Driveway Slab Concrete: How Many Yards

The formula

Cubic feet = Length (ft) × Width (ft) × (Thickness in inches ÷ 12)
Cubic yards = Cubic feet ÷ 27
Order = Cubic yards × (1 + waste allowance), rounded up to your supplier's order increment

About the waste allowance: adding about 5% to 10% for spillage, uneven subgrade and form flex is a common practice, not a rule. Your crew, your subgrade and your supplier's advice should set the number. This example uses 10%.

Worked example: 40 ft × 12 ft at 4 inches thick

Area = 40 × 12 = 480 sq ft
Cubic feet = 480 × (4 ÷ 12) = 480 × (1/3) = 160 cu ft
Cubic yards = 160 ÷ 27 = 5.93 cu yd
With 10% = 5.93 × 1.10 = 6.52 cu yd
Order = round up to your supplier's increment (for example, 6.75 yd if they sell in quarter yards)

Same slab at 5 inches: 480 × (5 ÷ 12) = 200 cu ft ÷ 27 = 7.41 cu yd, or 8.15 cu yd with 10% (8.25 yd if your supplier sells in quarter yards). One extra inch adds about a quarter more concrete. The example thicknesses are volume inputs only; they do not establish a suitable driveway design for a particular vehicle or site.

Thickness, base, reinforcement and joint spacing come from your job spec, local code and the loads the slab will carry. This page only covers volume.

Bag math (if you're mixing it yourself)

Commonly listed yields for standard concrete mix are about 0.60 cu ft per 80 lb bag and 0.45 cu ft per 60 lb bag. Use the yield listed on the bag.

Bags = Cubic feet (with waste) ÷ yield per bag, rounded up
One cubic yard = 27 ÷ 0.60 = 45 bags (80 lb)
Example slab: 160 cu ft × 1.10 = 176 cu ft ÷ 0.60 = 293.3 → 294 bags (80 lb)

At driveway size, that's a lot of bags. Bag math is more useful for small pads, patches and footings.

FAQ

Why divide by 27?
A cubic yard is 3 ft × 3 ft × 3 ft, which is 27 cubic feet.

Why divide inches by 12?
Length and width are in feet, so thickness has to be in feet too. 4 inches ÷ 12 ≈ 0.333 ft.

How much extra should I order?
Many crews add about 5% to 10%, but that's common practice, not a rule. Uneven subgrade, thicker edges and form flex all use more concrete than the flat math shows.

Does the thickened edge change the math?
Yes. If your spec calls for a thickened edge, work it out as a separate strip (length × width of the thickened part × extra depth) and add it in.

Quote it in the app

Put the yards, the forming and the finish on one quote in QuoteMaven. Its AI quantity helper shows its math, so check it before you send, and A/B/C options let you show the customer a broom finish next to an upgraded finish on the same quote. Customers sign by link with nothing to install, a signed quote becomes an invoice automatically, and card payments go to your bank account. Some features vary by plan.

Free to download, with 30 days free, no card needed. I'm a fence and concrete contractor in Slidell, LA. QuoteMaven is a quote app built by contractors.

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