Rebar Calculator
Bars, linear feet, weight & cost for any slab or wall
Table of Contents
What Is a Rebar Calculator and Why Do You Need One?
A rebar calculator estimates the number of reinforcing steel bars, total linear footage, weight, and cost for your concrete project. Whether you are pouring a driveway slab, a basement foundation, or a retaining wall, rebar is the skeleton that keeps concrete from cracking under tension.
Concrete handles compression well — it can support thousands of pounds per square inch. But it is weak in tension. That is exactly where rebar comes in. Steel reinforcing bars embedded inside the concrete absorb tensile forces, prevent crack propagation, and dramatically extend the life of your pour.
Why use a rebar calculator instead of eyeballing it?
- Accurate bar count — Buying too few bars means a trip back to Home Depot mid-pour. Buying too many wastes money at $7–$28 per bar.
- Correct spacing — ACI 318-19 limits rebar spacing in slabs to the lesser of 3× slab thickness or 18 inches. A calculator enforces that automatically.
- Weight planning — A #5 bar weighs 1.043 lb/ft. Forty 20-ft bars = 834 lbs. You need to know if your truck can handle that before you leave the yard.
- Budget control — Material + labor estimates before the first shovel hits dirt.
Our free rebar calculator gives you bar count, total linear feet, weight in pounds, estimated cost, 20-ft bar quantity, lap splice allowance, and tie wire estimate — all updating instantly as you type.
How to Calculate Rebar for a Concrete Slab (Step-by-Step)
Calculating rebar for a rectangular slab is straightforward once you understand the grid concept. Here is exactly how it works:
Step 1: Measure Your Slab Dimensions
Measure the length and width of your slab in feet. For our example, we will use a 20 ft × 12 ft driveway slab.
Step 2: Choose Your Bar Size
The most common rebar sizes for residential work are:
- #3 (⅜″ diameter) — Patios, sidewalks, light-duty slabs
- #4 (½″ diameter) — Driveways, garage floors, standard slabs
- #5 (⅝″ diameter) — Foundations, footings, structural slabs
- #6 (¾″ diameter) — Heavy foundations, commercial work
For a driveway, #4 rebar is the standard choice.
Step 3: Set Your Spacing
Rebar spacing is measured center-to-center (called “on center” or OC). Common residential spacings:
- 12″ OC — Heavy-duty (foundations, load-bearing slabs)
- 16″ OC — Standard (driveways, garage floors)
- 18″ OC — Light-duty (patios, sidewalks)
- 24″ OC — Minimum for temperature/shrinkage control
For a 4-inch driveway slab, ACI 318 allows up to 12 inches (3 × 4″ = 12″). We will use 16″ OC for a good balance of strength and cost.
Step 4: Calculate the Grid
The rebar grid runs in two directions — lengthwise bars and widthwise bars.
Lengthwise bars (running the 20-ft direction):
Number of bars = (Width ÷ Spacing) + 1 = (12 ft ÷ 1.33 ft) + 1 = 10 bars
Each bar = 20 ft long
Widthwise bars (running the 12-ft direction):
Number of bars = (Length ÷ Spacing) + 1 = (20 ft ÷ 1.33 ft) + 1 = 16 bars
Each bar = 12 ft long
Total bars = 10 + 16 = 26 bars
Total linear feet = (10 × 20) + (16 × 12) = 392 linear feet
Step 5: Add Lap Splice Length
Standard rebar comes in 20-foot lengths. If your slab is longer than 20 feet, you need to overlap (lap splice) the bars. The minimum lap splice for #4 rebar in 3,000 PSI concrete is 24 inches.
Add the lap splice length to your total when ordering. Our calculator handles this automatically.
Step 6: Calculate Weight and Cost
Using ASTM A615 weights:
- #4 rebar weighs 0.668 lb per foot
- 392 linear feet × 0.668 = 261.9 lbs total
- At ~$0.75/ft average: $294 in materials
Rebar Size Chart — ASTM A615 Standard Weights
This table shows every common rebar size used in residential and commercial concrete work. All weights follow ASTM A615 Grade 60 specifications — the standard for deformed steel reinforcing bars in the United States.
| Bar Size | Diameter (in) | Diameter (mm) | Weight (lb/ft) | Weight (kg/m) | Cross-Section Area (in²) | Common Use |
|---|---|---|---|---|---|---|
| #3 | 0.375 | 9.5 | 0.376 | 0.560 | 0.11 | Patios, sidewalks, temperature steel |
| #4 | 0.500 | 12.7 | 0.668 | 0.994 | 0.20 | Driveways, garage slabs, standard walls |
| #5 | 0.625 | 15.9 | 1.043 | 1.552 | 0.31 | Foundations, footings, structural slabs |
| #6 | 0.750 | 19.1 | 1.502 | 2.235 | 0.44 | Heavy foundations, columns, beams |
| #7 | 0.875 | 22.2 | 2.044 | 3.042 | 0.60 | Commercial foundations, bridge decks |
| #8 | 1.000 | 25.4 | 2.670 | 3.973 | 0.79 | Heavy structural, commercial piers |
Quick rule: To find rebar weight per foot, square the bar number and divide by 100. For #5: 5² ÷ 100 = 0.25… close to 0.31 — not exact but a useful field estimate. The ASTM values above are precise.
