How Many Bags of Concrete for an 8x8 Slab? (2026 Guide)
An 8x8 ft (64 sq ft) slab poured 4 in thick needs about 21.3 cu ft of concrete — 36 bags of 80-lb mix or 48 bags of 60-lb mix; at 3.5 in, about 32 bags of 80-lb.
How many bags for an 8x8 slab, by thickness
An 8x8 ft slab is 64 sq ft of surface area; thickness decides the volume. One 80-lb bag yields about 0.6 cu ft of mixed concrete and one 60-lb bag about 0.45 cu ft, so divide the volume by those figures and round up to the nearest full bag — a short bag means a skimpy pour.
| Slab thickness | Concrete needed | 80-lb bags | 60-lb bags |
|---|---|---|---|
| 3.5 in | 18.7 cu ft | 32 bags | 42 bags |
| 4 in | 21.3 cu ft | 36 bags | 48 bags |
| 5 in | 26.7 cu ft | 45 bags | 60 bags |
| 6 in | 32 cu ft | 54 bags | 72 bags |
The 4 in row is the practical default for a slab this size — stiff enough for a shed, a heat-pump pad or light foot traffic without pouring away money. 3.5 in works only on firm, well-drained ground under a light load. Choose 5–6 in when the slab must carry something heavier, such as a shed stocked with dense equipment, and confirm that thickness against the load before you order bags.
Worked example: an 8x8 ft slab, 4 in thick
Four inches is the standard pour for a slab at this size, so run that case once in full. Thickness in feet first: 4 in ÷ 12 = 0.333 ft. Volume is then 64 sq ft × 0.333 ft = 21.3 cu ft. With 80-lb bags: 21.3 ÷ 0.6 ≈ 35.6 → round up to 36 bags. With 60-lb bags: 21.3 ÷ 0.45 ≈ 47.4 → round up to 48 bags. The whole pour is about 0.8 cu yd, which is why bags beat a ready-mix truck at this size — a typical truck minimum is 1 cu yd, so delivery would leave you with spare concrete and a short-load fee.
Pouring an 8x8 slab that lasts
- Compact the base first — dig out soft topsoil and tamp 2–4 in of gravel level before anything else; a soft, uneven base is the main reason small slabs crack and settle.
- Set and square the forms — brace all four sides and pitch the finished surface about 1/4 in per ft so water runs off instead of pooling on the pad.
- Mix in batches, not one giant load — 36 bags of 80-lb mix is far more than any wheelbarrow or small mixer can batch at once, so keep a steady rhythm: one person mixing successive loads while the other places and screeds. Pull bags from the same batch number so every load matches in color and strength.
- Screed, then float — strike off along the forms with a straight 2x4, then bull-float once the water sheen disappears so the surface is flat and uniform.
- Cure with water — mist the slab and cover it with plastic for at least 3–5 days; keep it damp and stay off it for 24–48 hours so it gains strength instead of drying out.
Bigger or smaller than an 8x8 ft slab?
The same math scales in both directions. A 6x8 ft patio is the next size down at 48 sq ft, and a 4x8 ft slab at 32 sq ft suits an outdoor unit or a narrow walkway pad — follow those sizes to how many bags of concrete for a 6x8 patio and how many bags of concrete for a 4x8 slab. If you only know the footprint, start from surface area with how many bags of concrete per square foot. For anything much larger than this, it's worth checking delivery prices — a ready-mix truck becomes practical once a pour passes about 1 cu yd.
FAQ
How many bags of concrete for an 8x8 slab at 4 in?
An 8x8 ft slab is 64 sq ft, so at 4 in thick it needs about 21.3 cu ft of concrete: 21.3 ÷ 0.6 ≈ 35.6, rounded up to 36 bags of 80-lb mix — or 21.3 ÷ 0.45 ≈ 47.4, rounded up to 48 bags of 60-lb mix.
What is a 64 sq ft slab commonly used for?
64 sq ft (an 8x8 ft pour) is the classic footprint for a small tool shed pad, an outdoor heat-pump or air-conditioner pad, or a compact seating patio for a bistro or two-chair set. Poured 4 in thick, one 64 sq ft slab covers any of those jobs with a single continuous pour.
Can you mix the concrete for an 8x8 slab all at once?
Not by hand in one go. At 4 in thick the slab is about 0.8 cu yd (21.3 cu ft) of concrete — far more than a wheelbarrow or small mixer can batch at once. The practical route is bagged mix poured in successive batches: work in a steady rhythm so each fresh batch lands while the previous one is still plastic, and buy all bags from the same batch number so color and strength stay uniform.
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