How many bags of concrete for an 8×8 slab?
An 8 × 8 ft slab 4 inches thick holds 21.3 cubic feet of concrete (0.79 cubic yards). With 10% extra that is 40 bags of 80 lb mix, or 53 of 60 lb.
How to work it out
- 8 ft × 8 ft = 64 sq ft
- 64 sq ft × 4 in ÷ 12 = 21.3 cu ft ÷ 27 = 0.79 cu yd
- + 10% extra = 0.87 cu yd = 23.5 cu ft
- 23.5 cu ft ÷ 0.6 cu ft per 80 lb bag = 40 bags
Sixty-four square feet covers a small hot tub, a garden shed of 6 × 8 with a lip, or a landing at the foot of deck stairs. It is about the largest slab most people comfortably pour from bags in one session.
Adjust for your project
More options: Extra · Price
- Or bags of mix (incl. extra)
- 40 bags · 80 lb bag
- Concrete before the extra
- 0.79 cu yd (21.3 cu ft)
- Slab area
- 64 sq ft
- Weight of the concrete (≈ 145 lb/cu ft)
- 3,093 lb (2 tons)
- Total to buy counts this as bags. Switch “How will you buy it?” to ready-mix truck if you are ordering a delivery.
8×8 slab by thickness (10% extra included)
| Thickness | Or bags of mix (incl. extra) | Concrete to order | Weight of the concrete (≈ 145 lb/cu ft) |
|---|---|---|---|
| 3.5 in | 35 bags | 0.76 cu yd | 2,707 lb |
| 4 in | 40 bags | 0.87 cu yd | 3,093 lb |
| 5 in | 49 bags | 1.09 cu yd | 3,867 lb |
| 6 in | 59 bags | 1.3 cu yd | 4,640 lb |
Forty bags, one session
Forty 80 lb bags is 3,200 lb of mix, a little under a cubic yard of finished concrete with the extra. Two people with a rented drum mixer can place that in two to three hours: three bags per batch, about fourteen batches. Line the bags up beside the mixer before you start, and have a third person screeding and floating as the concrete goes in so the first batches don’t stiffen at the edge before the last ones arrive.
If a truck can reach the spot, a one-yard load is an alternative, but small orders usually carry a short-load fee. Compare the two on the bags vs ready-mix calculator.
Hot tubs and sheds
A filled hot tub is heavy; spa makers publish their own pad requirements, often a minimum thickness and a level, reinforced slab. Follow the tub’s installation guide and your local rules over any general figure. For a shed, check whether your local code treats it as an accessory structure that needs a particular foundation. At 8 ft, one control joint each way, cut about 1 in deep, divides the slab into four 4 ft squares. Smaller and larger pads are on how many bags of concrete for a 4×4 slab and how much concrete for a 10×12 shed.
Different size? Use the full Concrete Calculator.
What you’ll need
- Portable electric concrete mixerPremium price
A small drum mixer turns a long day of hand-mixing into batches of two or three 80 lb bags. Check the drum size against the bag count you need.
See options on Amazon (opens in a new tab) - Aluminum screedMid-range price
A straight, light screed strikes off the concrete level with the forms. A straight 2×4 works too if it really is straight.
See options on Amazon (opens in a new tab) - Magnesium hand floatBudget price
Closes the surface and pushes stones down after screeding, and prepares the slab for a broom or trowel finish.
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 many 60 lb bags of concrete for an 8×8 slab?
53 bags of 60 lb at 4 inches, including 10% extra.
How many bags of concrete for an 8×8 slab 6 inches thick?
59 bags of 80 lb with 10% extra, about 1.3 cubic yards.
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 much concrete for a 30×40 slab — 16.3 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 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.