Show the math
- Convert to feet (inches ÷ 12, centimeters ÷ 30.48): Length 30 ft; Width 30 ft; Thickness 4 in = 0.3333 ft
- Volume = 30 × 30 × 0.3333 × 1 = 300 ft³
- Net volume = 300 ft³ ÷ 27 = 11.1111 yd³ (× 0.0283168 = 8.4951 m³)
- Order quantity = 11.1111 yd³ × (1 + 10%) = 12.2222 yd³ = 330 ft³
- 80 lb bags = 330 ft³ ÷ 0.6 ft³ per bag = 550 → 550 bags (always round up)
- Weight = 330 ft³ × 150 lb/ft³ = 49,500 lb
What this slab is used for
Thirty by thirty feet (900 ft²) is a large detached garage for three vehicles, a shop, or a small barn. It is also a typical size for a basement or a farm outbuilding. At this scale, the slab is a structural element of the building, not a patio.
A 4 in slab is the minimum. Five or six inches, with steel and a thickened perimeter or footing, is common for vehicles and equipment. Large floors often get a saw-cut joint grid with panels of about 10 to 12 ft, and need an engineer’s plan if they carry a building.
Concrete for a 30×30 ft slab at each thickness
The volume is 12.22 yd³ at 4 in, 15.28 yd³ at 5 in and 18.33 yd³ at 6 in, or 2 to 2 truckloads. The slab weighs 24.75 tons. You need a pour plan: trucks spaced so that each arrives as the last is finishing, enough people for placing, screeding and floating, and a plan to saw-cut the joints in time.
| Thickness | Net yd³ | Order yd³ | Order m³ | 80 lb bags | 60 lb bags | Weight |
|---|---|---|---|---|---|---|
| 3 in | 8.33 | 9.17 | 7.01 | 413 | 550 | 37,125 lb |
| 4 in | 11.11 | 12.22 | 9.34 | 550 | 734 | 49,500 lb |
| 5 in | 13.89 | 15.28 | 11.68 | 688 | 917 | 61,875 lb |
| 6 in | 16.67 | 18.33 | 14.02 | 825 | 1100 | 74,250 lb |
The formula is 30 × 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 30×30 ft?
Bags: 550 bags of 80 lb mix at 4 in, a total of 44,000 lb of dry product. Ready-mix: 12.22 yd³, material about $1,528–$2,583 at the published range, last verified 2026-10-04, before delivery and short-load fees. Bagged concrete for this slab costs about $3,025–$4,400. 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
- Not booking enough trucks, or booking them to arrive together.
- Having no plan to reach the middle of a 30 ft span.
- Running out of finishing help as the surface begins to set.
- Not reading the weather. Heat, wind or rain can ruin a pour that takes hours.
Layout and joints
Many owners hire a concrete contractor for a floor this size; the calculator is still useful for checking the quote.
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 900 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 30×30 slab?
11.11 yd³ net at 4 in, 12.22 yd³ with 10% waste.
How many bags of concrete for a 30×30 slab?
550 bags of 80 lb mix or 734 bags of 60 lb mix at 4 in with 10% waste.
How much does a 30×30 slab weigh?
About 49,500 lb (24.75 US tons) at 4 in with 10% waste, using 150 lb/ft³.
How many yards for a 30×30 slab at 6 in?
18.33 yd³ with 10% waste.
How much does a 30×30 concrete slab cost in materials?
About $1,528–$2,583 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.