Rebar Weight & Spacing Calculator

Use this Rebar Weight & Spacing Calculator to estimate total steel tonnage, linear footage, and grid layout specs for any project. Whether pouring a backyard patio, driveway, or commercial foundation, getting your reinforcement measurements right prevents slab failure and eliminates expensive material waste.

How Much Rebar Do I Need for a Concrete Slab?

A rebar calculator determines the total bars, linear footage, and weight needed to reinforce concrete. Plain concrete resists heavy downward compression but cracks under tension. Embedding a tied steel grid absorbs tensile stress, controls temperature shrinkage cracks, and maintains structural integrity under live loads.

Your required steel volume depends on the concrete slab reinforcement ratio, bar size selection (#3 through #8 for most flatwork), and on-center spacing (inches). For footings, use this tool as a dedicated rebar spacing calculator for footings by setting 3-inch side clearances and dual-bar longitudinal runs.

Rebar Weight Per Foot Calculator & Grid Formulas

Calculate reinforcement runs, total footage, and steel mass using standard structural engineering equations:

Run Length = Slab Length - (2 × Edge Clearance)
Run Width = Slab Width - (2 × Edge Clearance)
Bars (Length Run) = 1 + Floor(Run Width / On-Center Spacing)
Bars (Width Run) = 1 + Floor(Run Length / On-Center Spacing)
Total Linear Feet = (Bars Length × Run Length) + (Bars Width × Run Width)
Total Steel Weight (lbs) = Total Linear Feet × Weight Per Linear Foot (lbs/ft)

The theoretical nominal weight per foot is derived from bar diameter: Weight (lbs/ft) = (Nominal Diameter in Inches / 8)² × 2.67. Standard supply yards deliver steel in ASTM A615 Grade 40 or Grade 60 rebar sticks.

Step-by-Step Example: How Many Rebar Bars for a 10x10 Slab?

Here is a practical calculation for a 10 ft × 10 ft patio slab using #4 Grade 60 rebar on 12-inch centers with a 3-inch edge clearance:

Calculate Net Grid Dimensions: Subtract 6 inches (3 in clearance on both sides) from each 10 ft side: 10 ft - 0.5 ft = 9.5 ft (114 inches).
Calculate Number of Bars Per Direction: Divide 114 inches by 12-inch on-center spacing: 114 / 12 = 9.5, rounded down to 9 spaces plus 1 starter bar = 10 bars per direction (20 bars total).
Calculate Total Linear Footage: Multiply 20 bars by their 9.5 ft cut length: 20 × 9.5 ft = 190 linear feet.
Calculate Total Weight: #4 rebar has a weight per linear foot of 0.668 lbs/ft: 190 ft × 0.668 lbs/ft = 126.92 lbs (approx. 10 twenty-foot stock bars).

Rebar Size Chart (#3–#11 Bar Sizes)

US Size	Nominal Diameter (in)	Weight per Linear Foot (lbs/ft)	Cross-Sectional Area (sq in)
#3	0.375	0.376	0.11
#4	0.500	0.668	0.20
#5	0.625	1.043	0.31
#6	0.750	1.502	0.44
#7	0.875	2.044	0.60
#8	1.000	2.670	0.79
#9	1.128	3.400	1.00
#10	1.270	4.303	1.27
#11	1.410	5.313	1.56

Frequently Asked Questions

How do you calculate rebar for a slab?

Subtract your edge clearances (usually 2 to 3 inches) from the slab dimensions. Divide the remaining length and width by your grid spacing, add 1 starter bar to each run, and multiply the count by the cut bar length to find total linear footage and steel weight.

How much does a piece of rebar weigh?

Weight depends on bar size and length. A standard 20-foot stick of #3 bar weighs 7.52 lbs, a 20-foot stick of #4 bar weighs 13.36 lbs, and a 20-foot stick of #5 bar weighs 20.86 lbs.

How far apart should rebar be spaced?

Residential slabs, patios, and garage floors typically space rebar 12 to 18 inches apart on-center. Continuous structural footings and heavy load-bearing foundations use tighter spacing between 8 and 12 inches.

What size rebar do I need for a driveway?

Standard passenger car driveways poured at 4 to 5 inches thick perform well with #3 (3/8-inch) or #4 (1/2-inch) Grade 60 rebar set on 12-inch or 16-inch centers. Driveways supporting RVs or heavy equipment should use #4 or #5 rebar on 12-inch centers.