Rebar Calculator — Length, Weight & Spacing
Estimate total rebar length, weight, and the number of bars for concrete slabs and footings. Supports #3 through #8 bar sizes (10–25 mm) with custom spacing and overlap. Includes weight per bar based on ASTM A615.
Rebar Calculator
Rebar Sizes (ASTM A615 / Metric)
| Size | Diameter | Metric | Weight (lb/ft) | Weight (kg/m) |
|---|---|---|---|---|
| #3 | 3/8″ (0.375″) | 10 mm | 0.376 | 0.560 |
| #4 | 1/2″ (0.500″) | 13 mm | 0.668 | 0.994 |
| #5 | 5/8″ (0.625″) | 16 mm | 1.043 | 1.552 |
| #6 | 3/4″ (0.750″) | 19 mm | 1.502 | 2.235 |
| #7 | 7/8″ (0.875″) | 22 mm | 2.044 | 3.042 |
| #8 | 1″ (1.000″) | 25 mm | 2.670 | 3.973 |
US bar size #N corresponds to a diameter of N/8 inch. Metric equivalents are nominal — actual diameters per ASTM differ slightly.
How to Calculate Rebar for a Slab
- Measure slab dimensions in feet (or meters): length and width.
- Choose bar size and spacing. Most residential slabs: #4 at 12″ (30 cm) on center, both directions.
- Bars in each direction: floor(slab dimension ÷ spacing) + 1. Add bars in BOTH directions for a grid (running lengthwise and widthwise).
- Length per bar: if your slab is longer than the standard bar (typically 20 ft / 6 m), add overlap (40× bar diameter) at each splice.
- Total weight: total length × weight per foot (from the table above).
Imperial example: 20×20 ft slab, #4 bar, 12″ spacing, 20-ft bars. Bars lengthwise = floor(20÷1) + 1 = 21. Same for widthwise = 21. Total = 42 bars × 20 ft = 840 ft. Weight = 840 × 0.668 = 561 lbs (254 kg).
Metric example: 6×6 m slab, 13 mm bar, 30 cm spacing, 6-m bars. Bars per direction = 21. Total = 42 bars × 6 m = 252 m. Weight = 252 × 0.994 = 250 kg.
Rebar Spacing Guide
| Application | Bar Size | Spacing |
|---|---|---|
| Walkway / patio (4″ slab) | #3 or #4 | 16–18″ (40–45 cm) |
| Residential driveway (5–6″) | #4 | 12″ (30 cm) both ways |
| Garage floor (5–6″) | #4 | 12″ (30 cm) both ways |
| Heavy load slab (8″+) | #5 | 10–12″ (25–30 cm) |
| Footings (continuous) | #4 or #5 | 6–12″ (15–30 cm) |
| Walls (vertical & horizontal) | #4 or #5 | 12″ (30 cm) both ways |
Overlap and Development Length
Where two bars meet end-to-end (a splice), they must overlap to transfer load. The standard rule per ACI 318:
- Lap length = 40 × bar diameter for tension splices in standard concrete.
- #3 bar: 40 × 0.375″ = 15″ (38 cm) overlap
- #4 bar: 40 × 0.5″ = 20″ (51 cm) overlap (24″ / 60 cm is common rule of thumb)
- #5 bar: 40 × 0.625″ = 25″ (64 cm) overlap
- #6 bar: 40 × 0.75″ = 30″ (76 cm) overlap
For high-stress applications or specific concrete strengths, consult ACI 318 Chapter 25 or your project engineer. Tie all crossings with rebar tie wire to maintain spacing during the pour.
Frequently Asked Questions
How much rebar for a 20×20 slab?
A 20×20 ft slab with #4 rebar at 12-inch spacing both directions needs about 21 bars × 20 ft per direction = 42 bars (840 ft total). Total weight: ~561 lbs (254 kg). Cost: $168–$252 at $4–$6 per 20-ft bar.
What size rebar for a 4-inch slab?
#3 (3/8″ / 10 mm) for residential 4″ slabs and walkways. #4 (1/2″ / 13 mm) for driveways, garages, and 5–6″ slabs. #5 (5/8″ / 16 mm) for heavy-load applications and footings. Always follow local building codes — engineered designs override these defaults.
How far apart should rebar be spaced?
Standard residential slabs: 12–18 inches (30–45 cm) on center, both directions. Driveways and heavy loads: 12 inches (30 cm). Footings: 6–12 inches (15–30 cm). Walls: 12 inches (30 cm) vertical and horizontal. Closer spacing increases strength but adds material cost.
How much does rebar cost?
Approximately $4–$6 per 20 ft (6 m) #4 bar in 2026 US. Larger bars cost more: #5 ~$6–$9, #6 ~$9–$13. Bulk delivery from steel suppliers reduces per-bar cost significantly. Add $50–$100 for delivery on small orders.
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Weights from ASTM A615/A615M standard for deformed steel reinforcing bars. Spacing recommendations are typical for residential applications; commercial and structural designs require engineered drawings and may use different spacings, sizes, or grade (e.g., grade 60 vs 80). Always follow local building codes.
