Concrete for a 30 × 40 slab
Short answer
A 30 × 40 ft slab at 4" thick needs about 14.81 cubic yards of concrete — roughly 734 80-lb or 978 60-lb pre-mix bags (including 10% waste).
Concrete
14.81 yd³
80-lb bags
734
60-lb bags
978
40-lb bags
1467
How much concrete for a 30 × 40 slab
A 30 × 40 ft slab is 1200 square feet. At the standard 4-inch thickness that's about 14.81 cubic yards of concrete once you add a 10% waste allowance — roughly 734 80-lb or 978 60-lb pre-mix bags. The "how much concrete do I need" answer is always area × thickness ÷ 27, plus that waste factor.
Thickness is the lever people miss. Bump the same 1200 sq ft from 4" to 6" — what a driveway or anything bearing vehicle loads needs — and you jump from 14.81 to 22.22 cubic yards, about 50% more concrete. Order to the real thickness, not the footprint.
Past roughly a cubic yard, bagging gets impractical — 734 80-lb bags is a lot of mixing. Most pours this size take delivered ready-mix instead. The table below gives the exact bag count for every size if you do mix it by hand.
What drives the amount
| Factor | Effect |
|---|---|
| Slab area | 30 × 40 ft = 1200 sq ft — the base for the volume |
| Thickness | 4" is standard; driveways and vehicle loads want 5–6", which adds concrete proportionally |
| Waste allowance | Add ~10% for spillage, uneven subgrade, and over-excavation |
| Bags vs ready-mix | Bags suit small pours; past ~1 cubic yard, delivered ready-mix is easier and usually cheaper |
| Subgrade & forms | A level, compacted gravel base and solid forms keep you from over-ordering |
| Rebar / wire mesh | Reinforcement doesn't change the concrete volume, but it's part of any real slab |
30 × 40 slab: bags vs ready-mix
| Option | You need | Notes |
|---|---|---|
| 40-lb bags | 1467 bags | 0.30 ft³ each |
| 50-lb bags | 1174 bags | 0.375 ft³ each |
| 60-lb bags | 978 bags | 0.45 ft³ each |
| 80-lb bags | 734 bags | 0.60 ft³ each |
| Ready-mix | 14.81 yd³ | delivered by the truck |
Pouring tips
- Order about 10% extra — running short mid-pour leaves a cold joint that weakens the slab.
- Compact the subgrade and set forms before you order; a soft or uneven base quietly raises how much you need.
- For a driveway, garage, or anything vehicles sit on, pour 5–6 inches, not 4 — and size the order to that thickness.
- Past a cubic yard, price delivered ready-mix against bags; mixing dozens of bags by hand rarely saves money.
Different size or thickness?
Open the concrete calculatorOther slab sizes
Frequently asked questions
How much concrete for a 30 × 40 slab?
A 30 × 40 ft slab at 4" thick needs about 14.81 cubic yards (400 cubic feet), including a 10% waste allowance.
How many bags of concrete for a 30 × 40 slab?
About 734 80-lb bags, 978 60-lb bags, or 1467 40-lb bags. Above roughly 1 cubic yard it is usually cheaper to order ready-mix than to buy bags.
Is a 4-inch slab thick enough?
Four inches is standard for patios, walkways, and shed floors. Go to 5–6 inches for driveways or anything carrying vehicle loads. Change the thickness in the calculator to see the new amount.