How do you calculate concrete for a slab?
Slab volume = length × width × thickness ÷ 27, in feet. A 24×24 ft garage slab at 6 inches thick: 24 × 24 × 0.5 ÷ 27 = 10.67 yards — order 11.5 with waste. Use 4 inches for patios and walkways, 5–6 inches for driveways and garages.
Thickness rules of thumb by slab use
| Slab type | Thickness | Yards per 100 ft² |
|---|---|---|
| Sidewalk / patio | 4 in | 1.23 yd³ |
| Residential driveway | 5–6 in | 1.54–1.85 yd³ |
| Garage floor | 6 in | 1.85 yd³ |
| RV / heavy vehicle pad | 6–8 in | 1.85–2.47 yd³ |
How to read this table
- RV / heavy vehicle pad sits at the top of the table (6–8 in) — 1.85–2.47 yd³. If your situation looks like this row, plan against the upper half of the range rather than the midpoint.
- Sidewalk / patio anchors the bottom (4 in) — 1.23 yd³. Treat this as the conservative case you should still be profitable at.
- The gap between the top and bottom row is roughly 2.0×. That spread is why a single blended average is close to useless here — pick the row that matches your setup instead of averaging the column.
- Most rows are ranges, not single figures. The low end usually reflects a weaker month, a softer audience geography, or an unoptimised setup; the high end reflects a well-run, well-targeted operation of the same size.
- With 4 reference points in the "thickness rules of thumb by slab use" table, the fastest way to use this page is to find the closest row, take its thickness, then stress-test it ±30% before you build a plan on it.
Context
Thickness is the decision that matters more than the exact volume: a 4-inch driveway that cracks under an SUV costs far more than the extra 0.6 yards per 100 ft² that 6 inches would have added. Reinforcement (rebar or wire mesh) and a compacted gravel base of 4 inches are standard for anything vehicle-bearing. Irregular shapes: split into rectangles, compute each, and add.
What moves this number
Regional pricing
Ready-mix, bag, and labor prices vary 30–50% between US metros. Sun Belt markets with many plants run well below Northeast and remote-area pricing.
Waste and site conditions
Uneven subgrade, spillage, and form bowing consume 5–15% extra material. Orders placed at exact volume routinely come up short mid-pour.
Local code requirements
Frost depth, footing dimensions, and reinforcement rules are set by county. The same project can require twice the concrete in a northern climate versus a southern one.
Methodology
Volume formula as in the cubic-yards answer; thickness guidance follows ACI 302 residential practice (4 in minimum for pedestrian slabs, 5–6 in for light vehicles). Yards per 100 ft² = thickness in feet × 100 ÷ 27.
Assumptions and caveats
- Volumes are theoretical; order quantities include a waste allowance but site conditions vary.
- Prices reflect 2026 US national ranges — get local quotes before budgeting a real pour.
- Structural dimensions and frost depths are governed by local building code; verify with your building department.
- This page was last reviewed on 2026-10-01. Ranges are updated as new data lands, so re-check before using them in a contract or a plan.
- Use these numbers as a starting range, not a guarantee — your own historical data always beats a benchmark.
Frequently asked questions
How do you calculate concrete for a slab?
Slab volume = length × width × thickness ÷ 27, in feet. A 24×24 ft garage slab at 6 inches thick: 24 × 24 × 0.5 ÷ 27 = 10.67 yards — order 11.5 with waste. Use 4 inches for patios and walkways, 5–6 inches for driveways and garages.
Which option pays the most in the thickness rules of thumb by slab use table?
RV / heavy vehicle pad, at 6–8 in (1.85–2.47 yd³). That row represents the strongest case in this dataset, so use it as an upper bound rather than an expectation.
What is a realistic low-end figure?
Sidewalk / patio at 4 in (1.23 yd³). Plan your costs so the low end still works, then treat anything above it as upside.
Why do the numbers vary so much?
The spread between the highest and lowest row is about 2.0×. Regional pricing and waste and site conditions explain most of that gap — see the drivers section above for the full list.
Where do these numbers come from?
Volume formula as in the cubic-yards answer; thickness guidance follows ACI 302 residential practice (4 in minimum for pedestrian slabs, 5–6 in for light vehicles). Yards per 100 ft² = thickness in feet × 100 ÷ 27.
How can I estimate my own number instead of using a benchmark?
Use the Concrete Slab Cost Calculator on RevenueLab — it takes your own inputs and returns a figure specific to your setup, which is always more accurate than a published range.
Model your own numbers
Related reading
More answers in this category
- How do you calculate cubic yards of concrete?
- How many bags of concrete make a cubic yard?
- How do you calculate concrete for a driveway?
- How do you calculate concrete for footings?
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Last updated 2026-10-01.