Published August 19, 2026 · Updated August 23, 2026 · By Tupep
Thickness first, because it drives everything else
On a driveway, thickness is not a rounding detail. Every extra inch across a two car pad is more than a cubic yard of concrete, and on a job this size a cubic yard is real money and real weight. So the order of operations is to settle thickness, then calculate volume, then decide how you are getting it there.
Here is the part we cannot answer for you. Driveway thickness, reinforcement, and the apron where the drive meets the street are all governed locally. In many areas a residential drive carrying passenger vehicles is built at around 4 inches, and 5 to 6 inches is common where an RV, a work truck, or a delivery vehicle will park. But your municipality, and sometimes an HOA, may have its own required thickness, and the apron in the public right of way is often a separate permitted spec with its own inspection. Call the building department before you form anything and confirm both the driveway requirement and the apron requirement.
The formula
Cubic yards = (length x width x thickness in inches / 12) / 27
Two adjustments to add before you order.
Thickened edges (cu ft) = edge length x (extra depth in inches / 12) x (edge width in inches / 12)
Order quantity = total yards x waste factor, rounded up to the next 1/4 yard, because plants batch in quarter yard increments.
Worked example, a 20 x 24 pad
20 x 24 = 480 square feet.
- At 4 inches: 480 x 0.3333 = 160 cubic feet = 5.93 cubic yards
- At 5 inches: 480 x 0.4167 = 200 cubic feet = 7.41 cubic yards
- At 6 inches: 480 x 0.5 = 240 cubic feet = 8.89 cubic yards
The useful takeaway is the step size. Each additional inch on 480 square feet is 40 cubic feet, or 1.48 cubic yards. That is why we always confirm the required thickness before pricing anything.
Now add thickened edges on the two long sides, a common detail so the drive does not spall where wheels track near the edge. Two runs of 24 feet is 48 linear feet. Turning a 5 inch slab down to 8 inches over an 8 inch wide band gives 48 x (3/12) x (8/12) = 8 cubic feet, or 0.30 yards.
Total: 200 + 8 = 208 cubic feet = 7.70 cubic yards. Add 5 percent for a machine graded, string checked subgrade and you order 8.25 yards. Add 10 percent for a hand graded subgrade and you order 8.5 yards.
| Driveway size | Square feet | 4 in (cu yd) | 5 in (cu yd) | 6 in (cu yd) |
|---|---|---|---|---|
| 10 x 20 single, short | 200 | 2.47 | 3.09 | 3.70 |
| 12 x 24 single car | 288 | 3.56 | 4.44 | 5.33 |
| 16 x 24 wide single | 384 | 4.74 | 5.93 | 7.11 |
| 20 x 24 two car | 480 | 5.93 | 7.41 | 8.89 |
| 10 x 60 long approach | 600 | 7.41 | 9.26 | 11.11 |
| 24 x 30 two car plus turn | 720 | 8.89 | 11.11 | 13.33 |
Why the waste factor is bigger than it looks
Half an inch of average over-excavation across 480 square feet is 20 cubic feet, which is 0.74 cubic yards of concrete you did not plan to buy. That is the whole reason a 5 percent allowance only works on a subgrade that has actually been checked with a string line or a laser. A hand dug, eyeballed subgrade routinely swallows more than that, and running short mid-pour on a driveway is the worst outcome in this article because it means a cold joint across your finished surface.
Weight is worth knowing too. Concrete runs roughly 4,000 lb per cubic yard, about two tons. Our 7.7 yard pour is around 31,000 lb. For scale, the same volume in 80 lb bags is 45 x 7.7 = about 347 bags, close to 28,000 lb of bags to buy, haul, and mix by hand. Driveways are ready-mix work, full stop.
Joint layout
A driveway will crack. Joints decide where. The common rule of thumb is spacing in feet of about two to three times the slab thickness in inches, with panels kept roughly square and the long dimension no more than about one and a half times the short one.
| Slab thickness | Typical joint spacing | Joint depth (1/4 of thickness) |
|---|---|---|
| 4 in | 8 to 12 ft | 1 in |
| 5 in | 10 to 15 ft | 1.25 in |
| 6 in | 12 to 18 ft | 1.5 in |
Apply that to the 20 x 24 pad at 5 inches. Twenty feet of width is wider than the spacing allows, so you run a longitudinal joint down the centerline to make two 10 foot lanes, then transverse joints roughly every 10 to 12 feet. That yields four panels near 10 x 12, which is comfortably inside the aspect guidance.
Timing matters as much as spacing. Conventional saw cutting is generally done within several hours of finishing, before the concrete has developed enough tensile strength to crack on its own, and early entry saws let you start much sooner. Cut late and the slab will have already made its own joints.
Where this stops being a quantity question
Reinforcement selection, subgrade improvement over poor soils, and thickness for anything heavier than normal household vehicles are engineering decisions, not estimating decisions. If you are parking an RV, a dump trailer, or expecting a concrete or delivery truck to drive on it, get a design. Likewise, frost depth affects any footing tied to this work, such as a garage foundation or a retaining curb, and it is set locally. Verify it with your building department rather than assuming.
On pour day, the practical items are access for a truck that may hold 10 or 11 yards, enough hands to screed 480 square feet before it sets, a plan for washout, and the discipline to refuse extra water at the chute. Slump is part of the mix design you paid for, and thinning it at the truck undoes the strength you ordered.
Related guides
- How Many Bags of Concrete for a 10×12 Shed Slab
- Concrete for a 4×8 Patio
- How Much Concrete Per Fence Post
- How Many Tons of Gravel for a 20×40 Driveway
Sources and Further Reading
Run your own numbers with the Concrete Slab Calculator, or browse every tool on the calculators index. Coverage figures and material constants are cross-checked against manufacturer technical data sheets before publication. See our Methodology page for how figures are selected and verified.
Planning estimate only. Results assume standard conditions and are intended for material planning. Verify against your product’s technical data sheet and your local building code before purchasing. Structural, electrical, and gas work should be specified by a licensed professional.
Last updated: August 23, 2026
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