How Many Bags of Concrete per Square Foot? (2026 Guide)
How many bags of concrete per square foot? At 4 in thick you need about 0.56 bags of 80-lb mix per sq ft — one 80-lb bag covers about 1.8 sq ft.
The math: from thickness to bag count
Concrete is sold by volume, so every estimate starts with cubic feet. Each square foot of slab holds thickness (in) ÷ 12 = cu ft per sq ft of concrete, and an 80-lb bag of mix yields about 0.60 cu ft of wet concrete, so bags per sq ft = cu ft per sq ft ÷ 0.60. A 60-lb bag yields about 0.45 cu ft — three-quarters of the 80-lb bag's volume — which is why 60-lb jobs need about 4/3 the bag count.
Worked at 4 in: 4 ÷ 12 = 0.333 cu ft per sq ft, and 0.333 ÷ 0.60 ≈ 0.56 bags of 80-lb mix — flip it and one bag covers about 1.8 sq ft.
Bags per square foot by slab thickness
Coverage for the common slab depths, using the 80-lb bag's 0.60 cu ft yield:
| Slab thickness | Cu ft per sq ft | 80-lb bags per sq ft | Sq ft per 80-lb bag |
|---|---|---|---|
| 3.5 in | 0.292 | 0.49 | 2.06 |
| 4 in | 0.333 | 0.56 | 1.80 |
| 5 in | 0.417 | 0.69 | 1.44 |
| 6 in | 0.500 | 0.83 | 1.20 |
Read across the 4-in row: each sq ft needs 0.333 cu ft, which is 0.56 of an 80-lb bag, and one bag covers 1 ÷ 0.56 ≈ 1.8 sq ft. Switching to 60-lb bags multiplies every count by about 4/3 — at 4 in that's about 0.75 bags (60-lb) per sq ft.
Worked example: a 10×10 slab at 4 in
A 10×10 ft slab is 100 sq ft. At 4 in it needs 100 × 0.333 = 33.3 cu ft of concrete — in 80-lb bags, 33.3 ÷ 0.60 ≈ 56 bags (or simply 100 ÷ 1.8 ≈ 56). With 60-lb bags the same slab needs 33.3 ÷ 0.45 ≈ 75 bags, about 4/3 of the 80-lb count. You can't buy a third of a bag, so round up and add one or two for spillage and a slightly uneven grade.
At this size, bagged mix is the practical route for a DIY crew with a wheelbarrow or a rental mixer. The full walkthrough lives in our 10×10 slab concrete guide; for a quick sanity check on any footprint, the concrete calculator does the volume math for you.
Concrete-buying tips
- Choose the pour depth for the load first. 4 in suits most patios, walkways and shed slabs; go 5–6 in where cars or trucks will drive. Depth drives the whole count, so set it first.
- Know the bag-to-truck crossover — about 1 cu yd. One cubic yard is 27 cu ft ≈ 45 bags of 80-lb mix, or about an 80 sq ft slab at 4 in. Once a pour approaches that size (and certainly by 100–150 sq ft at 4 in), quote the ready-mix delivery fee — one short truck run often beats hauling fifty bags.
- Don't ignore the subbase. Concrete only performs as well as what sits under it. Compact the soil or gravel base and set solid forms, or the slab cracks no matter how accurate the bag count.
Planning a related pour? The same thickness math drives concrete for a 10×10 slab and concrete for a 12×20 patio.
FAQ
How many 80-lb bags of concrete do I need per square foot for a 4-in slab?
About 0.56 bags per sq ft. A 4-in pour takes 4 ÷ 12 = 0.333 cu ft of concrete per sq ft, and one 80-lb bag yields about 0.60 cu ft, so 0.333 ÷ 0.60 ≈ 0.56 — roughly one 80-lb bag per 1.8 sq ft.
How thick or how large an area does one 80-lb bag of concrete cover?
It depends on the pour depth. An 80-lb bag holds about 0.60 cu ft of wet mix: spread 3.5 in deep it covers about 2.06 sq ft, at 4 in about 1.80 sq ft, at 5 in about 1.44 sq ft and at 6 in about 1.20 sq ft.
When should I order ready-mix concrete instead of mixing bags myself?
Start comparing ready-mix quotes once a pour needs about 1 cubic yard (27 cu ft) — roughly 45 bags of 80-lb mix, or a 4-in slab in the 100–150 sq ft range. Under that, bagged mix is usually cheaper; above it, the ready-mix delivery fee for your area usually wins, so price the truck before you haul bags.
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