Rebar Calculator

Q: How much does rebar weigh per foot?

#3=0.376 lb/ft, #4=0.668 lb/ft, #5=1.043 lb/ft, #6=1.502 lb/ft, #7=2.044 lb/ft, #8=2.670 lb/ft.

Q: How deep should rebar be placed in a slab?

Place rebar in the middle third of the slab depth. For a 4-inch slab, that is 2 inches from the bottom. ACI 318 requires a minimum 1.5-inch concrete cover for interior slabs and 2 inches for exterior slabs.

Q: Can I use wire mesh instead of rebar?

Wire mesh is an option for light slabs like sidewalks and small patios. For driveways, garage floors and any slab subject to vehicle or heavy load, rebar is strongly preferred as it provides better load distribution.

Q: How many pieces of rebar come in a bundle?

Standard mill bundles contain 40-foot bars in counts that vary by bar size and supplier. Most home improvement stores sell rebar in 10-foot, 20-foot or cut lengths individually.

Q: Does rebar rust in concrete, and is that a problem?

Light mill scale and surface rust is acceptable and improves bond. Severe flaking rust that reduces the bar cross-section is a concern and such bars should be rejected. Properly encased rebar in quality concrete will not continue to corrode due to the high pH passivating environment.

Get exact bar count, total linear footage, weight and 20/40 ft stick order quantity for concrete slabs, driveways, footings and walls.

Top-down illustration of a concrete slab with a perpendicular orange rebar grid showing 12-inch spacing

Updated 17 May 2026 ACI 318-19 + ASTM A615 sourced Formulas & sources

Quick presets — click to load typical project values:

Slab Length (ft)

Slab Width (ft)

Rebar Spacing (inches)

Overlap (inches)

Bar Size

Bars Along Length

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Bars Along Width

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Total Bars

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Total Linear Feet

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Total Weight (lbs)

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20-ft Sticks to Order

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40-ft Sticks to Order

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Stick counts assume individual 20-ft or 40-ft lengths sold at home centers; supplier mill-bundle quantities vary. For estimating only — verify against local code and your structural drawings.

Live top-down grid

Updates as you change inputs. Bars drawn to scale with 3″ edge cover and lap-splice indicator.

Gray = concrete slab · Brown = length-direction bars · Orange = width-direction bars · Yellow tint = 3″ edge cover zone · Red bar = lap splice region

📐 Fraction → decimal helper (tape measure conversions)

1/16 = 0.0625
1/8 = 0.125
3/16 = 0.1875
1/4 = 0.25
5/16 = 0.3125
3/8 = 0.375
7/16 = 0.4375
1/2 = 0.5
9/16 = 0.5625
5/8 = 0.625
11/16 = 0.6875
3/4 = 0.75
13/16 = 0.8125
7/8 = 0.875
15/16 = 0.9375
1 = 1.0

Type the decimal value into any input above. Example: 14-1/2 ft slab = 14.5 ; 6-3/4″ overlap = 6.75.

Anatomy of a Reinforced Slab (Cross-Section)

The live diagram above is a top-down view. This is the side view — showing where rebar sits inside the concrete, what holds it up, and how much cover protects it from rust.

Rebar sits at mid-depth of the slab, held in place by bar chairs or "dobies" (small concrete blocks). Cover protects steel from rust and gives concrete its strength. ACI 318-19 §20.5.1.

Best Practices — Bar Size, Cover, Spacing, Lap

Four reference scales every rebar layout uses. Each comes from ACI 318 or industry standard practice.

Reinforcement Patterns by Element

Five common reinforcement layouts. Each shows the cross-section and the typical bar arrangement.

Slab. Bars in both directions at the same depth (single mat). For thick slabs (≥8″) use two mats — top and bottom. Residential driveway = #4 @ 12–18″ OC.

Footing. Longitudinal #4–#5 bars (top + bottom), plus stirrups (#3 closed loops) every 10–12″. Bottom bars carry tension from soil reaction.

Wall. Vertical #4–#5 @ 12″ + horizontal #4 @ 18″ in both faces (basement / retaining). Walls ≥4 ft tall typically require ACI design.

Column. 4 vertical longitudinal bars (#5–#8 typical) with #3 ties at 12″ OC. Ties prevent buckling of longitudinal bars under axial load.

Beam. Heavier bottom bars (tension) + smaller top bars + #3 stirrups (closed loops) for shear. Stirrup spacing tightens near supports.

Installation Details — Chairs, Lap, Corner Hooks

Three details that determine whether your rebar actually works once concrete cures.

Chairs. Plastic chairs, wire bolsters, or concrete "dobies" hold rebar at mid-depth before the pour. Without them rebar sinks to the bottom and the slab fails.

Lap splice. When a bar isn't long enough, overlap two pieces by 40–60× the bar diameter and tie with wire. For #4 bar that's 20–30″. Stagger splices — don't put them all in the same plane.

Corner hooks. At outside corners and at the end of footings, bars get a 90° standard hook with bend radius ≥4× bar diameter. Maintains continuity around the corner so the slab acts as one piece.

