How to use this rebar calculator
- Enter the slab length and width.
- Choose the grid spacing: 12–18 in is common for driveways and patios.
- Keep bars about 3 in from the slab edges.
- Allow 10–15% extra for overlaps (typically 40 bar diameters) and waste.
Example
A 24 × 24 ft garage slab with an 18 inch rebar grid needs 827 ft (252 m) of bar, or 42 × 20 ft.
What to know before you buy
Rebar sizes
Rebar is numbered by eighths of an inch in diameter: #3 is 3/8 inch, #4 is 1/2 inch and #5 is 5/8 inch. Residential slabs and footings commonly use #3 or #4. Bars are sold in 10 ft and 20 ft lengths.
Rebar or wire mesh?
Welded wire mesh is quicker to lay and suits light-duty slabs like patios and walkways. Rebar provides more strength for driveways, garage floors and structural footings. Both only work if supported in the middle of the slab during the pour.
Rebar for square slabs (12 in grid, 3 in edges, 10% overlap)
| Slab | Rebar sticks | Total length |
|---|---|---|
| 10 × 10 ft | 11 × 20 ft | 209 ft (64 m) |
| 12 × 12 ft | 16 × 20 ft | 304 ft (93 m) |
| 20 × 20 ft | 43 × 20 ft | 858 ft (262 m) |
| 24 × 24 ft | 63 × 20 ft | 1,241 ft (378 m) |
| 30 × 30 ft | 98 × 20 ft | 1,947 ft (593 m) |
Formula
Bars lengthwise = (width − 2 × edge) ÷ spacing + 1, each (length − 2 × edge) long, and vice versa. Total length × (1 + overlap) ÷ stick length.
Tips from the pros
- Support rebar on chairs or blocks so it sits in the middle of the slab, not on the ground.
- Tie intersections with wire every few crossings to keep the grid in place during the pour.
- #3 (3/8 in) and #4 (1/2 in) bars are the most common for residential slabs.
Common mistakes to avoid
- Laying rebar directly on the ground, where it does nothing to strengthen the slab.
- Leaving bar ends too close to the slab edge, where they can rust.
Frequently asked questions
How far apart should rebar be in a slab?
Commonly 12 to 18 inches on center for residential slabs; follow your plans or local code.
How much should rebar overlap?
A common rule is 40 times the bar diameter, e.g. 20 inches for #4 (1/2 in) bar.
How much rebar for a 20x20 slab?
With a 12 inch grid and 3 inch edges, about 780 ft of bar plus overlap, roughly 43 sticks of 20 ft.
Is rebar necessary in a concrete patio?
Not always; many patios use wire mesh or fiber-reinforced concrete, but rebar reduces cracking and is recommended for driveways.