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