How this concrete slab calculator works
The calculator tells you how much concrete you need for a slab — in cubic yards of ready-mix or bags of concrete — and what it should cost. The math is the same one a batch plant uses, no mystery factors: volume is length times width times thickness, converted to cubic yards:
yd³ = ft³ ÷ 27 · order = yd³ × (1 + waste%) → rounded up to ¼ yd³
- Waste: 10% covers uneven subgrade, spillage, and over-excavation. Bump it to 15% on rough grade.
- Rounding: ready-mix is sold in ¼-yard increments, so we always round up — never short a pour.
- Bags: bag count applies the same waste factor, divided by the yield of your bag size, rounded up.
Worked example: how much concrete for a 12 × 10 slab
The default slab is 12 ft × 10 ft at 4 in thick: 12 × 10 × 0.333 = 40 ft³, which is 1.48 yd³. Adding 10% waste gives 1.63 yd³, and rounding up to the next quarter yard means you order 1.75 yd³. As bags, 44 ft³ ÷ 0.60 ft³ per 80 lb bag = 73.3, so you buy 74 bags — roughly $537 versus $280 for ready-mix.
How to measure a slab for concrete
Odd-shaped slabs
For an L- or T-shaped pad, split it into rectangles and run each one through the calculator. Add the exact volumes (the "before waste" line under the drawing) together, then round the total once — rounding each leg separately orders more cubic yards of concrete than you need. For several identical pads — deck footings, a pair of walkway pads — use the Slabs field instead.
Thickness is where estimates go wrong
A "4-inch slab" poured over a wavy subgrade can easily average 4½–5 in, and that difference in how much concrete you need is invisible until the truck runs dry. Measure down from your form boards at several points, use the deepest consistent figure, and raise the waste allowance to 15% on rough or hand-dug grade.
What goes under and into the slab
- Base: 4 in of compacted gravel is typical over soil; add a vapor barrier under interior and garage slabs.
- Forms: stake 2× lumber level and square — check with a diagonal measurement, not eyesight.
- Joints: cut or tool control joints every 8–12 ft, about ¼ of the slab depth, so it cracks where you chose.
| Use | Thickness |
|---|---|
| Walkway / patio | 4″ |
| Shed slab | 4″ |
| Driveway (cars) | 4–5″ |
| Driveway (trucks / RV) | 6″ |
| Garage | 4–6″ |
How many bags of concrete do I need?
Divide the slab's volume — waste included — by the bag's yield: an 80 lb bag of concrete makes 0.60 ft³ once mixed. A typical 10 × 10 ft slab at 4 in takes about 62 bags of 80 lb concrete, or 1.5 yd³ of ready-mix. The yield table covers every common bag size, and the sizes table below it answers the usual "how much concrete do I need" cases at both 4 in and 6 in thick.
| Bag size | Yield | Bags / yd³ |
|---|---|---|
| 40 lb | 0.30 ft³ | 90 |
| 50 lb | 0.375 ft³ | 72 |
| 60 lb | 0.45 ft³ | 60 |
| 80 lb | 0.60 ft³ | 45 |
| Slab | Order at 4″ | 80 lb bags at 4″ | Order at 6″ |
|---|---|---|---|
| 8 × 8 ft | 1.0 yd³ | 40 | 1.5 yd³ |
| 10 × 10 ft | 1.5 yd³ | 62 | 2.25 yd³ |
| 12 × 12 ft | 2.0 yd³ | 88 | 3.0 yd³ |
| 16 × 16 ft | 3.5 yd³ | 157 | 5.25 yd³ |
| 20 × 20 ft | 5.5 yd³ | 245 | 8.25 yd³ |
| 24 × 24 ft | 8.0 yd³ | 352 | 11.75 yd³ |
Concrete slab cost: what actually drives the price
- The mix: $150–175 per cubic yard of concrete, delivered, is typical in 2026 for 3,000–4,000 psi. Higher psi, fiber, and accelerators add per-yard.
- Short loads: orders under ~4 yd³ usually carry a $50–150 fee, and most plants have a 1 yd³ minimum — price the whole ticket.
- Access: if the truck can't reach the forms, wheelbarrowing is slow and a pump truck adds several hundred dollars.
- Bags: $5–8 each plus a mixer rental. Fine for pads and footings; painful past the ~45-bag crossover.
- The rest of the job: base gravel, forms, reinforcement and finishing labor usually cost more than the concrete itself — installed slabs run $6–12/ft².
Concrete slab FAQ
How many bags of concrete do I need for a 10 × 10 slab at 4 inches?
About 62 bags of 80 lb concrete. A 10 × 10 ft slab at 4 in is 33.3 ft³; add 10% waste (36.7 ft³) and divide by the 0.60 ft³ yield of an 80 lb bag: 61.1, so round up to 62. That is about 5,000 lb of concrete to mix by hand — at this size, get a ready-mix quote first.
How much does a 12 × 12 concrete slab cost?
At the calculator defaults, a 12 × 12 ft slab at 4 in takes 2 yd³ of ready-mix (about $320) or 88 bags of 80 lb concrete (about $638) — materials only. With base gravel, forms, reinforcement and finishing labor, installed slabs typically run $6–12 per square foot.
Should I order ready-mix or mix bags?
The crossover is around 1 cubic yard, which is about 45 bags at 80 lb. Below that, bags are usually cheaper once you count delivery fees. Above it, ready-mix wins on price and always wins on labor — a yard of concrete is over two tons of hand-mixing.
How much does a yard of concrete cost in 2026?
Most markets run $150–175 per cubic yard delivered for 3,000–4,000 psi mix, before extras. Orders under about 4 yd³ usually add a short-load fee of $50–150, so price the whole ticket, not just the per-yard rate — then type your quote into the price field above.
How thick should a concrete slab be?
Patios, walkways and shed slabs: 4 in. Driveways for cars: 4–5 in. Driveways that see trucks or RVs: 6 in. The IRC (R506.1) sets 3.5 in as the minimum for slabs-on-grade.
Do I need rebar or wire mesh in a slab?
Patios and shed slabs typically use welded-wire mesh or fiber mix. Driveways and garages do better with #3 or #4 rebar on an 18–24 in grid, plus control joints every 8–12 ft. Keep steel in the middle of the slab depth, and check local code — this tool estimates volume, not structure.
How long until you can use a new slab?
Keep foot traffic off for 24–48 hours and vehicles off for at least 7 days; concrete reaches its full design strength at about 28 days. Keep it damp while curing, and expect everything to take longer in cold weather.
Embed this concrete calculator on your site
Run the slab calculator on your own site, free — supplier pages, contractor sites, how-to posts. Paste the snippet where you want it; the visible credit link is the only condition.