Concrete Slab Calculator
Calculate concrete volume for driveways, garage floors, foundations, and structural slabs. Includes an optional rebar quantity estimate when you enter a grid spacing.
Need to include material cost? Try the Concrete Estimator →
This driveway concrete slab calculator is designed for structural pours — driveways, garage floors, shop floors, and foundations where load-bearing capacity matters. It also works as a concrete slab calculator with rebar: enter your planned grid spacing in inches and the calculator adds a linear footage estimate alongside your concrete volume. Enter your slab dimensions and thickness to get cubic feet, cubic yards, and bag counts. Leave rebar spacing blank if you're not adding steel reinforcement and only the concrete volume will display.
How to Use the Concrete Slab Calculator
- Enter the slab length and width in feet.
- Enter thickness in inches — 4 inches minimum for residential, 6 inches for driveways and vehicle areas.
- Enter rebar spacing in inches if you plan to add steel reinforcement (leave blank to skip).
- Click "Calculate Slab" to get volume results and, if spacing was entered, the rebar linear footage.
- Add 10% to your concrete total before ordering to account for waste.
Concrete Slab Example: 20×24 Garage Floor at 4 Inches with 12-Inch Rebar
A 20-foot by 24-foot garage floor poured at 4 inches thick with rebar on a 12-inch grid:
| Measurement | Value |
|---|---|
| Volume | 160.00 cubic feet |
| Cubic yards | 5.93 cu yd |
| 60lb bags needed | 356 bags |
| 80lb bags needed | 267 bags |
| Rebar needed | 1,004 linear feet |
At 5.93 cubic yards this is a full readymix job. The 1,004 linear feet of rebar covers a complete bidirectional grid — 25 runs of 20 feet plus 21 runs of 24 feet. Standard rebar comes in 20-foot lengths, so you would need approximately 51 sticks.
Rebar Sizing and Spacing Reference
#3 rebar (3/8 inch) — walkways, thin decorative slabs, light-duty applications.
#4 rebar (1/2 inch) — standard for residential driveways, garage floors, and most structural slabs.
#5 rebar (5/8 inch) — commercial floors, heavy equipment pads, structural foundations.
Common spacing: 12 inches on center for driveways and garage floors, 18 inches for residential slabs with lighter loads, 6–8 inches for high-load applications.
Frequently Asked Questions
How much concrete do I need for a 10x10 slab?
How much concrete for a 10×10 slab depends on thickness: at 4 inches it is 33.33 cubic feet, or 1.23 cubic yards — 75 60lb bags or 56 80lb bags. At 6 inches thick, the same slab needs 50 cubic feet, or 1.85 cubic yards — 112 60lb bags or 84 80lb bags. For structural applications, 4 inches is a bare minimum; most professionals use 4.5–6 inches for slabs that carry any real load.
Do I need rebar in a concrete slab?
Rebar is strongly recommended for any slab carrying vehicle weight or structural load — driveways, garage floors, shop floors, and foundations. For residential driveways, #4 rebar on 18-inch centers is a standard specification. Slabs that will only see foot traffic can often use wire mesh instead. Either way, steel reinforcement dramatically reduces crack propagation when the slab does flex under load or temperature changes.
How do I calculate rebar for a concrete slab?
For a standard grid layout, count runs in each direction: runs along the length direction = floor(width ÷ spacing) + 1, and runs along the width direction = floor(length ÷ spacing) + 1. Multiply runs by their respective span to get linear feet. A 20×20 slab with 12-inch (1-foot) spacing needs 21 runs × 20 feet + 21 runs × 20 feet = 840 linear feet. This calculator handles that arithmetic automatically when you provide a spacing value.
How thick should a concrete slab be for a driveway?
How thick should a concrete slab be starts with the load it will carry. Residential driveways need a minimum of 4 inches, with 6 inches recommended for trucks and SUVs. Thin 4-inch driveways are prone to cracking under vehicle weight, especially without rebar. A 20×40 driveway at 4 inches is 9.88 cubic yards; at 6 inches it is 14.81 cubic yards — a significant difference, but the additional durability is usually worth the cost on a project that will last 30+ years.
How many cubic yards of concrete do I need for a 20x20 driveway slab?
A 20×20 slab at 4 inches thick is 4.94 cubic yards. At 6 inches it is 7.41 cubic yards. This concrete floor calculator in cubic yards works identically for garage floors and shop floors — just enter your floor dimensions and thickness. With 10% overage, order 5.44 yards (4-inch) or 8.15 yards (6-inch). At $150 per cubic yard, that is approximately $816 or $1,223 in concrete material respectively — before delivery, labor, forms, and rebar.