Rebar (Reinforcing Steel) Calculator

Enter your slab or footing dimensions and rebar spacing to instantly calculate total linear feet, weight in pounds, number of bars, and material cost estimate.

Free to use No sign-up required Formulas verified against ACI 318 Imperial & metric supported

✓ Linear feet & bar count ✓ Weight in pounds & kg ✓ Cost estimator included ✓ Last verified May 2026

Reviewed by the AllConcreteCalculator.com editorial team — formulas cross-checked against ACI 318 standards, May 2026.

Enter Your Slab & Rebar Details

Slab / Footing Length

The longest dimension of the area to be reinforced. Please enter a valid length greater than 0.

Slab / Footing Width

The shorter dimension. For a one-way layout, set width to the footing depth. Please enter a valid width greater than 0.

Rebar Spacing (on-center)

Residential slabs: 12–18 in. Driveways: 12 in. Footings & structural: 6–12 in. Please enter a valid spacing greater than 0.

Rebar Size (Bar Number)

#4 is standard for residential slabs. #5–#6 for driveways and footings. #7+ for structural work.

Waste / Overage Factor (%)

Add 10% for cuts, laps, and waste. Add 15%+ for complex shapes.

Stock Bar Length

Used to calculate number of full bars to purchase.

Price per Linear Foot (optional)

Leave blank to skip cost estimate. US average: $0.40–$0.75/ft for #4 rebar (material only).

Results appear instantly. No sign-up required.

Your Rebar Estimate

Total Rebar Required (with waste)

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Linear Feet

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Weight (lbs)

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Weight (kg)

Bar & Grid Breakdown

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Bars Along Length

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Bars Along Width

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Stock Bars to Buy

— Area (sq ft)

— Spacing OC

— Bar Size

— Waste Factor

Estimated Material Cost

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Rebar material cost only. Add labor, tie wire, bar chairs, and delivery for a full project budget. Use our Full Project Estimator for a complete breakdown.

Step 1: Convert all dimensions to feet
Step 2: Bars along width = FLOOR(Length ÷ Spacing) + 1 — these run the full width
Step 3: Bars along length = FLOOR(Width ÷ Spacing) + 1 — these run the full length
Step 4: Linear feet (net) = (Bars along width × Width) + (Bars along length × Length)
Step 5: Linear feet (with waste) = Net LF × (1 + waste% ÷ 100)
Step 6: Weight (lb) = Final LF × lbs-per-foot for selected bar size
Step 7: Stock bars = CEIL(Final LF ÷ stock bar length)

Standard bar weights (lb/ft): #3=0.376 | #4=0.668 | #5=1.043 | #6=1.502 | #7=2.044 | #8=2.670 | #9=3.400 | #10=4.303

How to Use This Rebar Calculator

Measure your slab or footing. Use a tape measure to get the exact length and width of the area to be reinforced. For a rectangular slab, you need two dimensions. For a strip footing, length is the run of the footing and width is the footing's actual width. Select the correct unit from the dropdown — the calculator handles all conversions automatically.
Set your rebar spacing and bar size. Choose an on-center spacing — use the quick-select buttons for the three most common spacings, or type a custom value. Then select your bar number. For most residential flatwork, #4 at 12-inch centers is the industry default. Use the spacing guide table below this section to confirm what's right for your application.
Adjust waste factor and stock bar length. The default 10% waste accounts for cuts, lap splices, and the inevitable short pieces. Increase to 15% for L-shaped slabs or complex footings. The stock bar length (20 ft is standard at most suppliers) tells the calculator how many full bars you need to order.
Use your results to order materials. Give your supplier the total linear feet figure and bar size — they can confirm the bar count and weight. If you entered a price per linear foot, the cost estimate shows your rebar material budget. Always confirm availability with your steel supplier before your concrete pour date.

⚠ Pro Tip: Never cut rebar to exact field dimensions and skip lap splices. All rebar must overlap a minimum of 40 bar diameters at joints — for #4 rebar, that's 20 inches. This lap length is why your actual linear footage is always more than a simple grid calculation suggests. The 10% waste factor covers standard laps; bump it to 15% if you have many joints.

