How Many Bags of Concrete for a 4x8 Slab? (2026 Guide)
A 4x8 ft (32 sq ft) slab poured 4 in thick needs about 10.7 cu ft of concrete — 18 bags of 80-lb mix or 24 bags of 60-lb mix; at 3.5 in, about 16 bags of 80-lb.
How many bags for a 4x8 slab, by thickness
A 4x8 ft slab is 32 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 | 9.33 cu ft | 16 bags | 21 bags |
| 4 in | 10.67 cu ft | 18 bags | 24 bags |
| 5 in | 13.33 cu ft | 23 bags | 30 bags |
| 6 in | 16 cu ft | 27 bags | 36 bags |
The 4 in row is the default answer for a small pad — AC condenser pads, generator pads and similar equipment bases are usually poured 3.5–4 in. Choose 5–6 in when the slab carries machinery or repeated foot traffic, and always confirm a heavier pad against the equipment manufacturer's base spec.
Worked example: an AC condenser pad (4x8 ft, 4 in)
Outdoor AC units are the most common reason homeowners pour a 4x8 ft slab, so run the numbers once in full. Thickness in feet first: 4 in ÷ 12 = 0.333 ft. Volume is then 32 sq ft × 0.333 ft = 10.67 cu ft. With 80-lb bags: 10.67 ÷ 0.6 ≈ 17.8 → round up to 18 bags. With 60-lb bags: 10.67 ÷ 0.45 ≈ 23.7 → round up to 24 bags. A 4x8 slab at 4 in is only about 0.4 cu yd, which is why bags beat a ready-mix truck at this size.
Pouring a 4x8 slab that stays flat
- Compact a 2–4 in gravel base — tamp it level first; a soft, uneven base is the main reason small slabs crack and settle.
- Set and brace the forms — square up the corners, level the top edges, and stake the boards so they hold under the weight of wet concrete.
- Pour it in one go — 18 bags is a solid morning with one helper mixing while the other places and screeds; if you must pause, stop at a straight edge rather than feathering a ragged end.
- Screed, then float — strike off along the forms with a straight 2x4, then bull-float once the water sheen disappears so the surface stays level as it sets.
- 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 4x8 ft?
The same math scales in both directions. A 4x4 ft pad needs a quarter of this volume, and a 10x10 ft slab is roughly three times bigger — follow those sizes to how many bags of concrete for a 4x4 slab and how much concrete for a 10x10 slab. If you only know the footprint, start from the 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 a 4x8 slab at 4 in?
A 4x8 ft slab is 32 sq ft, so at 4 in thick it needs about 10.7 cu ft of concrete: 10.7 ÷ 0.6 ≈ 17.8, rounded up to 18 bags of 80-lb mix — or 10.7 ÷ 0.45 ≈ 23.7, rounded up to 24 bags of 60-lb mix.
Is 3.5 in or 4 in thick better for a 4x8 slab?
For a non-load-bearing equipment pad, both work: 3.5 in is a light but serviceable pad (about 16 bags of 80-lb mix), while 4 in (about 18 bags of 80-lb mix) gives a stiffer surface with more margin over soft ground. Pour 4 in or more when the slab supports a shed, steps or concentrated weight.
Bags or a ready-mix truck for a 4x8 slab?
Bags. A 4x8 ft slab at 4 in is only about 0.4 cu yd (10.7 cu ft) — less than half a typical 1 cu yd truck minimum — so ready-mix wastes concrete and adds a short-load fee. Stick with 80-lb or 60-lb bags for a 32 sq ft pad and consider a truck only once the job exceeds about 1 cu yd.
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