27.
- Slab, patio or driveway: length (ft) × width (ft) × thickness (in) ÷ 12 = cubic feet
- Footing or wall: length (ft) × width (in) ÷ 12 × depth (in) ÷ 12 = cubic feet
- Round column, post hole or Sonotube: 3.1416 × (diameter in inches ÷ 24)² × height (in) ÷ 12 = cubic feet
- Cubic yards = cubic feet ÷ 27
- Bags = cubic feet ÷ bag yield, rounded up
How many bags of concrete in a yard?
| Bag size | Yield per bag | Bags per cubic yard | Bags per 10 ft³ |
|---|---|---|---|
| 80 lb | 0.60 ft³ | 45 | 17 |
| 60 lb | 0.45 ft³ | 60 | 23 |
| 50 lb | 0.375 ft³ | 72 | 27 |
| 40 lb | 0.30 ft³ | 90 | 34 |
One cubic yard of bagged concrete weighs roughly 3,600 lb (45 bags × 80 lb). That is a useful reality check before you plan to mix it all by hand.
Concrete slab chart (cubic yards and 80 lb bags)
| Slab size | 4 in thick | 80 lb bags (4 in, +10%) | 6 in thick | 80 lb bags (6 in, +10%) |
|---|---|---|---|---|
| 10 × 10 ft | 1.23 yd³ (1.36 with 10%) | 62 | 1.85 yd³ (2.04 with 10%) | 92 |
| 12 × 12 ft | 1.78 yd³ (1.96 with 10%) | 88 | 2.67 yd³ (2.93 with 10%) | 132 |
| 10 × 20 ft | 2.47 yd³ (2.72 with 10%) | 123 | 3.70 yd³ (4.07 with 10%) | 184 |
| 20 × 20 ft | 4.94 yd³ (5.43 with 10%) | 245 | 7.41 yd³ (8.15 with 10%) | 367 |
| 24 × 24 ft | 7.11 yd³ (7.82 with 10%) | 352 | 10.67 yd³ (11.73 with 10%) | 528 |
Worked examples
1. A 10 × 10 ft patio, 4 inches thick
- Volume = 10 × 10 × (4 ÷ 12) = 33.33 ft³
- Cubic yards = 33.33 ÷ 27 = 1.23 yd³
- 80 lb bags = 33.33 ÷ 0.60 = 55.6, round up to 56 bags
- Add 10% for waste: 62 bags (or 1.36 yd³ of ready-mix)
2. A 40 ft footing, 16 in wide and 8 in deep
- Volume = 40 × (16 ÷ 12) × (8 ÷ 12) = 35.56 ft³ = 1.32 yd³
- With 10% extra: 1.45 yd³, or 66 bags of 80 lb concrete
- At this size a ready-mix delivery is usually simpler than 66 bags
3. Four 12 in Sonotubes, 4 ft (48 in) deep
- One tube = 3.1416 × (12 ÷ 24)² × (48 ÷ 12) = 3.14 ft³
- Four tubes = 12.57 ft³; with 5% extra, 13.19 ft³
- 80 lb bags = 22 bags
Bags or a ready-mix truck?
- Under about 1 yd³ (roughly 45 bags of 80 lb): bags are practical. Posts, small pads, steps and repairs.
- Around 1–2 yd³: either works. Bags mean hours of mixing; a mixer rental helps.
- Above about 2 yd³: ready-mix is usually the better choice. It arrives evenly mixed, so the slab cures more uniformly. Ask about the minimum order and any short-load fee for small loads.
How thick should the concrete be?
| Use | Typical thickness | Notes |
|---|---|---|
| Sidewalk, walkway, patio | 4 in | On a compacted gravel base |
| Shed or hot-tub pad | 4–6 in | Thicken under heavy point loads |
| Driveway (cars) | 4–5 in | 5 in for frequent use |
| Driveway (trucks, RVs) | 6 in | Consider rebar or wire mesh |
| Garage slab | 4–6 in | Check local code and the expected loads |
Tips for an accurate order
- Measure depth in several places. A base that is ½ in low over a whole slab adds up fast.
- Always add 5–10%. Spillage, form bulge and an uneven base eat concrete. Running short mid-pour leaves a weak cold joint.
- Split odd shapes into rectangles and circles, calculate each, and add them up.
- Pricing your slab? See our guide to the cost to pour a concrete slab per square foot.
FAQ
How many 80 lb bags of concrete are in a yard?
About 45. An 80 lb bag yields roughly 0.60 ft³, and a cubic yard is 27 ft³ (27 ÷ 0.60 = 45).
How many 60 lb bags of concrete are in a yard?
About 60. A 60 lb bag yields roughly 0.45 ft³ (27 ÷ 0.45 = 60).
How much concrete do I need for a 10 x 10 slab?
At 4 in thick, a 10 × 10 ft slab needs 1.23 cubic yards, which is 56 bags of 80 lb concrete, or 62 bags with 10% extra. At 6 in thick it needs 1.85 cubic yards.
How many square feet does a yard of concrete cover?
About 81 sq ft at 4 in thick, 65 sq ft at 5 in and 54 sq ft at 6 in.
Should I use bags or order ready-mix?
Bags suit jobs under about 1 cubic yard. Above about 2 cubic yards, ready-mix is usually easier and gives a more uniform slab.
Bag yields are typical values printed on US bagged concrete mixes; check the product you buy. Thickness guidance is common residential practice. Your local building code governs.
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