Rebar Spacing Guide by Project Type
Not sure what spacing to use? This table shows recommended rebar spacing for every common residential concrete project, based on ACI 318-19 and standard construction practice.
| Project Type | Slab Thickness | Recommended Bar Size | Spacing (OC) | ACI Max Spacing | Notes |
|---|---|---|---|---|---|
| Sidewalk | 4″ | #3 | 18–24″ | 12″ | Fiber mesh often used instead |
| Patio | 4″ | #3 | 18–24″ | 12″ | Wire mesh is common alternative |
| Driveway | 4–6″ | #4 | 12–18″ | 12–18″ | #4 @ 16″ is most common |
| Garage Floor | 4–6″ | #4 | 12–16″ | 12–18″ | Heavier if storing vehicles |
| Foundation Wall | 8–12″ | #5 | 12″ | 18″ | Horizontal + vertical bars |
| Footing | 8–12″ | #5 | 12″ | 18″ | Continuous bars, minimum 3″ cover |
| Retaining Wall | 8–12″ | #5–#6 | 8–12″ | 18″ | Heavier on tension face |
| Pool Deck | 4–5″ | #4 | 12–16″ | 12–15″ | Expansion joints every 10–12 ft |
ACI 318-19 Rule: Maximum rebar spacing in a structural slab shall not exceed the lesser of 3× the slab thickness or 18 inches. For a 4-inch slab, that means 12-inch maximum. For a 6-inch slab, that means 18-inch maximum.
Rebar Lap Splice Length Table
When your slab or wall is longer than a standard 20-foot rebar bar, you need to overlap the bars. This overlap is called a lap splice. The required lap length depends on bar size, concrete strength, and splice class.
| Bar Size | 3,000 PSI Concrete | 3,500 PSI Concrete | 4,000 PSI Concrete | 4,500 PSI Concrete | 5,000 PSI Concrete |
|---|---|---|---|---|---|
| #3 | 12″ | 12″ | 12″ | 12″ | 12″ |
| #4 | 24″ | 22″ | 21″ | 19″ | 18″ |
| #5 | 30″ | 28″ | 26″ | 24″ | 23″ |
| #6 | 36″ | 33″ | 31″ | 29″ | 28″ |
| #7 | 42″ | 39″ | 36″ | 34″ | 32″ |
| #8 | 54″ | 50″ | 47″ | 44″ | 42″ |
These are Class B tension lap splices per ACI 318-19 Section 25.5. Class B is the default for most residential work because more than 50% of the bars are typically spliced at the same location.
Pro tip: Stagger your lap splices. Don’t overlap all the bars at the same point — offset them by at least 24 inches. This prevents a weak plane in your slab.
Rebar vs Wire Mesh — Which Should You Use?
This is one of the most common questions homeowners ask. Here is a direct comparison:
| Feature | Rebar Grid | Wire Mesh (WWR) | Winner |
|---|---|---|---|
| Tensile Strength | 60,000 PSI (Grade 60) | 70,000–80,000 PSI | Wire mesh (per wire) |
| Crack Control | Excellent — holds cracks tight | Good for shrinkage only | Rebar |
| Structural Capacity | High — carries real loads | Low — temperature/shrinkage only | Rebar |
| Installation | Tie individual bars to chairs | Roll out and cut | Wire mesh (easier) |
| Cost (materials) | $0.50–$1.00/sq ft | $0.15–$0.40/sq ft | Wire mesh (cheaper) |
| Best For | Driveways, foundations, structural | Sidewalks, patios, light-duty | Depends on project |
| Code Requirement | Required for structural concrete | Acceptable for non-structural | Rebar (for code) |
My recommendation: Use rebar for any slab that will carry vehicle loads (driveways, garage floors) or support structure (foundations, footings). Wire mesh is fine for sidewalks and patios where the concrete is purely decorative or carries only foot traffic. When in doubt, go with rebar — the cost difference on a residential project is usually only $100–$300, and you get significantly better crack control.
