Quick answer: Cubic yards of concrete = (Square feet × Thickness in inches ÷ 12) ÷ 27. A 10 × 12 ft patio poured 4 in thick needs 120 × 0.333 ÷ 27 = 1.48 cubic yards. Order about 10% extra, so 1.63 cubic yards.
Ordering concrete is one of the few jobs where running short is worse than having too much: a cold joint in the middle of a slab is permanent. Here’s how to go from square footage to an order you can trust.
Step 1: Find the square footage
Measure the slab outline and calculate its area. For rectangles that’s length × width; for anything else, use our square footage calculator or split the shape as shown in how to calculate square footage.
Step 2: Multiply by thickness to get cubic feet
Convert the thickness to feet first (inches ÷ 12):
| Thickness | In feet |
|---|---|
| 3 in | 0.25 ft |
| 4 in | 0.333 ft |
| 5 in | 0.417 ft |
| 6 in | 0.5 ft |
| 8 in | 0.667 ft |
Cubic feet = Square feet × Thickness (ft)
Step 3: Divide by 27 to get cubic yards
Ready-mix concrete is sold by the cubic yard, and one cubic yard is 3 × 3 × 3 = 27 cubic feet.
Cubic yards = Cubic feet ÷ 27
Worked example: backyard patio
- Area: 10 ft × 12 ft = 120 sq ft
- Volume: 120 × 0.333 = 40 cu ft
- Cubic yards: 40 ÷ 27 = 1.48 cu yd
- Plus 10% for spillage, uneven subgrade and form bulge: 1.63 cu yd
Coverage chart: how far does one cubic yard go?
| Slab thickness | Area one cubic yard covers |
|---|---|
| 3 in | 108 sq ft |
| 4 in | 81 sq ft |
| 5 in | 64.8 sq ft |
| 6 in | 54 sq ft |
| 8 in | 40.5 sq ft |
Divide your square footage by the number in the right-hand column for a fast estimate. A 400 sq ft driveway at 5 in is about 400 ÷ 64.8 = 6.2 cu yd before extra.
Bags or ready-mix?
For small pours (roughly under a cubic yard), premixed bags are practical. Typical yields (check the bag, as brands vary):
| Bag size | Approx. yield |
|---|---|
| 40 lb | 0.30 cu ft |
| 60 lb | 0.45 cu ft |
| 80 lb | 0.60 cu ft |
Bags needed = Cubic feet ÷ Yield per bag
For the 40 cu ft patio above: 40 ÷ 0.60 = 66.7, so about 67 bags of 80 lb mix, or 74 bags with 10% extra. At that size most people switch to a ready-mix truck. Ask your supplier about minimum orders and short-load fees.
Circular slabs and footings
Use π × r² for the area, then multiply by depth as usual.
Example: a round fire-pit pad 10 ft in diameter, 4 in thick → 3.1416 × 5² = 78.54 sq ft × 0.333 = 26.18 cu ft ÷ 27 = 0.97 cu yd.
Round footings or post holes: π × r² × depth for each hole, then multiply by the number of holes.
Typical slab thicknesses
- Walkways and patios: 4 in is common.
- Driveways: 4–6 in depending on vehicle weight and local practice.
- Garage floors and heavy-use pads: 5–6 in or more.
Always follow your local building code and any engineer’s specification. These are general guidelines, not structural advice.
Gravel: from cubic yards to tons
Gravel bases and paths use exactly the same volume formula. Suppliers often sell by the ton, though, so multiply cubic yards by the material’s density. Many common gravels weigh roughly 1.4–1.7 tons per cubic yard. Ask your supplier for the exact figure for their stone.
Example: a 200 sq ft path, 4 in deep → 200 × 0.333 = 66.7 cu ft ÷ 27 = 2.47 cu yd × 1.4 = about 3.5 tons.
Asphalt: from square footage to tons
Hot-mix asphalt weighs about 145 lb per cubic foot once compacted, which is close to 2 tons per cubic yard.
Tons of asphalt = Square feet × Compacted thickness (ft) × 145 ÷ 2,000
Example: a 12 × 40 ft driveway (480 sq ft) paved 3 in thick → 480 × 0.25 = 120 cu ft × 145 = 17,400 lb ÷ 2,000 = 8.7 tons.
How much extra should you order?
- Concrete: 5–10%. Go toward 10% on uneven ground or with complex forms.
- Gravel: 10% or more, because it compacts.
- Asphalt: confirm with your paving contractor, who will account for compaction.
Let Areo do the volume math
In the Areo app, draw the slab, patio or driveway outline, enter the depth, and the concrete and asphalt calculator returns the volume and the total cost at your price per unit. Gravel estimates work the same way. It’s quicker than a spreadsheet and you can do it on site while the client is standing next to you.
Skip the formulas next time
Draw any shape in Areo and get square feet, square meters, acres and the total cost instantly. Try it free for 3 days.