Rebar Calculator
Use this free rebar calculator to find out exactly how much rebar you need for any concrete slab, driveway, foundation, or retaining wall. Enter your slab dimensions, select your rebar spacing and bar size, and the calculator instantly gives you total linear feet, number of 20 ft bars to order, estimated weight, and material cost — all based on a standard two-way grid layout with lap splice overlap included.
Total Linear Ft = (Rows × Width + Rows × Length) × Lap Factor
Calculates a two-way grid pattern (rows in both directions) · Based on standard 20 ft rebar bars · #4 rebar weighs 0.668 lbs/ft · Always add 10% for waste and overlaps
Estimates based on 2026 US average pricing. Rebar prices fluctuate — always confirm current pricing at your local supplier.
How Does the Rebar Calculator Work?
This rebar calculator estimates the total linear feet of rebar needed for any concrete slab or foundation, based on your area dimensions, rebar spacing, and bar size. It calculates a standard two-way grid pattern (rows running in both directions) and includes lap splice overlap allowance for joints where bars connect.
Steps:
- Enter your slab length and width in feet.
- Select your rebar spacing — 12" on center is standard for most residential concrete slabs.
- Select your rebar size — #4 (½ inch) is the most common for driveways, patios, and slabs.
- Select your lap splice overlap — use 8% for standard residential work.
- Get instant results — total linear feet, number of 20 ft bars, estimated weight, and material cost.
Always order 10% more rebar than calculated. This accounts for cutting waste, additional overlaps at corners, and any errors during placement. Rebar is inexpensive — running short mid-pour is a serious problem.
Rebar Size & Weight Chart
Rebar is identified by bar number, which corresponds to the diameter in eighths of an inch. Here's a complete reference chart for common rebar sizes used in residential and commercial construction.
| Bar # | Diameter | Weight (lbs/ft) | Weight (lbs/20 ft bar) | Common Use |
|---|---|---|---|---|
| #3 | 3/8 inch | 0.376 lbs/ft | 7.5 lbs | Sidewalks, small slabs |
| #4 | 1/2 inch | 0.668 lbs/ft | 13.4 lbs | Driveways, patios, slabs |
| #5 | 5/8 inch | 1.043 lbs/ft | 20.9 lbs | Foundations, retaining walls |
| #6 | 3/4 inch | 1.502 lbs/ft | 30.0 lbs | Heavy foundations, columns |
| #7 | 7/8 inch | 2.044 lbs/ft | 40.9 lbs | Commercial, bridge decks |
| #8 | 1 inch | 2.670 lbs/ft | 53.4 lbs | Heavy structural |
*Standard rebar bars are sold in 20 ft lengths in the US. Some suppliers also carry 40 ft and 60 ft lengths for large projects.
Rebar Spacing Guide
The correct rebar spacing depends on what you're building, the thickness of the concrete, and local building codes. Here's a general guide for common residential applications.
| Application | Recommended Spacing | Rebar Size | Concrete Thickness |
|---|---|---|---|
| Sidewalk / Path | 18 – 24 inches OC | #3 | 4 inches |
| Residential Patio | 18 inches OC | #3 or #4 | 4 inches |
| Driveway | 12 – 18 inches OC | #4 | 4 – 6 inches |
| Garage Floor Slab | 12 inches OC | #4 | 4 – 6 inches |
| House Foundation | 12 inches OC | #4 or #5 | 8 – 12 inches |
| Retaining Wall | 12 inches OC | #4 or #5 | 8 – 12 inches |
| Pool Deck | 12 inches OC | #4 | 4 – 6 inches |
| Commercial Slab | 6 – 12 inches OC | #5 or #6 | 6+ inches |
Rebar spacing requirements vary by municipality and project type. Always check your local building code and permit requirements before placing rebar. Many jurisdictions require a structural engineer's drawings for foundations, retaining walls, and any slab over a certain size.
Rebar Cost by Size (2026)
Rebar is priced per linear foot or per 20 ft bar. Here's a breakdown of 2026 US average pricing for common rebar sizes.
| Size | Diameter | Price per Linear Ft | Price per 20 ft Bar | Price per Ton |
|---|---|---|---|---|
| #3 | 3/8" | $0.25 – $0.45 | $5 – $9 | $700 – $1,000 |
| #4 | 1/2" | $0.35 – $0.60 | $7 – $12 | $650 – $950 |
| #5 | 5/8" | $0.50 – $0.85 | $10 – $17 | $600 – $900 |
| #6 | 3/4" | $0.70 – $1.15 | $14 – $23 | $580 – $880 |
*Prices are for standard Grade 60 deformed rebar (ASTM A615). Epoxy-coated rebar costs 25–40% more. Stainless steel rebar costs 8–10× more. Prices fluctuate with steel market conditions.
For projects requiring 1+ ton of rebar, contact steel distributors directly for bulk pricing — you can typically save 15–25% compared to home improvement store pricing. Always buy the same grade and manufacturer batch to ensure consistent strength ratings.
Example Calculation
You want to reinforce a 20 ft × 15 ft concrete driveway slab with #4 rebar at 12" on center, standard 8% lap overlap.
Length direction: (20 ft × 12 in) ÷ 12 in + 1 = 21 rows
Width direction: (15 ft × 12 in) ÷ 12 in + 1 = 16 rows
Length rows: 21 rows × 15 ft = 315 linear ft
Width rows: 16 rows × 20 ft = 320 linear ft
Subtotal: 635 linear ft
635 × 1.08 = 686 linear ft total
Step 4 — Convert to 20 ft bars:686 ÷ 20 = 34.3 → 35 bars (round up) + 10% waste = 39 bars
Step 5 — Estimated weight:686 ft × 0.668 lbs/ft = ~458 lbs (~0.23 tons)
Step 6 — Estimated cost (#4 at $0.35–$0.60/ft):686 × $0.35 = $240 | 686 × $0.60 = $412
Material cost: $240 – $412
Rebar Installation Tips
Concrete Cover Requirements
Rebar must be positioned inside the concrete with adequate cover — the distance from the rebar surface to the edge of the concrete. Too little cover causes the rebar to rust and spall the concrete. Use plastic rebar chairs or wire spacers to maintain proper positioning.
| Application | Minimum Cover |
|---|---|
| Slab on grade (not exposed to weather) | ¾ inch |
| Slab exposed to weather | 1½ inches |
| Foundation wall exposed to soil | 3 inches |
| Footing cast against soil | 3 inches |
Key Installation Tips
- Tie intersections — use wire ties at every grid intersection to keep the rebar grid in place during the pour.
- Maintain proper cover — use rebar chairs to keep bars the correct distance from the bottom of the slab. Never let rebar sit directly on the ground.
- Lap splices correctly — overlap bars by at least 24 bar diameters at joints (24 × ½" = 12" for #4 rebar).
- Keep rebar clean — remove any mud, oil, or loose rust before placing concrete. Light surface rust is acceptable but flaky rust is not.
- Never cut rebar with an angle grinder near fresh concrete — sparks can damage the concrete surface and surrounding formwork.
For light-duty applications like sidewalks and small patios, wire mesh (welded wire fabric) is a cheaper and faster alternative to rebar — costing $0.10–$0.30/sq ft vs $0.35–$0.60/linear ft for #4 rebar. For driveways, foundations, and structural slabs, rebar provides significantly better crack resistance and is required by most building codes.