How to Place Rebar in a Concrete Slab
Getting the rebar into the right position matters just as much as using the right amount. Here is the correct placement process:
| Step | Action | Key Detail |
|---|---|---|
| 1 | Set rebar chairs/supports | Place every 3–4 ft to hold bars at correct height |
| 2 | Lay lengthwise bars first | Space per your plan (12″, 16″, or 18″ OC) |
| 3 | Lay widthwise bars on top | Same spacing, perpendicular to first layer |
| 4 | Tie intersections | Use 16-gauge tie wire, wrap twice diagonally |
| 5 | Check concrete cover | Minimum 3″ from soil, 1.5″ from formed edges |
| 6 | Verify bar position | Rebar should sit at middle or lower third of slab |
Concrete cover is the distance between the rebar and the nearest concrete surface. ACI 318-19 requires:
- 3 inches minimum — concrete cast against and permanently exposed to earth
- 1.5 inches minimum — concrete exposed to weather (formed surfaces)
- 0.75 inches minimum — interior concrete not exposed to weather
Insufficient cover is the #1 cause of rebar corrosion. The steel rusts, expands, and spalls the concrete from inside. Always use proper rebar chairs — never let the bars sit on the ground and hope the pour lifts them.
Rebar Cost Guide — Materials and Labor (2026 Prices)
Rebar prices vary by region, bar size, and quantity. Here are current 2026 averages for the US market:
| Bar Size | Price per 20-ft Bar | Price per Linear Foot | Price per Pound | Where to Buy |
|---|---|---|---|---|
| #3 | $4.50–$7.00 | $0.23–$0.35 | $0.60–$0.90 | Home Depot, Lowe’s |
| #4 | $7.00–$12.00 | $0.35–$0.60 | $0.55–$0.85 | Home Depot, Lowe’s |
| #5 | $10.00–$16.00 | $0.50–$0.80 | $0.50–$0.80 | Home Depot, supply yards |
| #6 | $14.00–$22.00 | $0.70–$1.10 | $0.48–$0.75 | Steel supply yards |
| #7 | $18.00–$26.00 | $0.90–$1.30 | $0.45–$0.70 | Steel supply yards |
| #8 | $22.00–$28.00 | $1.10–$1.40 | $0.42–$0.55 | Steel supply yards |
Labor costs: Professional rebar installation runs $0.30–$0.50 per linear foot for residential work. A typical 20×12 ft driveway slab with #4 @ 16″ OC costs roughly $120–$200 in labor on top of materials.
Bulk discount: If you need more than 50 bars, call a local steel supply yard instead of buying from Home Depot or Lowe’s. You will save 20–30% and often get free delivery over certain quantities.
8 Pro Tips for Working with Rebar
Buy 10% Extra
Always order 10% more rebar than your calculator shows. You will lose material to cuts, bends, and overlap. Running short mid-pour is not an option — you cannot pause a concrete pour to run to the store.
Use the Right Chair Height
For a 4-inch slab, your rebar should sit about 2 inches from the bottom — roughly center of the slab. Use 2-inch rebar chairs (also called “dobies” or “bolsters”). Do not use bricks, rocks, or scrap wood as supports — they shift during the pour.
Tie Every Other Intersection
You do not need to tie wire at every single grid intersection. Tying every other one in a checkerboard pattern saves time and uses less wire without sacrificing grid stability. For residential work, this is standard practice.
Keep Bars Clean
Light surface rust (mill scale) is actually fine — it improves concrete bond. But heavy flaking rust, oil, grease, or mud on the bars will weaken the bond. Knock off anything loose with a wire brush before placing.
Never Weld Without Checking the Grade
Standard ASTM A615 rebar is not guaranteed weldable. If you need to weld, specify ASTM A706 (weldable grade) when ordering. Welding A615 can create brittle spots that crack under load.
Stagger Your Lap Splices
When bars need to overlap, never splice all of them at the same cross-section. Offset every other splice by at least 24 inches. This distributes the stress transfer zone and prevents a weak line across your slab.
Check Local Codes First
ACI 318 is the national standard, but your local building department may have stricter requirements. Some jurisdictions require #4 minimum for driveways or 12″ maximum spacing regardless of slab thickness. Pull the requirement before you order materials.
Store Bars Off the Ground
Stack rebar on wooden dunnage or concrete blocks, at least 6 inches off the ground. This prevents moisture contact, keeps bars clean, and makes them easier to pick up. Bars stored in mud for weeks develop heavy rust that needs cleaning.
Rebar Calculator Methods Compared
There are several ways to estimate rebar for a project. Here is how they compare:
| Method | Accuracy | Speed | Best For | Limitation |
|---|---|---|---|---|
| Online Calculator (this tool) | High | Instant | Rectangular slabs, standard grids | Cannot handle complex shapes |
| Manual Formula | High | 5–10 min | Learning, verification | Prone to math errors |
| Takeoff Software | Very High | 30+ min | Commercial projects, blueprints | Expensive, steep learning curve |
| Contractor Estimate | High | 1–3 days | Complex projects, custom shapes | Costs money, takes time |
| Rule of Thumb | Low–Medium | Instant | Rough budgeting only | Can be off by 20–30% |
Our recommendation: Use our free rebar calculator for all standard rectangular projects. It handles bar count, weight, cost, lap splices, and tie wire in one step — no formulas needed. For irregular shapes (L-shaped slabs, curved walls), break the area into rectangles and calculate each section separately, then add the totals.