Rebar Calculation Formula

The rebar grid calculation counts bars in both directions independently, then sums the total linear footage. Here's the process step by step with a worked example:

Step	Formula	Example (20 × 20 ft, 12 in OC, #4)
1. Bars along width (run full width)	FLOOR(L ÷ spacing) + 1	FLOOR(20 ÷ 1) + 1 = 21 bars
2. Bars along length (run full length)	FLOOR(W ÷ spacing) + 1	FLOOR(20 ÷ 1) + 1 = 21 bars
3. Net linear feet	(bars × W) + (bars × L)	(21 × 20) + (21 × 20) = 840 ft
4. Add 10% waste	Net LF × 1.10	840 × 1.10 = 924 ft
5. Weight (#4 = 0.668 lb/ft)	Final LF × 0.668	924 × 0.668 = 617 lbs
6. Stock bars (20 ft each)	CEIL(924 ÷ 20)	CEIL(46.2) = 47 bars

Common Rebar Job Reference Table

Pre-calculated rebar quantities — no waste factor applied. Add 10% for real-world ordering.
Slab Size	Bar Size	Spacing OC	Linear Feet	Weight (lbs)	20-ft Bars
10 × 10 ft	#4	12 in	220 ft	147 lbs	11 bars
12 × 12 ft	#4	12 in	312 ft	208 lbs	16 bars
20 × 20 ft	#4	12 in	840 ft	561 lbs	42 bars
20 × 20 ft	#4	18 in	560 ft	374 lbs	28 bars
20 × 20 ft	#5	12 in	840 ft	876 lbs	42 bars
24 × 24 ft	#5	12 in	1,200 ft	1,252 lbs	60 bars
30 × 30 ft	#5	12 in	1,860 ft	1,940 lbs	93 bars
30 × 30 ft	#5	18 in	1,240 ft	1,293 lbs	62 bars
20 × 40 ft	#4	12 in	1,260 ft	842 lbs	63 bars
10 × 100 ft	#4	12 in	1,320 ft	882 lbs	66 bars

Values calculated using the grid formula above. Add 10% waste before ordering. Lap splices not included.

Which Rebar Size and Spacing Do I Need?

Bar size and spacing are the two most consequential decisions in rebar design. Too small a bar or too wide a spacing and the concrete will crack; over-reinforcing wastes money and can interfere with concrete consolidation. Use this table as a starting point — always confirm with your local building code and engineer for structural applications.

Rebar size and spacing recommendations by application type.
Application	Recommended Bar	Spacing OC	Layout	Notes
Residential patio / walkway	#3 or #4	18 in	Two-way grid	4 in slab min.; consider wire mesh alternative
Residential slab-on-grade	#4	12–18 in	Two-way grid	ACI 318 minimum for 4–6 in slabs
Residential driveway	#4	12 in	Two-way grid	6 in slab; place rebar at mid-depth
Garage floor	#4	12 in	Two-way grid	Consider #5 for heavy vehicles
Strip footing (typical)	#4 or #5	—	2–3 bars longitudinal	Minimum 2 bars; check frost depth
Continuous wall footing	#5	12 in	Two-way	Engineer required if >2 stories
Commercial floor slab	#5 or #6	12 in	Two-way grid	ACI 318 structural; engineer required
Structural beam / column	#7–#10	Per design	Per structural drawings	Engineer required — do not estimate

Rebar must be placed at or near mid-depth of the slab — not on the ground. Use plastic bar chairs or rebar supports rated for the slab thickness. Rebar resting on the subgrade provides almost no tensile reinforcement because it ends up in the compression zone, not the tension zone where cracks initiate.

Common Rebar Estimation Mistakes

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Forgetting to add lap splice length. Rebar must overlap at joints — the minimum lap is 40 bar diameters per ACI 318. For #4 rebar (½-inch diameter), that's a 20-inch overlap every time two bars meet. A 10% waste factor covers typical residential laps. For long runs with many splices, increase to 15%.
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Measuring the outside of formwork instead of the net pour area. Rebar sits inside the slab, not at its edges. Concrete cover requirements (typically 1½ to 2 inches on residential slabs) mean your rebar grid is actually shorter and narrower than the slab perimeter. This is minor for large slabs but can matter on footings where the bar starts and stops near the edge.
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Selecting the wrong bar size for the application. #3 rebar is too light for anything but light decorative pads. Most residential contractors default to #4, which is correct for standard flatwork. Using #3 to save money on a driveway or garage floor is a common mistake that leads to premature cracking under vehicle loads.
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Confusing on-center spacing with clear spacing. "12 inches on-center" means 12 inches from the center of one bar to the center of the next — not 12 inches of open gap between bars. On-center spacing is the standard and what this calculator uses. If your drawings show clear spacing, add the bar diameter to get on-center spacing before entering it.
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Ordering in linear feet but forgetting to confirm bar lengths. Steel suppliers sell rebar in standard lengths, most commonly 20-foot sticks. If your total is 850 linear feet, you need 43 sticks of 20-foot bar — not 850 individual pieces. Confirm with your supplier whether they stock the bar size you need and in what lengths, as longer bars reduce cutting waste.

