Concrete Slab Calculator: Yards, Bags and Cost
A 10×10 ft concrete slab at 4 in needs 1.23 cubic yards, or 1.36 yd³ with 10% extra, which is 62 bags of 80 lb mix.
Advanced options: waste, prices
NRMCA advises ordering 4 to 10 percent over the estimate.
Concrete needed (with 10% extra)
1.96 yd³
1.50 m³ · 48 ft³ before extra
Slab: 12 ft × 12 ft × 4 in. Over about 1 yd³: order ready-mix.
Premix bags
- 80 lb88
- 60 lb118
- 50 lb141
- 40 lb176
- Ready-mix cost
- $313
- Weight
- 7,200 lb
Full breakdown
| Volume before extra | 1.78 yd³ · 1.36 m³ |
|---|---|
| With 10% extra | 1.96 yd³ |
| Weight (150 lb/ft³) | 7,200 lb · 3.60 tons |
| Pallets of 40 lb (80 bags each) | 3 |
| Pallets of 60 lb (56 bags each) | 3 |
| Pallets of 80 lb (42 bags each) | 3 |
| Truckloads at 12 yd³ | 1 |
| Truckloads at 8 yd³ | 1 |
Bag counts round up, as Quikrete's own calculator does. Cost uses your prices; delivery and short-load fees are extra.
Key takeaways
- A 10×10 ft slab at 4 in needs 1.36 yd³ with 10% extra, or 62 bags of 80 lb mix.
- Slabs on the ground must be at least 3½ in thick (2021 IRC R506.1).
- Above about 1 yd³, order ready-mix instead of bags (this calculator's own rule).
- Installed slabs cost $6.50–10.50 per sq ft (Concrete Network).
How Much Concrete Do I Need for a Slab?
You need the slab's length × width × thickness in cubic feet, divided by 27 for cubic yards, plus 10% extra. A 10×10 ft slab 4 in thick holds 33.33 ft³, which is 1.23 yd³ (0.94 m³). With 10% extra, the order is 1.36 yd³, or 62 bags of 80 lb concrete mix.
The extra 10% is this calculator's own rule. It sits inside the 4 to 10 percent that NRMCA tells buyers to order above the estimate, because uneven subgrade and spills use up concrete.
Concrete Slab Formula
The concrete slab formula multiplies length and width in feet by thickness in feet, then divides by 27.
Cubic yards = Length (ft) × Width (ft) × (Thickness (in) ÷ 12) ÷ 27
Metric: m³ = Length (m) × Width (m) × Thickness (m)Thickness is the step most people get wrong: 4 in is 0.333 ft, not 4 ft. A 12×12 ft slab at 4 in gives 12 × 12 × 0.333 = 48 ft³, and 48 ÷ 27 = 1.78 yd³.
How Many Bags of Concrete for a Slab?
A 4 in slab needs 45 bags of 80 lb concrete per cubic yard, before extra. The table gives bag counts for common slab sizes at 4 in, with 10% extra and every count rounded up, as Quikrete's own calculator does.
| Project | yd³ with 10% | 80 lb bags | 60 lb bags | 50 lb bags | 40 lb bags |
|---|---|---|---|---|---|
| 8×8 ft, 4 in | 0.87 | 40 | 53 | 63 | 79 |
| 10×10 ft, 4 in | 1.36 | 62 | 82 | 98 | 123 |
| 12×12 ft, 4 in | 1.96 | 88 | 118 | 141 | 176 |
| 16×20 ft, 4 in | 4.35 | 196 | 261 | 313 | 392 |
| 20×20 ft, 4 in | 5.43 | 245 | 326 | 392 | 489 |
Every size from 8×8 to 40×60 is on the slab sizes table.
How Thick Should a Concrete Slab Be?
A concrete slab on the ground must be at least 3½ in (89 mm) thick under the 2021 International Residential Code (R506.1). Most patios, sheds and car driveways are poured at 4 in (10 cm), and slabs for trucks or RVs at 5 to 6 in (13 to 15 cm), according to Concrete Network.
Each extra inch adds 25% to a 4 in slab's concrete. A 10×10 ft slab needs 1.23 yd³ at 4 in and 1.85 yd³ at 6 in. The full chart is in our guide on how thick a concrete slab should be.
Concrete Pad for a Shed, AC Unit or Hot Tub
A concrete pad is a small slab, so it uses the same formula. Pad size follows the footprint of what sits on it, plus a margin on each side.
| Pad | Example size | Thickness | yd³ with 10% | 80 lb bags |
|---|---|---|---|---|
| AC unit / mini split | 3×3 ft | 4 in (check the equipment manual) | 0.12 | 6 |
| Generator | 4×4 ft | 4 in (check the equipment manual) | 0.22 | 10 |
| Hot tub | 8×8 ft | At least 4 in, reinforced | 0.87 | 40 |
| Shed | 10×12 ft | 4 in | 1.63 | 74 |
Bullfrog Spas advises a hot tub pad at least 4 in thick, reinforced with rebar or mesh, and 6 in or more for larger tubs and swim spas. The AC and generator rows use 4 in only as a starting value; the equipment manual sets the real size.