How the Rebar Calculator Works

This rebar calculator determines the number of steel reinforcing bars needed for a concrete slab by computing a two-direction (bidirectional) grid. Bars running parallel to the width are spaced across the slab length; bars running parallel to the length are spaced across the width. The bar count in each direction is the dimension in inches divided by the chosen spacing, rounded up to a whole number, plus one additional end bar. Every bar's length adds an overlap (splice) allowance — 6 inches is the standard residential minimum per ACI 318; commercial and structural work may require 12–24 inches. Total linear footage (all bars × their individual lengths) multiplied by the weight per linear foot gives you the total steel weight, which drives both cost and delivery logistics.

Standard Rebar Sizes — Quick Reference

Bar Size	Diameter (in)	Cross-Section (in²)	Weight (lb/ft)	Common Use
#3	0.375	0.11	0.376	Walkways, slabs-on-grade (light)
#4	0.500	0.20	0.668	Residential driveways, patios, footings
#5	0.625	0.31	1.043	Commercial slabs, retaining walls, columns
#6	0.750	0.44	1.502	Heavy structural, bridge decks

Recommended Rebar Spacing by Project

Project Type	Bar Size	Spacing (in)	Slab Thickness
Residential patio	#3 or #4	18	4 in
Residential driveway	#4	12–18	4–6 in
Garage floor	#4	12	4–6 in
Pool deck	#4	12	4 in
Commercial floor slab	#5	12	6 in
Retaining wall	#5–#6	8–12	8–12 in

Code & Best-Practice Notes

ACI 318 (the primary US concrete design code) requires a minimum concrete cover of 1.5 inches over rebar for interior slabs and 2 inches for slabs exposed to weather. Keep rebar at least 3 inches from the slab edge. For driveways and patios, position rebar in the middle third of the slab depth — not on the ground. Always use plastic bar chairs or dobies to hold rebar at the correct height before the pour.

Frequently Asked Questions

What size rebar do I need for a concrete slab?

#4 rebar (1/2 inch diameter, 0.668 lb/ft) is the standard choice for residential driveways, garage floors and patios. Use #3 for lightly loaded walkways and #5 or #6 for heavier commercial slabs, retaining walls and footings.

How far apart should rebar be spaced?

12–18 inches on center (OC) is typical for residential slabs. High-traffic or load-bearing areas such as garage floors and driveways use 12-inch spacing; lighter patios and walkways can use 18 inches. Commercial slabs and structural work usually call for 8–12 inches OC.

How much does rebar weigh per foot?

#3 = 0.376 lb/ft, #4 = 0.668 lb/ft, #5 = 1.043 lb/ft, #6 = 1.502 lb/ft, #7 = 2.044 lb/ft, #8 = 2.670 lb/ft. The bar size number represents eighths of an inch in diameter (#4 = 4/8 = 1/2 inch).

Do I need rebar in a 4-inch concrete slab?

Yes for most residential applications — driveways, garage floors, patios. Use #4 at 18-inch spacing. A plain 4-inch slab without reinforcement can crack from ground movement and freeze-thaw cycles. Lightly loaded walkways and sidewalks can use 6×6 wire mesh instead of rebar as a lower-cost alternative.

How deep should rebar be placed in a slab?

Place rebar in the middle third of the slab depth, ideally at the mid-point for thin slabs. For a 4-inch slab, that is 2 inches from the bottom. ACI 318 requires a minimum 1.5-inch concrete cover over rebar for interior slabs and 2 inches for exterior slabs exposed to weather or freeze-thaw.

What is the required rebar overlap (splice length)?

The minimum lap splice for residential slabs is typically 6 inches, but structural applications follow ACI 318 Class B splice requirements — roughly 40–60 bar diameters. For #4 rebar (0.5 inch diameter), a Class B splice is about 20–30 inches. Always check your local building code and structural drawings.

How far from the edge should rebar be?

Keep rebar at least 3 inches from any slab edge and 2–3 inches from form boards. Getting too close to the edge leaves insufficient concrete cover and can cause the edge to spall or crack along the rebar line after a few freeze-thaw cycles.

Can I use wire mesh instead of rebar?

Wire mesh (welded wire fabric) is an option for light residential slabs like sidewalks and small patios. It provides crack control but less structural reinforcement than rebar. For driveways, garage floors and any slab subject to vehicle or heavy load, rebar is strongly preferred. Rebar provides better load distribution, especially if the subgrade settles unevenly.

How many pieces of rebar come in a bundle?

Standard mill bundles contain 40-foot bars, typically packed in bundles of varying count depending on bar size and supplier. Most home improvement stores sell rebar in 10-foot, 20-foot or cut lengths individually. Call ahead to confirm bundle sizing when ordering large quantities for contractor pricing.

Does rebar rust in concrete, and is that a problem?

Light mill scale and surface rust on rebar is actually acceptable and even beneficial — it improves the bond between steel and concrete. Severe flaking rust that reduces the bar's cross-sectional area is a concern and such bars should be rejected. Properly encased rebar in quality concrete (low water-cement ratio) will not continue to corrode because the high pH environment of cured concrete passivates the steel surface.