01 Concrete slab rebar calculator
Enter your slab dimensions, on-center spacing and edge cover to calculate exact lengthwise and widthwise runs, 40-diameter lap splices, stock bars to purchase, tie intersections and support chairs.
Need the concrete yardage too? Open our concrete slab volume calculator or driveway cost estimator.
02 Rebar grid & lap splice formula
Parallel bar runs
Runs = ⌊(Span − 2 × Cover) ÷ Spacing⌋ + 1
Splice overlap (40d rule)
Lap = 40 × Bar Diameter
Always add + 1 to the division result to account for the starter bar along the first edge. Whenever a slab dimension exceeds your supplier’s stock bar length (typically 20 ft in the US or 6 m in the UK, Australia and New Zealand), add a 40-bar-diameter lap splice at every joint.
03 Step-by-step rebar takeoff
- Deduct concrete cover. Subtract twice your edge clearance (2 × 3 in = 6 in, or 150 mm) from both slab length and slab width.
- Count runs each way. Divide the usable width by on-center spacing, round down and add 1 for lengthwise runs; repeat with usable length for widthwise runs.
- Include lap splices. Any run longer than one 20-ft (or 6-m) stock bar gains 40 bar diameters of overlap per splice (20 in for #4 bar; 480 mm for 12 mm bar).
- Add 5–10% offcut waste and divide total linear length by stock bar length, rounding up to whole bars.
- Count chairs & ties. Multiply lengthwise × widthwise runs for wire ties, and budget roughly one rebar chair per 9 ft² (0.85 m²).
04 #4 rebar needed for common slab sizes
| Slab size | Spacing | Grid runs | Linear ft (+10%) | 20-ft #4 bars | Chairs |
|---|---|---|---|---|---|
| 10 × 10 ft | 18 in o.c. | 7 × 7 | 146 ft | 8 bars | 12 |
| 12 × 16 ft | 18 in o.c. | 11 × 8 | 276 ft | 14 bars | 22 |
| 20 × 20 ft | 18 in o.c. | 14 × 14 | 601 ft | 31 bars | 45 |
| 20 × 20 ft | 12 in o.c. | 20 × 20 | 858 ft | 43 bars | 45 |
| 24 × 24 ft | 18 in o.c. | 16 × 16 | 886 ft | 45 bars | 64 |
| 40 × 16 ft | 18 in o.c. | 11 × 27 | 979 ft | 49 bars | 72 |
05 Rebar size, diameter & weight reference
US bar numbers count eighths of an inch (so #4 = 4/8 in = ½ in). UK, Australian and New Zealand steel uses metric diameter in millimetres:
| Designation | Diameter | Unit weight | 40d splice lap |
|---|---|---|---|
| #3 (3/8 in · 10 mm) | 0.375 in | 0.376 lb/ft | 15 in |
| #4 (1/2 in · 13 mm) | 0.5 in | 0.668 lb/ft | 20 in |
| #5 (5/8 in · 16 mm) | 0.625 in | 1.043 lb/ft | 25 in |
| #6 (3/4 in · 19 mm) | 0.75 in | 1.502 lb/ft | 30 in |
| H10 / N10 (10 mm) | 10 mm | 0.617 kg/m | 400 mm |
| H12 / N12 (12 mm) | 12 mm | 0.888 kg/m | 480 mm |
| H16 / N16 (16 mm) | 16 mm | 1.578 kg/m | 640 mm |
06 Rebar grid vs welded wire mesh
Deformed rebar grid (#3–#5 / H12)
- High tensile capacity for vehicle loads & expansive soils
- Rigid enough to stay on chairs when crews walk the pour
- Easy to add perimeter corner hairpins and dowels
Welded wire fabric (WWF / SL72 / A142)
- Fast sheet installation on flat patios & warehouse floors
- Controls fine surface shrinkage cracking
- Must use flat sheets on chairs—rolled mesh curls and sinks
07 Chairs, cover & jobsite tips
08 Frequently asked questions
How much rebar do I need for a 20×20 concrete slab?
A 20 × 20 ft slab with #4 rebar spaced 18 inches on center and 3 inches of edge clearance takes 14 bars in each direction (28 runs × 19.5 ft = 546 linear feet). With 10% waste, order 31 standard 20-foot bars (about 401 lb of steel).
What size and spacing of rebar is best for a residential slab or driveway?
Light patios and walkways often use #3 (3/8 in) or #4 (1/2 in) rebar at 18 to 24 inches on center. Residential driveways and garage slabs typically specify #4 rebar (or metric H12 / N12) at 12 to 18 inches on center, subject to local code and engineer specs.
How far should rebar overlap when splicing two bars together?
A common residential rule of thumb is overlapping spliced bars by 40 bar diameters (40d): 15 inches for #3 bar, 20 inches for #4 bar, 25 inches for #5 bar and 480 mm for 12 mm metric bar, tied securely with anneal wire.
How far from the edge of the forms should slab rebar stop?
Maintain 3 inches (75 mm) of clear concrete cover between rebar ends and earth or perimeter forms so moisture cannot reach the steel and cause rust spalling.
Where should rebar sit vertically inside a 4-inch or 6-inch slab?
Single-mat slab-on-grade rebar belongs near the middle-to-upper third of the slab depth, supported on plastic or steel rebar chairs every 3 feet—never lying flat on the dirt or gravel where it provides virtually zero tensile help.
Is rebar better than welded wire mesh for a driveway?
Welded wire mesh helps control hairline plastic shrinkage cracks, provided workers do not step it down onto the gravel during the pour. A chaired #4 rebar grid offers substantially higher load-carrying and crack-bridging strength for driveways and heavy vehicles.
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