Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 30 ft; Width 40 ft; Thickness 4 in = 0.3333 ft
- Volume = 30 × 40 × 0.3333 × 1 = 400 ft³
- Net volume = 400 ft³ ÷ 27 = 14.8148 yd³ (× 0.0283168 = 11.3267 m³)
- Order quantity = 14.8148 yd³ × (1 + 10%) = 16.2963 yd³ = 440 ft³
- 80 lb bags = 440 ft³ ÷ 0.6 ft³ per bag = 733.33 → 734 bags (always round up)
- Weight = 440 ft³ × 150 lb/ft³ = 66,000 lb
What this slab is used for
A 30×40 ft slab (1,200 ft²) is the size of a small barn, a workshop, a three-car garage with storage or a pole-barn floor. It is a significant structure that calls for a design, a permit and professional help in most places.
Heavy use favors 5 to 6 in with reinforcement. Pole barns and shops that carry loaded equipment, lifts or heavy storage often use thicker slabs, fibers and engineered base layers. Follow the plans.
Concrete for a 30×40 ft slab at each thickness
A 4 in slab is 16.30 yd³ with waste, 5 in is 20.37 yd³ and 6 in is 24.44 yd³, which is 2 to 3 truckloads. The concrete alone weighs 33.00 tons at 4 in. The cost of the concrete is in the thousands, so a 5% error is meaningful. Check the quantity twice and ask the contractor for a breakdown.
| Thickness | Net yd³ | Order yd³ | Order m³ | 80 lb bags | 60 lb bags | Weight |
|---|---|---|---|---|---|---|
| 3 in | 11.11 | 12.22 | 9.34 | 550 | 734 | 49,500 lb |
| 4 in | 14.81 | 16.30 | 12.46 | 734 | 978 | 66,000 lb |
| 5 in | 18.52 | 20.37 | 15.57 | 917 | 1223 | 82,500 lb |
| 6 in | 22.22 | 24.44 | 18.69 | 1100 | 1467 | 99,000 lb |
The formula is 30 × 40 × thickness ÷ 12 ÷ 27 for the net volume. The order adds 10% for waste and rounds the bag counts up. Use the calculator above for another thickness or waste allowance.
Bags or ready-mix for 30×40 ft?
Bags: 734 bags of 80 lb mix at 4 in, a total of 58,720 lb of dry product. Ready-mix: 16.30 yd³, material about $2,037–$3,378 at the published range, last verified 2026-10-04, before delivery and short-load fees. Bagged concrete for this slab costs about $4,037–$5,872. Because the order is above two cubic yards, a truck is usually the practical choice, and at 6 in you may need 3 loads. See bagged vs ready-mix.
What to watch for
- Underestimating the volume of thickened edges and footings, which can add several yards.
- Poor communication with the plant on spacing of trucks.
- A saw-cut schedule that starts too late, so the slab cracks before it is cut.
- Finishing without enough crew or equipment such as a power trowel.
Layout and joints
Plan access to the site for several trucks, with a place to turn around and wash out.
For a 4 in slab, joint spacing of 8 to 12 ft is the usual range, with a 15 ft maximum for any thickness. Read the control joints guide for depth and timing.
Before you order
- Measure the forms, not the excavation, and check the thickness in several places.
- Add the volume of any thickened edge, apron or step separately.
- Plan the base: use the gravel base calculator to estimate stone for 1200 ft².
- If the design calls for steel, estimate it with the rebar calculator.
- Put the quantity on the Pour List to combine it with footings and steps.
Frequently asked questions
How many yards of concrete for a 30×40 slab?
14.81 yd³ net at 4 in, 16.30 yd³ with 10% waste.
How many bags of concrete for a 30×40 slab?
734 bags of 80 lb mix or 978 bags of 60 lb mix at 4 in with 10% waste.
How much does a 30×40 slab weigh?
About 66,000 lb (33.00 US tons) at 4 in with 10% waste, using 150 lb/ft³.
How many yards for a 30×40 slab at 6 in?
24.44 yd³ with 10% waste.
How much does a 30×40 concrete slab cost in materials?
About $2,037–$3,378 for ready-mix concrete at 4 in, before delivery fees, based on the ranges on the cost page (verified 2026-10-04).
Sources
Estimate only. Results depend on the numbers you enter, ground conditions and your supplier. For structural work consult a licensed engineer and follow local codes. Brand names such as Quikrete and Sakrete are used only to cite published product data. This site is not affiliated with any brand.