Frequently Asked Questions

To calculate rebar for a two-way grid: divide the slab length by your on-center spacing and add 1 — that gives you the number of bars running across the width. Multiply by the width to get linear feet for that direction. Repeat for the other direction, then add both totals together. Multiply by 1.10 to add 10% waste for cuts and lap splices. This calculator automates that process — enter your dimensions, bar size, and spacing above to get instant results.
The most common spacing for residential slabs is 12 inches on-center in both directions using #4 rebar (½-inch diameter). This meets ACI 318 requirements for temperature and shrinkage reinforcement in 4 to 6-inch slabs. For patios and lightly loaded walkways, 18-inch spacing with #3 or #4 bar is sometimes used. For driveways and garage floors that will see vehicle traffic, 12-inch spacing with #4 is the minimum; many contractors use #4 at 12 inches or #5 at 18 inches for additional durability.
In the US, rebar is designated by a number that represents the diameter in eighths of an inch. So #4 rebar is 4/8 = ½ inch in diameter, #5 is 5/8 = ⅝ inch in diameter, and so on. A #3 bar is ⅜ inch. The bar number also roughly corresponds to weight per foot: a #4 bar weighs 0.668 pounds per linear foot, a #5 bar weighs 1.043 pounds per linear foot. This is the weight data used to calculate the total steel tonnage in this calculator.
Both control shrinkage cracking, but they behave differently. Rebar (deformed steel bar) provides structural tensile strength and is required anywhere the slab must carry load — driveways, garage floors, footings, and anything with a vehicle or structural load. Welded wire mesh (wire mesh or WWF) is cheaper and faster to install but provides less tensile reinforcement; it's acceptable for lightly loaded patios and sidewalks. ACI 318 requires rebar for structural applications. For residential driveways and garage floors, most building codes and contractors specify #4 rebar at 12 inches on-center rather than wire mesh.
As of 2025–2026, #4 rebar (the most commonly used size) typically costs $0.40–$0.75 per linear foot for material at retail, or roughly $700–$900 per ton at contractor pricing. Prices fluctuate significantly with steel market conditions — they can shift 20–30% within a year. #5 and larger bars cost proportionally more. Prices at big-box stores (Home Depot, Lowe's) are generally higher per foot than ordering direct from a steel service center or concrete supplier. For accurate current pricing, call two or three local suppliers and compare quotes on a per-ton or per-stick basis.
Rebar should be placed at approximately mid-depth of the slab, with a minimum concrete cover of 1½ inches from the bottom of the slab for slabs not exposed to weather, and 2 inches for slabs exposed to weather or ground contact (per ACI 318). In a standard 4-inch slab, rebar sits about 2 inches up from the bottom. Use plastic bar chairs or rebar support chairs — never rely on rocks, wood scraps, or other improvised supports. The cover is critical: too little cover and the rebar corrodes and causes spalling; too much cover and the bar ends up in the compression zone where it provides no benefit.
A lap splice is the overlap between two bars where they meet. ACI 318 requires a minimum lap of 40 bar diameters for tension splices in class A conditions (most residential slabs). For common bar sizes: #3 = 15 inches minimum, #4 = 20 inches minimum, #5 = 25 inches minimum, #6 = 30 inches minimum. In practice, most contractors use 24 inches as a standard lap for residential #4 rebar. Lap splices should be staggered — don't line all your laps up on the same line across a slab. The 10% waste factor in this calculator covers standard lap lengths for typical residential grids.
Yes, for any driveway that will see regular vehicle traffic, rebar is strongly recommended. A 6-inch slab with #4 rebar at 12-inch centers is the standard for residential driveways that accommodate passenger cars and light trucks. Wire mesh alone is insufficient for vehicle loads and does not meet most municipal driveway permit requirements. For driveways that will support heavy trucks, RVs, or trailers, use #5 rebar at 12-inch centers in a 6-inch (minimum) slab, with 4,000 PSI concrete and air entrainment in freeze-thaw climates. Always check your local building department requirements before permitting.
Rebar intersections are tied with 16- or 18-gauge annealed tie wire, twisted tight with a rebar tier tool or pliers. The wire holds the grid together during concrete placement — it doesn't transfer structural load, so the tie pattern matters less than ensuring bars don't shift when the concrete truck pours. A common pattern is to tie every other intersection in a checkerboard pattern for large grids, and every intersection near edges and at corners. For residential slabs, a standard 50-foot coil of tie wire handles roughly 100–150 intersections. Mechanical rebar tying tools (like the Makita RTF300) dramatically speed this up on large pours.
Epoxy-coated rebar (ECR) is specified in environments prone to chloride-induced corrosion — bridge decks, coastal structures, parking garages, and slabs that will be exposed to road deicing salts. It's not required for typical residential slabs, patios, or driveways unless the slab is close to the coast or will see heavy salt exposure. ECR costs roughly 20–40% more than plain black rebar. The coating reduces bond strength slightly, which is why ACI 318 requires slightly longer lap splices (typically 20% longer) for epoxy-coated bars. For most homeowners, standard black rebar with adequate concrete cover is sufficient.