Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 24 ft; Width 30 ft; Thickness 4 in = 0.3333 ft
- Volume = 24 × 30 × 0.3333 × 1 = 240 ft³
- Net volume = 240 ft³ ÷ 27 = 8.8889 yd³ (× 0.0283168 = 6.796 m³)
- Order quantity = 8.8889 yd³ × (1 + 10%) = 9.7778 yd³ = 264 ft³
- 80 lb bags = 264 ft³ ÷ 0.6 ft³ per bag = 440 → 440 bags (always round up)
- Weight = 264 ft³ × 150 lb/ft³ = 39,600 lb
What this slab is used for
A 24×30 ft slab (720 ft²) is a big two-car-plus-workshop garage, a three-bay shed or a barn floor. The 30 ft side is long enough for two vehicles in tandem or a workshop bay behind the cars.
Five inches is the working minimum for a shop floor with vehicles, and 6 in is common when heavy equipment is parked. Use reinforcement, a compacted base and a vapor barrier if the floor is enclosed. An engineered design is worthwhile at this size, especially on weak or sloped soil.
Concrete for a 24×30 ft slab at each thickness
At 4 in the order is 9.78 yd³. At 5 in it is 12.22 yd³, which is more than one 10 yd³ truck. This is the size at which the question of how many trucks matters: an order of about 10 yd³ is on the edge, and running a few cubic feet short means a second truck and a delivery fee. Round up to two loads or ask the plant to send a full truck and a partial load.
| Thickness | Net yd³ | Order yd³ | Order m³ | 80 lb bags | 60 lb bags | Weight |
|---|---|---|---|---|---|---|
| 3 in | 6.67 | 7.33 | 5.61 | 330 | 440 | 29,700 lb |
| 4 in | 8.89 | 9.78 | 7.48 | 440 | 587 | 39,600 lb |
| 5 in | 11.11 | 12.22 | 9.34 | 550 | 734 | 49,500 lb |
| 6 in | 13.33 | 14.67 | 11.21 | 660 | 880 | 59,400 lb |
The formula is 24 × 30 × 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 24×30 ft?
Bags: 440 bags of 80 lb mix at 4 in, a total of 35,200 lb of dry product. Ready-mix: 9.78 yd³, material about $1,222–$2,107 at the published range, last verified 2026-10-04, before delivery and short-load fees. Bagged concrete for this slab costs about $2,420–$3,520. Because the order is above two cubic yards, a truck is usually the practical choice, and at 6 in you may need 2 loads. See bagged vs ready-mix.
What to watch for
- Treating the order as one truck when it needs one and a bit.
- Letting the first load stiffen while waiting for the second. Make sure the gap between loads is short.
- A wide, long slab with too few joints. Aim for panels no larger than 12×12 ft.
- Ignoring the slope and the drainage around the building.
Layout and joints
On a slab this size a pump or a conveyor is often worth the fee. It also reduces the number of people needed to move concrete by barrow.
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 720 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 24×30 slab?
8.89 yd³ net at 4 in, 9.78 yd³ with 10% waste.
How many bags of concrete for a 24×30 slab?
440 bags of 80 lb mix or 587 bags of 60 lb mix at 4 in with 10% waste.
How much does a 24×30 slab weigh?
About 39,600 lb (19.80 US tons) at 4 in with 10% waste, using 150 lb/ft³.
How many yards for a 24×30 slab at 6 in?
14.67 yd³ with 10% waste.
How much does a 24×30 concrete slab cost in materials?
About $1,222–$2,107 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.