How do you calculate cubic yards of concrete?
Multiply length × width × thickness in feet to get cubic feet, then divide by 27. A 20×10 ft slab at 4 inches (0.333 ft) thick: 20 × 10 × 0.333 = 66.7 cubic feet ÷ 27 = 2.47 cubic yards — order 2.75 yards with waste.
Worked example: 20×10 ft patio slab, 4 in thick
| Step | Math | Result |
|---|---|---|
| Convert thickness | 4 in ÷ 12 | 0.333 ft |
| Volume in cubic feet | 20 × 10 × 0.333 | 66.7 ft³ |
| Convert to yards | 66.7 ÷ 27 | 2.47 yd³ |
| Add 10% waste | 2.47 × 1.10 | 2.72 → order 2.75 yd³ |
How to read this table
- Convert to yards sits at the top of the table (66.7 ÷ 27) — 2.47 yd³. If your situation looks like this row, plan against the upper half of the range rather than the midpoint.
- Add 10% waste anchors the bottom (2.47 × 1.10) — 2.72 → order 2.75 yd³. Treat this as the conservative case you should still be profitable at.
- The gap between the top and bottom row is roughly 11×. 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.
- With 4 reference points in the "worked example: 20×10 ft patio slab, 4 in thick" table, the fastest way to use this page is to find the closest row, take its math, then stress-test it ±30% before you build a plan on it.
Context
The two mistakes that blow up orders: forgetting to convert inches to feet (4 inches is 0.333 ft, not 4 ft), and ordering the exact volume with no margin. Ready-mix trucks have minimum loads (often 1–2 yards) and short-load fees, so a 2.47-yard need is usually cheaper ordered as 2.75 than risk a second delivery. For anything over ~2 yards, ready-mix beats bags on both price and labor.
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 = L × W × D in consistent units; 1 cubic yard = 27 cubic feet. The 10% waste factor covers subgrade unevenness, spillage, and form bowing — the standard contractor allowance for slabs on grade.
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 cubic yards of concrete?
Multiply length × width × thickness in feet to get cubic feet, then divide by 27. A 20×10 ft slab at 4 inches (0.333 ft) thick: 20 × 10 × 0.333 = 66.7 cubic feet ÷ 27 = 2.47 cubic yards — order 2.75 yards with waste.
Which option pays the most in the worked example: 20×10 ft patio slab, 4 in thick table?
Convert to yards, at 66.7 ÷ 27 (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?
Add 10% waste at 2.47 × 1.10 (2.72 → order 2.75 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 11×. 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 = L × W × D in consistent units; 1 cubic yard = 27 cubic feet. The 10% waste factor covers subgrade unevenness, spillage, and form bowing — the standard contractor allowance for slabs on grade.
How can I estimate my own number instead of using a benchmark?
Use the Concrete 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 many bags of concrete make a cubic yard?
- How do you calculate concrete for a slab?
- How do you calculate concrete for a driveway?
- How do you calculate concrete for footings?
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Last updated 2026-10-01.