Rebar Calculator

Calculate rebar quantity, weight, lap splices, and cost for slabs, mats, and footings.

Area Length (ft)

Area Width (ft)

Bar Size

Spacing (inches OC)

Mode

Total Rebar Needed

840 LF

42 stock lengths (20 ft each)

Bars (length direction)	21 bars x 20 ft
Bars (width direction)	21 bars x 20 ft
Total bars	42
Linear feet (net)	840 LF
Lap splices	0 (20" each)
Adjusted LF (with laps)	840 LF
Total weight	561 lbs
Estimated cost	$420.84

info

Sarah Torres:“Lap splice length is 40 bar diameters minimum per ACI 318. For #5 bar that’s 25 inches. Mark your laps before the pour crew arrives — inspectors check this.”

Methodology

Bars in each direction = ceil(perpendicular dimension in inches / spacing) + 1. Linear feet = bars x run length. Lap splices calculated assuming 20-ft stock lengths with 40-bar-diameter lap per ACI 318. Weight uses CRSI standard bar weights: #3=0.376, #4=0.668, #5=1.043, #6=1.502, #7=2.044, #8=2.670 lbs/ft. Cost estimated at $0.75/lb for Grade 60 rebar, 2026 pricing.

Frequently Asked Questions

What size rebar for a concrete slab?

For a standard 4-inch residential slab, #3 or #4 rebar at 18 to 24 inches on center is typical. For a 6-inch slab or one carrying heavier loads, #4 or #5 rebar at 12 inches on center is common. Garage slabs and driveways usually use #4 at 12-18 inches OC. Always follow the structural engineer’s specification when available — these are general guidelines only.

How far apart should rebar be spaced?

Common spacings are 12, 16, 18, and 24 inches on center. Tighter spacing (12 inches) provides more steel area and is used for structural slabs, footings, and retaining walls. Wider spacing (18-24 inches) is used for lightly loaded slabs and temperature/shrinkage reinforcement. ACI 318 limits maximum spacing to 18 inches for structural slabs or 5 times the slab thickness, whichever is less.

What is a lap splice?

A lap splice is where two rebar bars overlap to maintain structural continuity. Since rebar comes in 20 or 60 foot stock lengths, longer runs require splicing. The minimum lap length per ACI 318 is 40 bar diameters for a Class B tension splice — for example, 25 inches for #5 bar (40 x 0.625"). Laps must be tied with wire and staggered so that not all splices occur at the same cross-section.

#4 vs #5 rebar — when to use which?

#4 rebar (1/2" diameter, 0.668 lbs/ft) is the workhorse of residential construction — used in slabs, footings, and light retaining walls. #5 rebar (5/8" diameter, 1.043 lbs/ft) provides 56% more cross-sectional steel area and is used for structural slabs, taller retaining walls, grade beams, and commercial footings. #5 is harder to bend by hand but provides significantly more strength per bar.

How the Rebar Calculator works

The rebar calculator estimates bar count, total linear feet, weight, lap splices, 20-foot stock lengths, and material cost for a two-way mat in a slab or footing.

Bars are counted in each direction. Bars running the length of the slab are spaced across the width: ceil(width in inches ÷ spacing) + 1. Bars running the width are spaced across the length: ceil(length in inches ÷ spacing) + 1. The +1 accounts for the bar at the starting edge. Total linear feet = (length-direction bars × slab length) + (width-direction bars × slab width).

Because rebar ships in 20-foot sticks, any run longer than 20 feet needs a lap splice. The lap length is 40 bar diameters per ACI 318, so for a #4 bar (0.5-inch diameter) the lap is 20 inches. The tool adds the extra length from every required splice, then recomputes weight and stock count. Weight uses CRSI standard bar weights: #3 = 0.376, #4 = 0.668, #5 = 1.043, #6 = 1.502, #7 = 2.044, #8 = 2.670 lbs per foot. Stock lengths = ceil(adjusted LF ÷ 20). Material cost = total weight × $0.75 per pound for Grade 60 rebar.

Worked example: a 20 ft × 20 ft slab with #4 bar at 12 inches on center. Length-direction bars = (20 × 12) ÷ 12 + 1 = 21; width-direction bars = 21; total = 42 bars. Linear feet = 21 × 20 + 21 × 20 = 840 LF. Neither run exceeds 20 feet, so no lap splices are needed. Weight = 840 × 0.668 = 561 lbs, which is 42 twenty-foot sticks, and at $0.75 per pound the steel costs about $421.

Frequently Asked Questions

How does the calculator count bars in each direction?

For each direction it divides the perpendicular dimension (converted to inches) by the spacing, rounds up, and adds 1 for the edge bar. A 20-foot width at 12-inch spacing gives (240 ÷ 12) + 1 = 21 bars running the other way. It does this for both directions and sums them for the total bar count.

How is rebar weight calculated?

Total linear feet (including lap-splice length) is multiplied by the bar unit weight. The tool uses CRSI standard weights such as 0.668 lbs/ft for #4 and 1.043 lbs/ft for #5. So 840 linear feet of #4 weighs 840 × 0.668 = 561 lbs.

When does the calculator add lap splices?

Only when a run is longer than a 20-foot stock length. Each splice adds 40 bar diameters of overlap, which is 20 inches for #4 and 25 inches for #5. The extra length is added back into the linear-foot and weight totals so your order accounts for the overlap.

How many 20-foot sticks of rebar will I need?

The tool divides the adjusted linear feet (bars plus splices) by 20 and rounds up. For 840 linear feet that is 42 sticks. Ordering full sticks lets you cut on site and stagger your splices, which ACI 318 requires so laps do not all land at the same cross-section.