Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 40 ft; Width 40 ft; Thickness 4 in = 0.3333 ft
- Volume = 40 × 40 × 0.3333 × 1 = 533.3333 ft³
- Net volume = 533.3333 ft³ ÷ 27 = 19.7531 yd³ (× 0.0283168 = 15.1023 m³)
- Order quantity = 19.7531 yd³ × (1 + 10%) = 21.7284 yd³ = 586.6667 ft³
- 80 lb bags = 586.6667 ft³ ÷ 0.6 ft³ per bag = 977.78 → 978 bags (always round up)
- Weight = 586.6667 ft³ × 150 lb/ft³ = 88,000 lb
What this slab is used for
A 40×40 ft slab (1,600 ft²) is a commercial-sized pour: a large workshop, barn, warehouse bay or farm equipment shed. It is also in the range of a backyard sport court, such as a basketball half-court, although those usually have special surface requirements.
Floors this large normally use 5 or 6 in or an engineered section, with steel or fibers, a prepared base and a plan for joints. Warehouse floors and heavy-equipment shops may need more. Obtain a design from an engineer rather than guessing.
Concrete for a 40×40 ft slab at each thickness
A 4 in slab is 21.73 yd³ with waste, which is 3 truckloads. At 5 in it is 27.16 yd³ and at 6 in 32.59 yd³. The slab weighs 44.00 tons. Contractors pour a floor like this in sections or with laser screeds and power trowels, and the plant schedules loads continuously. Use the calculator for budgeting and to check the quote.
| Thickness | Net yd³ | Order yd³ | Order m³ | 80 lb bags | 60 lb bags | Weight |
|---|---|---|---|---|---|---|
| 3 in | 14.81 | 16.30 | 12.46 | 734 | 978 | 66,000 lb |
| 4 in | 19.75 | 21.73 | 16.61 | 978 | 1304 | 88,000 lb |
| 5 in | 24.69 | 27.16 | 20.77 | 1223 | 1630 | 110,000 lb |
| 6 in | 29.63 | 32.59 | 24.92 | 1467 | 1956 | 132,000 lb |
The formula is 40 × 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 40×40 ft?
Bags: 978 bags of 80 lb mix at 4 in, a total of 78,240 lb of dry product. Ready-mix: 21.73 yd³, material about $2,716–$4,437 at the published range, last verified 2026-10-04, before delivery and short-load fees. Bagged concrete for this slab costs about $5,379–$7,824. Because the order is above two cubic yards, a truck is usually the practical choice, and at 6 in you may need 4 loads. See bagged vs ready-mix.
What to watch for
- Treating it as a DIY job. Placing and finishing 20 to 30 yd³ requires a crew and equipment.
- Letting the concrete in the slab set at different rates in sun and shade.
- Not planning the joint saw cuts and curing across 1,600 ft².
- Not accounting for subgrade variations that change thickness and volume.
Layout and joints
Joint every 10 to 12 ft in both directions gives a grid of about 16 panels. Check that the plan keeps panels close to square.
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 1600 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 40×40 slab?
19.75 yd³ net at 4 in, 21.73 yd³ with 10% waste.
How many bags of concrete for a 40×40 slab?
978 bags of 80 lb mix or 1304 bags of 60 lb mix at 4 in with 10% waste.
How much does a 40×40 slab weigh?
About 88,000 lb (44.00 US tons) at 4 in with 10% waste, using 150 lb/ft³.
How many yards for a 40×40 slab at 6 in?
32.59 yd³ with 10% waste.
How much does a 40×40 concrete slab cost in materials?
About $2,716–$4,437 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.