How Many Bags of Concrete for a 4x4 Slab? (2026 Guide)
A 4×4 ft slab (16 sq ft) poured 4 in thick needs about 5.3 cu ft of concrete — roughly 9 bags of 80-lb mix; at 3.5 in, about 8 bags.
Bags of concrete for a 4×4 slab, by thickness
The 4×4 slab is 16 sq ft, so cubic feet = 16 × (thickness ÷ 12). Divide by the 0.6 cu ft an 80-lb bag yields, then round up to the next whole bag:
| Slab thickness | Cubic feet | 80-lb bags (rounded up) |
|---|---|---|
| 3.5 in | 4.67 | 8 |
| 4 in | 5.33 | 9 |
| 5 in | 6.67 | 12 |
| 6 in | 8 | 14 |
Read across: a 4-in slab at 5.33 cu ft takes 9 bags — 5.33 ÷ 0.6 = 8.9, rounded up. Always round up: a partial bag isn't available, and a thin edge is worse than one spare bag.
Worked example: a 4×4 slab at 4 in
Say you're pouring a small patio pad, a stepping-stone base, or a pad for a grill or AC unit. Four inches is the standard thickness for these, and it makes the math easy.
1. Find the area
4 × 4 = 16 sq ft.
2. Convert thickness to feet and multiply
4 in = 4 ÷ 12 = 0.333 ft, so 16 × 0.333 = 5.33 cu ft. For any pad, cubic feet = area (sq ft) × thickness in feet.
3. Divide by the bag yield
An 80-lb bag of mix yields about 0.6 cu ft. 5.33 ÷ 0.6 = 8.9, so buy 9 bags of 80-lb concrete mix.
That's the same arithmetic the concrete calculator runs for you — enter length, width and thickness and it returns bags for any bag size. Curious about larger pads? The 20×20 slab guide shows the same method scaled up.
Bagged concrete is the practical call at this size
5.33 cu ft is only about 0.2 cu yd — far below the delivery minimum most ready-mix plants charge for. Nine 80-lb bags mix up by hand or in a small mixer with no truck, no minimum order and no leftover yard. For small projects like a 4×4 pad, bagged mix is usually the cheaper and easier route; see the per-square-foot breakdown in bags of concrete per square foot to estimate any other footprint.
Tips for pouring a 4×4 slab
- Bagged mix handles a small project fine — for 9 bags, rent or borrow a small mixer (or use a wheelbarrow and hoe) and batch in consistent mixes rather than ordering ready-mix for a fraction of a yard.
- Compact the base first — dig out the depth plus a few inches, add and tamp a gravel or compacted-soil base, then set level forms; a soft base settles and cracks a thin 4×4 slab.
- Pour it in one go — mix and place consecutive batches without long pauses so the pad cures as a single piece; stopping halfway leaves a cold joint that weakens the slab.
FAQ
How many bags of concrete do I need for a 4×4 slab?
About 9 bags of 80-lb mix at 4 in thick. The slab is 16 sq ft, so it needs 16 × (4 ÷ 12) = 5.33 cu ft; at 0.6 cu ft per 80-lb bag that's 5.33 ÷ 0.6 ≈ 9 bags. Poured 3.5 in thick, it needs about 8 bags.
What is the minimum thickness for a 4×4 concrete slab?
3.5 to 4 in for a small slab on grade. A 3.5-in pour needs about 4.67 cu ft (8 bags of 80-lb mix); 4 in needs 5.33 cu ft (9 bags) and is the common choice for small pads and equipment bases. Go 5–6 in only when the slab must support heavy concentrated loads.
Is bagged concrete cost-effective for a 4×4 slab?
Yes — for small areas. A 4×4 slab at 4 in is only about 0.2 cu yd, under the delivery minimum for most ready-mix orders, and buying 9 bags costs less than a truck with no leftover material. Ready-mix concrete only becomes economical once the pour reaches a couple of cubic yards or more.
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