Rebar Calculator - Grid Bars, Pieces, Lap & Weight
A slab rebar grid is counted in two directions from slab dimensions and spacing. The calculator adds lap/waste, rounds to stock bar pieces, and estimates steel weight.
Percent added for cuts, waste, settlement, or field loss.
Cost uses rounded stock bar pieces.
Change any value and the results, formula, and diagram update immediately. Use the same unit system throughout one estimate.
Rebar Quick Reference
Bar counts and total length for common slab grids using #4 (1/2 in) bars at 12 in spacing - the residential default for driveways and footings. Use this for the planning conversation; the calculator handles your real size and spacing.
| Slab size | Bars (width) | Bars (length) | Total linear ft | 20 ft stock pieces |
|---|---|---|---|---|
| 10 x 10 ft | 11 | 11 | 220 | 11 |
| 15 x 20 ft | 16 | 21 | 635 | 32 |
| 20 x 30 ft | 21 | 31 | 1,250 | 63 |
| 30 x 40 ft | 31 | 41 | 2,470 | 124 |
| 40 x 60 ft (commercial) | 41 | 61 | 4,900 | 245 |
Add 5-10% for lap splices and cutting waste. The calculator handles lap automatically when you set the lap-length field.
Rebar Sizes - US, India, and Metric
The size system is regional. The US uses bar number (8ths of an inch); India and most of the world use millimeters of nominal diameter.
| US bar | Diameter (in) | Metric equivalent (mm) | Weight (lb/ft) | Weight (kg/m) | Common use |
|---|---|---|---|---|---|
| #3 | 3/8 in (0.375) | 10 mm | 0.376 | 0.560 | Stirrups, ties, light residential |
| #4 | 1/2 in (0.500) | 12 mm / 13 mm | 0.668 | 0.994 | Residential slabs, walls (standard) |
| #5 | 5/8 in (0.625) | 16 mm | 1.043 | 1.552 | Driveways, light footings, structural |
| #6 | 3/4 in (0.750) | 20 mm | 1.502 | 2.235 | Columns, beams, heavy footings |
| #8 | 1 in (1.000) | 25 mm | 2.670 | 3.973 | Commercial columns, large structural |
| #10 | 1-1/4 in | 32 mm | 4.303 | 6.404 | Heavy commercial, bridges |
India: TMT bars (Thermo-Mechanically Treated) come in grades Fe-415, Fe-500, Fe-550, Fe-600. Fe-500 (500 N/mm² yield) is the standard structural grade.
Spacing and Cover
- Slab on grade: #4 at 16-18 in OC each way is standard residential. Heavier loads need #5 at 12-16 in OC.
- Structural slab: #4 to #5 at 12 in OC each way, top and bottom mat.
- Walls: #4 at 16-24 in OC vertical, #4 at 16-24 in OC horizontal.
- Columns: 4-8 vertical bars (#5 to #8) with #3 ties at 12 in OC.
- Footings: 2-4 longitudinal #4 or #5 bars plus #3 ties.
Concrete cover (distance from bar to nearest concrete face):
- 3 in for concrete cast against earth (footings).
- 2 in for concrete exposed to weather and earth (slabs against earth, walls).
- 1.5 in for concrete exposed to weather only.
- 3/4 in for interior, dry concrete.
Lap Length and Splicing
Bars come in fixed stock lengths (20 ft or 40 ft in US, 12 m in India). When the slab is longer than the stock, bars need overlapping (lap splice) so the load transfers from one bar to the next.
- Standard lap length: 40-60 bar diameters. A #4 (1/2 in) bar needs 20-30 in of overlap; a #5 needs 25-37 in.
- Tension lap: longer (60 bar diameters) for bars carrying primary tension.
- Compression lap: shorter (30 bar diameters) in columns under compression.
- Mechanical couplers: threaded couplers eliminate lap, save steel on big jobs. Cost more per piece but no waste from lap.
- Welded splices: only when specified, and only by certified welders. Most projects skip welding.
Reading A Rebar Schedule
Structural drawings include a rebar schedule like "#4 @ 12 OC EW T&B" - decoded:
- #4: bar size (1/2 in diameter).
- @ 12: spacing (12 inches on centre).
- OC: on centre.
- EW: each way (perpendicular bars).
- T&B: top and bottom (two mats of steel).
For Indian drawings: "12 mm @ 150 c/c BW" reads as 12 mm bars at 150 mm centre-to-centre, both ways.
