How much concrete for a 30×40 slab?
A 30 × 40 ft slab 4 inches thick is 14.8 cubic yards of concrete, or 16.3 cubic yards with 10% extra. At 6 inches it is 22.22 cubic yards before extra. Either way it is two or more truckloads.
How to work it out
- 40 ft × 30 ft = 1,200 sq ft
- 1,200 sq ft × 4 in ÷ 12 = 400 cu ft ÷ 27 = 14.8 cu yd
- + 10% extra = 16.3 cu yd = 440 cu ft
- 440 cu ft ÷ 0.6 cu ft per 80 lb bag = 734 bags
1,200 square feet is the classic pole barn or shop footprint. The volume is easy; the planning is not. A slab this size is placed from several trucks in one go, and the thickness and reinforcement are set by the building plans.
Adjust for your project
More options: Extra · Price
- Or bags of mix (incl. extra)
- 734 bags · 80 lb bag
- Concrete before the extra
- 14.8 cu yd (400 cu ft)
- Slab area
- 1,200 sq ft
- Weight of the concrete (≈ 145 lb/cu ft)
- 58,000 lb (29 tons)
- 734 bags is a lot of mixing. Compare a ready-mix delivery (ask about minimum loads and short-load fees) with the bags vs ready-mix calculator.
- Total to buy counts this as ready-mix. Switch “How will you buy it?” to bags if you are mixing it yourself.
- Plan control joints about 1 in deep every 6–8 ft (Quikrete’s guidance) so shrinkage cracks follow your lines.
30×40 slab by thickness (10% extra included)
| Thickness | Concrete to order | Concrete before the extra | Weight of the concrete (≈ 145 lb/cu ft) |
|---|---|---|---|
| 4 in | 16.3 cu yd | 14.8 cu yd | 58,000 lb |
| 5 in | 20.4 cu yd | 18.5 cu yd | 72,500 lb |
| 6 in | 24.4 cu yd | 22.2 cu yd | 87,000 lb |
Yards and trucks for a 30×40
At 4 in the slab is 14.81 yards, 16.3 with the extra; at 5 in, 18.52 yards before extra; at 6 in, 22.22. Truck mixers carry 8 to 12 yards each by NRMCA’s figures, so a 4 in floor is two loads and a 6 in floor two or three, depending on the trucks your plant runs. Ask the plant to stagger them about 30–45 minutes apart (or whatever matches your crew’s pace) so the edge of the first load is still workable when the next arrives.
For a shop that will see vehicles or heavy equipment, the thickness, the reinforcement and any thickened edges come from your plans and local code, and the slab may need inspection before the pour. The calculator will give you the yards for whatever those specify.
Crew, joints and access
A 30 ft span needs a long screed or a screed rail down the middle, a bull float and a power trowel if you want a smooth shop finish. Plan for a crew of four to six. If the truck can’t reach every corner with its chute, add a pump or a concrete buggy to the order. Control joints at roughly 10 ft spacing on a 4 in slab is too far by Quikrete’s rule of thumb for its bag mix (6–8 ft); many slab designers set joint spacing from slab thickness, so follow your plans.
The steel for a slab this size adds up quickly: see the rebar calculator for the bar count on a grid you choose. Smaller footprints are on how many yards of concrete for a 24×24 slab and how much concrete for a 20×20 slab.
Different size? Use the full Concrete Calculator.
What you’ll need
- Bull float with handleMid-range price
Smooths and levels a slab you can’t reach across, right after screeding. Rent or buy one for anything wider than about 6 ft.
See options on Amazon (opens in a new tab) - Rebar chairsBudget price
Hold rebar or mesh off the ground so it ends up inside the concrete, not under it. Match the chair height to where your plans place the steel.
See options on Amazon (opens in a new tab) - Concrete knee boardsMid-range price
Spread your weight so you can kneel on a slab that is firm but not yet hard, to reach the middle with a float or trowel.
See options on Amazon (opens in a new tab)
As an Amazon Associate we earn from qualifying purchases. Links to products are affiliate links — they cost you nothing extra and help keep these calculators free.
Frequently asked questions
How much concrete for a 30×40 slab 6 inches thick?
22.2 cubic yards before extra and 24.4 cubic yards with 10%.
How much concrete for a 30×40 slab 5 inches thick?
18.5 cubic yards, or 20.4 cubic yards with 10% extra.
Other common sizes
- How much concrete for a 10×10 slab — 1.36 cu yd with 10% extra (62 bags of 80 lb)
- How much concrete for a 12×12 slab — 1.96 cu yd with 10% extra (order 2 yd)
- How much concrete for a 20×20 slab — 5.43 cu yd with 10% extra at 4 in
- How many yards of concrete for a 24×24 slab — 7.82 cu yd with 10% extra at 4 in
- How many bags of concrete for a 4×4 slab — 10 bags of 80 lb (14 of 60 lb), 4 in thick
- How many bags of concrete for an 8×8 slab — 40 bags of 80 lb, 4 in thick
- How much concrete for a 10×12 shed — 1.63 cu yd with 10% extra; about 2 cu yd with a thickened edge
Sources
- Quikrete Concrete Mix (No. 1101) data sheet (opens in a new tab) — Yields: 40 lb ≈ 0.30, 50 lb ≈ 0.375, 60 lb ≈ 0.45, 80 lb ≈ 0.60, 90 lb ≈ 0.675 cu ft; unit weight ≈ 140 lb/cu ft; for pours 2 in thick or more; 6–9 pints of water per 80 lb bag; control joints every 6–8 ft; cure 5 days at 70 °F or above, 7 days at 50–70 °F; protect from freezing for 48 hours.
- NRMCA Concrete in Practice 31 — Ordering Ready Mixed Concrete (opens in a new tab) — Truck mixers carry 8 to 12 cubic yards (about 6 to 9 m³); order 4–10% more than the plan estimate; a loaded truck weighs about 80,000 lb; fresh concrete can cause severe chemical burns.
- NRMCA Concrete in Practice 2 — Scaling Concrete Surfaces (opens in a new tab) — Order air-entrained concrete (about 6% air) for exterior slabs exposed to freezing and de-icers; avoid de-icing chemicals in the first winter.
- Iowa DOT Instructional Memorandum 340 — Weight per cubic foot, yield and air content of concrete (opens in a new tab) — Normal-weight concrete is typically 140–150 lb per cu ft at 5 to 8 percent air.
- OSHA 3351 — Preventing Skin Problems from Working with Portland Cement (opens in a new tab) — Wet portland cement can cause caustic burns that may not hurt at first and in severe cases reach the bone.
- NIST Handbook 44, Appendix C — General Tables of Units of Measurement (opens in a new tab) — 1 cubic yard = 27 cubic feet; 1 cubic foot = 1,728 cubic inches.