How Much Concrete for a 20x30 Slab?
A 20x30 slab poured 4 in thick needs 7.41 yd³ of concrete, or 8.15 yd³ to order with a waste allowance of 10%[1]; at 6 in thick the order is 12.22 yd³.
Estimates for planning, not professional advice. Read the full notice.
Work it out for your own sizes
Work out the yards for your own garage slabExample to try: Length 20 ft, Width 30 ft, Thickness 4 in
Concrete for a 20x30 slab at three thicknesses
Cubic yards for a 20x30 slab: the slab itself, the amount to order, truck loads and bags, at each thickness
| Slab thickness | Concrete in a 20 ft × 30 ft slab | To order, with the waste allowance | Truck loads at the smallest truck (8 yd³) | Truck loads at the largest truck (12 yd³) | 80 lb bags for the same order |
|---|---|---|---|---|---|
| 4 in | 7.41 yd³ | 8.15 yd³ | 2 loads | 1 load | 367 × 80 lb bags |
| 5 in | 9.26 yd³ | 10.19 yd³ | 2 loads | 1 load | 459 × 80 lb bags |
| 6 in | 11.11 yd³ | 12.22 yd³ | 2 loads | 2 loads | 550 × 80 lb bags |
Estimate for planning, from what you entered and the product data shown. Check product labels and your supplier before you buy.
The waste allowance is added once, on the total. Truck loads and bags are each rounded up once, from the order.
Truck loads for a 20x30 slab at the small and large end of the truck range
Loads of ready-mixed concrete for the order at each thickness, counted at the smallest and the largest truck capacity the ready-mix industry body gives
| Slab thickness | To order, with the waste allowance | Loads at the smallest truck | Loads at the largest truck |
|---|---|---|---|
| 4 in | 8.15 yd³ | 2 loads | 1 load |
| 5 in | 10.19 yd³ | 2 loads | 1 load |
| 6 in | 12.22 yd³ | 2 loads | 2 loads |
Each load count is the order divided by one truck's capacity, rounded up once.
One truck or more at this size
This slab is the size at which the order starts to be counted against a truck. The ready-mix industry body gives truck mixers as carrying 8 yd³[1] to 12 yd³[1]. At 4 in the order of 8.15 yd³ is 1 load at the largest truck and 2 loads at the smallest.
Going from 4 in to 5 in takes the order from 8.15 yd³ to 10.19 yd³, and to 12.22 yd³ at 6 in. Whether a given thickness fits in one load or needs a second depends on the capacity of the truck the plant sends, which is why the table shows both ends of the range.
In bags the 4 in slab is 367 × 80 lb bags. The 20x20 slab page shows the smaller footprint; the 30x40 slab page the shop-size one.
The truck capacity range in the source's words
The capacity of truck mixers varies between 8 to 12 cubic yards (5 to 9 m3).
Questions
How many yards of concrete do I need for a 20x30 slab?
The first table gives the cubic yards for the slab itself and the amount to order with the waste allowance, at three thicknesses.
How many trucks does a 20x30 slab take?
The second table counts the loads at the smallest and the largest truck capacity in the cited range, for each thickness, because the capacity of the truck the plant sends varies.
Can a 20x30 slab be poured from bags?
The last column of the first table counts the bags of the default size for each thickness, so the count can be set against the yards.
Does the order include waste?
The slab column does not; the order column does. The yards calculator adds the same allowance and lets you change it.
Calculate it for your project
Enter your own sizes to get the amount to buy or the layout figures, with the working shown and a page to print.
Enter your garage size in the yards calculatorSources (1)
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CIP 31, Ordering Ready Mixed Concrete (opens in new tab), National Ready Mixed Concrete Association, 2000, 2016, 2023.
Standards or industry body.
“Quantity of concrete ordered should be 4% to 10% more than an estimate calculated from the plan dimensions to account for contingencies.”
“The capacity of truck mixers varies between 8 to 12 cubic yards (5 to 9 m3).”
Checked November 12, 2026. Figures and codes change; verify with the source. Back to where this is used (Source 1)
These are summaries of what the sources say, not professional advice. Codes and product data change. Check with your building department and the product label about your own project.