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.
At the defaults — a 12 × 10 ft patio at 4 in — that’s an order of 1.75 yd³ of ready-mix, about $280, or 74 bags of 80 lb mix. The line-by-line math is in the pour math section below.
What goes under a concrete slab?
A slab-on-grade is a sandwich, and the concrete is only the visible layer: compacted subgrade, a gravel base, plastic where moisture matters, steel where cracks matter, then the pour itself, jointed so it cracks where you decided. What each layer does:
Subgrade
The native soil, stripped of sod and organics and compacted flat. Soft spots telegraph straight up through a slab as settled, cracked corners years later — fix them with compacted fill before the pour, not with extra concrete.
Gravel base
Four inches of compacted crushed stone gives the slab uniform bearing, drains water away from the underside, and breaks capillary rise. Size the stone order with the gravel calculator — base gravel is bought by the ton, and it’s not in this concrete estimate.
Vapor barrier
A 6-mil or heavier poly sheet under any slab that will be lived on, parked on, or covered with flooring — interiors and garages, not usually patios. It stops ground moisture from migrating up through the concrete for the life of the building.
Forms
2× lumber staked level and square around the perimeter — the top of the form is the top of your slab, which is why thickness gets measured down from it. Square is a diagonal measurement, not an eyeball: equal diagonals or it’s a parallelogram.
Reinforcement
Welded-wire mesh or fiber for patios and sheds; #3 or #4 rebar on an 18–24 in grid for driveways and garages. Steel belongs mid-depth on chairs — mesh pulled up during the pour or left on the base does nothing. Thickened edges and frost footings are their own pour; the footing calculator handles those.
Control joints
Concrete cracks; joints decide where. Tool or saw-cut them every 8–12 ft, about a quarter of the slab depth, in panels as close to square as the layout allows.
What concrete mix for a slab?
Ask for 3,000–4,000 psi with a 4–6 in slump and you’ve ordered correctly for almost any residential slab — the extras below are what the dispatcher will offer, and what they cost:
- 3,000 psi: the walkway/patio standard in mild climates.
- 4,000 psi: driveways, garages, and anywhere with freeze–thaw — pair it with air entrainment outdoors in cold country.
- Air entrainment: microscopic bubbles that give freezing water somewhere to push — cheap insurance north of the frost line.
- Fiber mix: synthetic fibers that replace mesh for shrinkage control in flatwork (not a rebar substitute).
- High-early / accelerated: sets fast in cold weather or when the schedule is tight — order it, don’t improvise it.
- Bagged mix: standard 4,000 psi in 40–80 lb bags; crack-resistant and fast-setting variants exist for pads and posts.
| Mix | Typical range |
|---|---|
| Ready-mix, 3,000 psi | $150–165 /yd³ |
| Ready-mix, 4,000 psi | $160–175 /yd³ |
| Air entrainment | +$5–10 /yd³ |
| Fiber reinforcement | +$10–15 /yd³ |
| High-early mix | +$10–20 /yd³ |
| 80 lb bag | $5–8 /bag |
How to measure a slab for concrete
- Measure length and width inside the forms — that’s the concrete you’ll actually pour. Dimension fields take feet-inches (12, 12 ft 6 in) or metric (4 m).
- Measure thickness down from the form tops at several points across the excavation and use the deepest consistent figure — subgrade is never as flat as it looks.
- Count identical pads in the Slabs field — a pair of walkway pads or four shed piers order as one ticket instead of four rounded ones.
- Leave waste at 10% on formed, compacted grade; go 15% on rough or hand-dug subgrade where the average depth is a guess.
- Enter the plant’s delivered price — with the short-load fee if your order is under about 4 yd³ — or your store’s bag price for small pours.
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. Total odd footprints first with the square footage calculator if that’s easier.
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.
| 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 |
Worked example: concrete for a 12 × 10 ft slab
The default pour, worked line by line — 12 × 10 ft at 4 in, 10% waste, $160 ready-mix against $7.25 bags:
yards = 40 ÷ 27 = 1.48 yd³
waste = 1.48 × 1.10 = 1.63 yd³
order = next ¼ yd³ up = 1.75 yd³
ready-mix = 1.75 × $160 = $280
bags = (40 × 1.10) ÷ 0.60 = 73.3 → 74 bags ≈ $537
weight = 40 × 150 lb/ft³ = 6,000 lb cured, exact volume
Two things worth noticing. The quarter-yard rounding bought the safety margin — 1.63 yd³ of need became 1.75 yd³ on the truck, and running short mid-pour is the one mistake a slab never forgives. And the bag line prices the same concrete at nearly double: 74 bags is three tons of hand mixing, which is why the crossover to ready-mix comes at about a yard.
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.
What this concrete estimate doesn’t include
The calculator prices the concrete in the forms. A finished slab spends money on every layer of the assembly above — budget these lines too:
- Base gravel and fabric: 4 in of compacted crushed stone under the pour, bought by the ton.
- Forming lumber and stakes: 2× boards, stakes, screws — and the hours to set them level and square.
- Reinforcement: mesh sheets, rebar and chairs, or the fiber upcharge at the plant.
- Vapor barrier: 6-mil+ poly under interior and garage slabs.
- Finishing labor: screeding, floating, troweling, jointing — the skill half of the job; hired flatwork commonly lands installed slabs at $6–12/ft² all-in.
- Pump truck: commonly $600–900 when the chute can’t reach the forms.
- Excavation and haul-off: digging out sod and soil, and paying to dump it.
- Cure and seal: curing compound or plastic now, penetrating sealer after 28 days.
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.
How much concrete do I need for a 24x24 slab?
At 4 in thick, a 24 × 24 ft slab takes 8 yd³ of ready-mix with 10% waste — about $1,280 at $160 per yard — or 352 bags of 80 lb, which is firmly ready-mix territory. Garages and shops that will see trucks pour 6 in instead: 11.75 yd³.
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.
Sources and standards
The rules and yields on this page come from the documents batch plants and inspectors actually work to:
- IRC R506: slabs-on-grade — the 3.5 in minimum thickness, base and vapor-retarder requirements.
- ASTM C94: the standard specification ready-mixed concrete is batched and delivered under.
- ACI 302.1R: the guide to concrete floor and slab construction behind the base, joint and curing practice here.
- Manufacturer-published bag yields: the 0.30 / 0.375 / 0.45 / 0.60 ft³ figures for 40–80 lb bags come straight from the bag makers’ data sheets.
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