Concrete for a Patio or Garage Slab
Patio and garage slabs are larger, so they usually pass the 1 yd³ point where ready-mix becomes the better buy.
| Slab | Size | Thickness | yd³ with 10% | Trucks at 12 yd³ |
|---|---|---|---|---|
| Patio | 12×16 ft | 4 in | 2.61 | 1 |
| One-car garage | 12×24 ft | 4 in | 3.91 | 1 |
| Two-car garage | 24×24 ft | 4 in | 7.82 | 1 |
| Two-car garage | 24×24 ft | 6 in | 11.73 | 1 |
Garage floor slabs need 3,500 psi concrete in severe weathering regions (2021 IRC Table R402.2).
Concrete for a Sidewalk
A sidewalk is a long, narrow slab, so it uses the same formula: a 4 ft wide, 20 ft long walk at 4 in needs 1.09 yd³ with 10% extra, or 49 bags of 80 lb mix. Two inches is not enough for a new sidewalk on the ground, since the 2021 IRC minimum for slabs-on-ground is 3½ in.
Concrete for a Garage or Basement Floor
A garage or basement floor is a slab-on-ground at least 3½ in thick (2021 IRC R506.1); garage floors need 3,500 psi in severe weathering regions and interior basement slabs 2,500 psi (Table R402.2). Use the garage rows above or enter your floor size.
Bags or Ready-Mix for a Slab?
Bags make sense for a slab up to about 1 cubic yard, and ready-mix makes sense above it; the 1 yd³ switch is this calculator's own rule. One cubic yard is 45 bags of 80 lb mix, or 3,600 lb to lift and mix by hand. A 20×20 ft slab at 4 in needs 5.43 yd³ with extra, which is one truck. Compare prices on bags or ready-mix.
How Much Does a Concrete Slab Cost?
A poured concrete slab costs $6.50 to $10.50 per square foot installed, according to Concrete Network's national range. A 20×20 ft slab therefore runs about $2,600–$4,200. The concrete alone costs $869–$1,059 at $160 to $195 per yd³. Sizes and options are on the concrete slab cost calculator.
How Much Concrete per Square Foot?
One square foot of slab at 4 in holds 0.333 ft³ of concrete, or 0.0123 yd³, so one cubic yard covers 81 sq ft at 4 in.
How Much Extra Concrete Should You Order?
Order 4 to 10 percent more concrete than the estimate, as NRMCA advises. A slab poured 1/8 in thicker than planned on a 4 in design uses 3 percent more concrete, so a small form error uses up most of a thin margin.
Do You Need Gravel Under a Concrete Slab?
Yes for house slabs below grade: the 2021 IRC (R506.2.2) calls for a 4 in base course. Patios and sheds on firm, well-drained soil are often poured on compacted ground, but a gravel base reduces settling.
Does a 4-Inch Concrete Slab Need Rebar?
No, not by code: the 2021 IRC treats reinforcement in slabs-on-ground as optional, and where it is used, it must sit between the center and the upper third of the slab (R506.2.4). Hot tub pads are an exception in manufacturer guidance, which asks for rebar or mesh.
Worked Examples: Three Slab Sizes
12×16 ft patio at 4 in. 12 × 16 × 0.333 = 64 ft³ = 2.37 yd³; with 10% extra, 2.61 yd³, or 118 bags of 80 lb mix, so a small ready-mix load is simpler.
24×24 ft garage at 4 in. 24 × 24 × 0.333 = 192 ft³ = 7.11 yd³; with 10% extra, 7.82 yd³, which fits one 8–12 yd³ truck. At 6 in, the same slab needs 11.73 yd³.
L-shaped slab. Split it into two rectangles, such as 20×12 ft and 10×8 ft at 4 in, and add them as parts: 80 + 26.67 = 106.67 ft³, or 4.35 yd³ with extra.
Common Slab Mistakes
Three mistakes cause most short slab orders.
- Thickness typed as feet: 4 in is 0.333 ft.
- Uneven subgrade: every extra 1/8 in on a 4 in slab adds 3 percent (NRMCA CIP 8).
- No waste allowance: order 4 to 10 percent extra.
Concrete Slab Calculator FAQ
How many yards of concrete for a 20×20 slab?
A 20×20 ft slab at 4 in needs 4.94 cubic yards of concrete, or 5.43 yd³ with 10% extra.
How many bags of concrete for a 12×12 slab?
A 12×12 ft slab at 4 in needs 88 bags of 80 lb concrete or 118 bags of 60 lb, with 10% extra.
How big of a pad can I pour with 1 yard of concrete?
One cubic yard of concrete pours an 81 sq ft pad at 4 in thick, such as 9×9 ft, before extra for waste.
Is it cheaper to buy bags of concrete or mix your own?
Bags are cheaper only for small pours under about 1 yd³; compare with your prices on bagged concrete vs ready-mix.
Next step
Price your slab with the concrete slab cost calculator, check the base on gravel under a concrete slab, then read ready-mix ordering guide.
Official sources
Last reviewed 2026-11-05. Numbers come from our tested engine; see how we calculate and our sources.