How Much Concrete for a 30x40 Slab?
A 30x40 slab poured 4 in thick needs 14.81 yd³ of concrete, or 16.3 yd³ to order once a waste allowance of 10%[1] is added; at 6 in thick the order is 24.44 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 shop slabExample to try: Length 30 ft, Width 40 ft, Thickness 4 in
Concrete for a 30x40 slab at three thicknesses
Cubic yards for a 30x40 slab: the slab itself, the amount to order, truck loads and bags, at each thickness
| Slab thickness | Concrete in a 30 ft × 40 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 | 14.81 yd³ | 16.3 yd³ | 3 loads | 2 loads | 734 × 80 lb bags |
| 5 in | 18.52 yd³ | 20.37 yd³ | 3 loads | 2 loads | 917 × 80 lb bags |
| 6 in | 22.22 yd³ | 24.44 yd³ | 4 loads | 3 loads | 1,100 × 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.
Loads and weight of a 30x40 slab, by thickness
Truck loads at the smallest and the largest truck capacity the ready-mix industry body gives, and the weight of the poured slab, at each thickness
| Slab thickness | Loads at the smallest truck | Loads at the largest truck | Weight of the order |
|---|---|---|---|
| 4 in | 3 loads | 2 loads | 29 tons |
| 5 in | 3 loads | 2 loads | 36.25 tons |
| 6 in | 4 loads | 3 loads | 43.5 tons |
The weight is the order, with its waste allowance, at the midpoint of the density range for normalweight concrete. Loads are rounded up once.
A job for trucks, and for more than one
At 4 in thick the order is 16.3 yd³. Truck mixers carry 8 yd³[1] to 12 yd³[1], so that is 2 loads at the largest truck and 3 loads at the smallest. At 6 in it is 24.44 yd³, which is 3 loads to 4 loads.
In bags the 4 in slab is 734 × 80 lb bags, set against the loads above. The 20x30 slab page works the smaller garage footprint the same way.
The third column of the second table is the weight of the order: 29 tons at 4 in and 36.25 tons at 5 in. It uses the middle of the range the ready-mix industry body gives for the density of normalweight concrete, 140 pcf[2] to 150 pcf[2].
Base and reinforcement under the pour
The gravel calculator works out the base under a pad, and the rebar calculator the reinforcement.
Questions
How much concrete do I need for a 30x40 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 much concrete is a 30x40 slab at 4 inches thick?
The first row of the first table: the yards in the slab itself, and the yards to order with the waste allowance added.
Does that include waste?
The slab column does not; the order column does. The yards calculator adds the same allowance by default and lets you change it.
How many trucks does a 30x40 slab take?
The second table counts the loads at the smallest and the largest truck capacity in the cited range, for each thickness.
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 shop size in the yards calculatorSources (2)
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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) -
CIP 36, Structural Lightweight Concrete (opens in new tab), National Ready Mixed Concrete Association, CIP 36 (sheet in 2021/01 upload folder).
Standards or industry body.
“normalweight concrete with a density in the range of 140 to 150 lb/ft3 (2240 to 2400 kg/m3)”
Checked November 12, 2026. Figures and codes change; verify with the source. Back to where this is used (Source 2)
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.