How much rebar do I need for a slab?
Rebar in a slab runs in two directions to form a grid. Bars in one direction = (slab width in inches ÷ spacing) + 1, each cut to the slab length minus edge cover. Repeat for the other direction.
Worked example: a 20 × 12 ft slab with #4 bar at 18 in on centre and 3 in cover. Usable dimensions are 19.5 × 11.5 ft. Across the width: (11.5 × 12) ÷ 18 = 7.7, so 8 bars at 19.5 ft. Along the length: (19.5 × 12) ÷ 18 = 13, so 14 bars at 11.5 ft. Total is 156 + 161 = 317 linear feet, about 212 lb of steel.
Add lap splices wherever a run is longer than a stock bar. Standard stock is 20 ft, and the lap is 40 bar diameters — 20 in for #4, 25 in for #5.
How the calculation works
Bar count per direction = floor(usable dimension in inches ÷ spacing) + 1. The "+1" catches the bar at the far edge, and forgetting it is the most common way rebar orders come up short.
Usable dimension is the slab dimension less twice the edge cover, since bars stop short of the forms.
Lap length is 40 × bar diameter, per ACI 318 for typical slab conditions. This calculator adds one lap for each stock length a run exceeds.
Unit weights are the ASTM standards: #3 at 0.376 lb/ft, #4 at 0.668, #5 at 1.043, #6 at 1.502, #7 at 2.044 and #8 at 2.670. Rebar is priced by weight, so linear feet × lb/ft is what the supplier bills against.
Typical rebar cost, mid-2026
| Material | Unit | Typical range |
|---|---|---|
| #3 rebar (3/8") | per linear ft | $0.40 – $0.90 |
| #4 rebar (1/2") | per linear ft | $0.70 – $1.60 |
| #5 rebar (5/8") | per linear ft | $1.10 – $2.40 |
| #6 rebar (3/4") | per linear ft | $1.60 – $3.40 |
| Rebar chairs | each | $0.30 – $1.20 |
| Tie wire | per 3.5 lb roll | $8.00 – $18.00 |
| Welded wire meshAlternative for light-duty slabs | per sq ft | $0.25 – $0.60 |
| Placement labour | per lb | $0.40 – $0.90 |
Rebar is a steel commodity — pricing moves with the market and can swing 30% in a year. Confirm before ordering large quantities.
What goes wrong on this job
- Support the steel on chairs. Rebar lying on the ground does nothing at all — it needs to sit near the middle of the slab depth to resist tension. This is the single most common failure in owner-poured slabs.
- Keep 3 in of cover anywhere concrete is cast against earth, 2 in for formed surfaces exposed to weather. Less cover means the steel corrodes, and corroding steel expands enough to break the slab apart.
- Lap splices need 40 bar diameters of overlap, tied — not butted end to end. A butted joint transfers no load and is effectively a planned crack.
- Order 20 ft stock rather than 10 ft where you can handle it. Fewer laps means less waste, less tying and a stronger mat.
- For light residential slabs, welded wire mesh is often specified instead. It is cheaper but far easier to get wrong, because it tends to end up flat on the ground under foot traffic during the pour.
Frequently asked questions
What size rebar for a concrete slab?
#4 (½ in) at 18 in on centre is the usual residential standard for a 4 in slab. Driveways and slabs carrying vehicles typically go to #4 at 12 in, or #5 for heavy loads.
How much does rebar weigh per foot?
#3 is 0.376 lb/ft, #4 is 0.668, #5 is 1.043, #6 is 1.502, #7 is 2.044 and #8 is 2.670 lb/ft.
How far apart should rebar be in a slab?
Eighteen inches on centre each way for a typical residential slab on grade. Twelve inches for driveways, garage floors and anything supporting vehicles.
How long should a rebar lap splice be?
Forty bar diameters as a general rule — 15 in for #3, 20 in for #4, 25 in for #5. Always tie the overlap rather than butting the ends.