Cost Estimates Around The World
2026 retail prices for Grade 60 (Fe-500) deformed bar, delivered.
| Region | Per pound / kg | Per #4 (12 mm) bar 20 ft (6 m) | Per ton / tonne |
|---|---|---|---|
| United States (USD) | $0.70-1.50 / lb | $9-20 per 20 ft #4 | $1,400-2,500 / ton |
| Canada (CAD) | C$1.00-2.00 / lb | C$13-26 per 20 ft | C$2,000-3,500 / tonne |
| United Kingdom (GBP) | £0.80-1.50 / kg | £5-9 per 6 m T12 | £800-1,500 / tonne |
| Eurozone (EUR) | €0.90-1.70 / kg | €6-10 per 6 m B500B | €900-1,700 / tonne |
| Australia (AUD) | A$1.50-2.50 / kg | A$9-15 per 6 m N12 | A$1,500-2,500 / tonne |
| India (INR) | ₹55-80 / kg (Fe-500) | ₹300-450 per 12 m of 12 mm bar | ₹55,000-80,000 / tonne |
| Mexico (MXN) | MX$22-40 / kg | MX$130-240 per 9 m #4 | MX$22,000-40,000 / tonne |
| Philippines (PHP) | PHP 65-100 / kg | PHP 400-700 per 6 m 12 mm | PHP 65,000-100,000 / tonne |
Brand reference: US - Nucor, Gerdau, Commercial Metals (CMC), Steel Dynamics; UK / EU - Celsa, ArcelorMittal, British Steel; India - Tata Tiscon, JSW Neosteel, SAIL, Vizag Steel, Kamdhenu, Jindal Panther; Mexico - DeAcero, AceroPan. Steel prices are volatile - re-confirm at order time.
Common Mistakes
- Counting bars in one direction only - rebar grid is both ways.
- Forgetting lap splices on long slabs - they can add 5-10% steel.
- Wrong cover - rebar too close to the surface rusts and spalls the concrete.
- Mixing US and metric sizes - #4 is not 12 mm exactly (it is 12.7 mm).
- Tying with too few wire ties - bars sag during pour and concrete cover drops.
- Using chairs / dobies too far apart - bars sag between supports.
- Substituting Grade 40 for Grade 60 without engineering review - lower yield strength.
- Ordering exact length without stock-length rounding - cuts waste 5-10%.
Rebar Calculator FAQ
How much rebar for a 20 x 30 ft slab?
At #4 bars 12 in OC each way: 21 bars across the 20 ft direction (each 30 ft long), 31 bars across the 30 ft direction (each 20 ft long). Total: 21x30 + 31x20 = 1,250 lf. At 20 ft stock, that is 63 pieces. With 10% lap and waste: 70 pieces. Weight: 1,250 lf x 0.668 lb/ft = 835 lb.
What size rebar should I use?
#4 (1/2 in / 12 mm) is the residential default for slabs and walls. #5 for driveways with vehicle loads. #6 for footings and columns. #8+ for commercial structural work. Always per engineer or local code.
How far apart should rebar be spaced?
12-18 inches on centre for residential slabs. 6-12 inches for heavy structural. Walls: 16-24 in OC. Columns: 4-8 vertical bars with closely-spaced ties (4-12 in).
What is the lap length?
40-60 bar diameters. A #4 bar (1/2 in) laps 20-30 in. A 12 mm bar laps 480-720 mm. In tension zones use 60 diameters; in compression 30 is enough. Engineering drawings always show the project-specific value.
How much concrete cover do I need over rebar?
3 in over earth (footings). 2 in for slabs and walls against earth. 1.5 in exposed to weather. 3/4 in for interior dry concrete. Less cover means corrosion risk; more cover wastes concrete.
How much does rebar weigh?
#3: 0.38 lb/ft. #4: 0.67 lb/ft. #5: 1.04 lb/ft. #6: 1.50 lb/ft. A 20 ft length of #4 weighs about 13.4 lb. A bundle of 60 bars weighs about 800 lb - lift carefully.
What is the difference between Grade 40 and Grade 60 rebar?
Grade refers to minimum yield strength in ksi. Grade 40 = 40,000 psi, Grade 60 = 60,000 psi. Grade 60 is the modern US standard. India uses Fe-415, Fe-500, Fe-550 (in N/mm²).
Can I use wire mesh instead of rebar?
For light residential slabs (driveways under 4 in, patios), welded wire fabric (WWF or WWM) is a code-approved alternative. For structural work, footings, columns, and loaded slabs, only rebar meets the strength requirements.
How much does rebar cost?
US: $0.70-1.50 per lb. UK: £0.80-1.50 per kg. India: ₹55-80 per kg for Fe-500 TMT. A 20-yard concrete slab with #4 grid at 16 in OC needs about $400-800 of US rebar.
How is rebar tied together?
16 or 18 gauge tie wire, twisted around each intersection with a tie tool. Roughly every other intersection needs a tie on horizontal slab grids. Fully-tied grids tie every intersection.
Related Construction Calculators
For the slab or footing the rebar reinforces, use the Slab Calculator or Footing Calculator. For concrete volume, see the Concrete Calculator. For cement-only bag math, the Cement Calculator. More tools live on the Construction Calculators hub.
Sources
- Concrete Reinforcing Steel Institute (CRSI) - design and detailing resources
- ACI 318 - Building Code Requirements for Structural Concrete
- ASTM A615 standard specification for deformed steel bars
- Bureau of Indian Standards - IS 1786 (high-strength deformed bars), IS 1139 (hot-rolled mild steel)
- BS 4449 - UK reinforcing steel standard
- NIST Handbook 44 unit conversion tables
This calculator is for planning and ordering conversations. Local code, project drawings, engineered design, and manufacturer instructions control the final work.