For a 10×10 ft slab with #4 rebar at 12″ on center, you need about 22 bars (11 lengthwise + 11 widthwise). At 16″ spacing, that drops to about 16 bars. Use our calculator above — enter your dimensions and spacing and it gives you the exact count instantly.
#3 rebar (⅜″ diameter) is sufficient for light-duty 4-inch slabs like patios and sidewalks. For driveways and garage floors, use #4 rebar (½″ diameter) for better crack resistance under vehicle loads. The bar size matters less than proper spacing and concrete cover.
The most common driveway rebar spacing is #4 bars at 16 inches on center (OC) in both directions. ACI 318-19 limits spacing to the lesser of 3× slab thickness or 18 inches. For a standard 4-inch driveway, the maximum is 12 inches — but many contractors use 16″ for cost savings on thicker 5–6 inch slabs.
#4 rebar weighs 0.668 pounds per linear foot per ASTM A615. A standard 20-foot bar weighs 13.36 lbs. A quick rule: #4 at 12 inches both ways gives you roughly 1 pound of rebar per square foot of slab area.
In 2026, #4 rebar costs $0.35–$0.60 per linear foot at Home Depot or Lowe’s. A 20-foot bar runs $7–$12. Bulk orders from steel supply yards are 20–30% cheaper. Professional installation labor adds $0.30–$0.50 per foot on top of materials.
A lap splice is where two rebar bars overlap to transfer tensile forces across the joint. For #4 bars in 3,000 PSI concrete, the minimum Class B tension lap splice is 24 inches. Higher concrete strengths allow shorter splices. Always stagger lap splices — never overlap all bars at the same cross-section.
Sidewalks don’t usually require rebar by code since they carry only foot traffic. Many contractors use 6×6 welded wire mesh (WWR) or fiber mesh instead. However, adding #3 rebar at 24″ OC significantly reduces cracking, especially in freeze-thaw climates or on clay soils that shift. The extra cost is about $0.15–$0.25 per square foot.
There is no fixed ratio — rebar quantity depends on slab dimensions and spacing, not concrete volume. A 4-inch slab with #4 at 16″ OC uses roughly 0.5–0.7 lbs of rebar per square foot. For a rough budget: plan about 100–150 lbs of rebar per cubic yard of concrete in residential flatwork.
Wire mesh (welded wire reinforcement) works for non-structural slabs like patios and sidewalks. But for driveways, garage floors, foundations, or any slab that carries vehicle or structural loads, rebar is required by most building codes. Wire mesh only controls shrinkage cracking — rebar provides actual structural reinforcement.
Standard bundle sizes vary by bar size. #3 and #4 bars typically come in bundles of 10 at Home Depot and Lowe’s. Steel supply yards sell by weight in larger bundles — a 2,000-lb bundle of #4 contains about 150 twenty-foot bars. Individual bars are also sold at most home improvement stores.
In a 4-inch slab on grade, rebar should sit approximately 2 inches from the bottom — roughly the center or lower third of the slab. Use 2-inch rebar chairs (dobies) spaced every 3–4 feet. The bars must maintain minimum 3 inches of concrete cover from any surface exposed to soil per ACI 318-19.
Plan for roughly 1 tie per every other intersection (checkerboard pattern). Each tie uses about 10–12 inches of 16-gauge wire. A standard 3.5-lb roll contains roughly 200 feet and covers about 200 ties — enough for a typical residential slab up to 400 sq ft. Our calculator estimates tie wire automatically.
#3 rebar is sufficient for most patios since they carry only foot traffic and light furniture. The cost difference is small — #3 at $4.50/bar versus #4 at $7–$12/bar. If you are placing a hot tub, heavy planter, or fire pit on the patio, upgrade to #4. For a basic patio on stable soil, #3 at 18″ OC is standard practice.
Standard ASTM A615 rebar is not guaranteed weldable — welding can create brittle heat-affected zones. If your project requires welded rebar connections, specify ASTM A706 rebar, which is formulated for weldability. For residential work, mechanical couplers or standard lap splices are safer and more common than welding.
Break the irregular shape into rectangles. For an L-shaped slab, split it into two rectangles and calculate each one separately using the rebar calculator. Add the totals together, then add 15% extra for the cuts and waste at the joints. The bars running through the joint between sections should be continuous for structural continuity.