Rebar Calculator — Slab Grid Cut List & Weight
Bar count, total length, and weight for a two-way rebar mat in a slab. Supports any bar size from #3 (3/8") through #11 (1-3/8").
Total weight
—
— kg
Bars (long way)
—
each — ft
Bars (short way)
—
each — ft
Total linear ft
—
ft
Tie wire
—
intersections
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How to lay out a slab rebar grid
For a basic slab-on-grade mat, you run bars in two perpendicular directions at a chosen spacing, tied together at the intersections. The number of bars in each direction equals the perpendicular dimension divided by the spacing, plus one (for the bar at the start).
Bars (long way) = floor((W − 2 × cover) / spacing) + 1
Bars (short way) = floor((L − 2 × cover) / spacing) + 1
Length per bar = orthogonal dimension − 2 × cover
Tie intersections = (bars long) × (bars short)
Common slab reinforcement
- Residential slab-on-grade (4–5"): #3 or #4 at 16–24" o.c. each way, or 6×6 W2.9×W2.9 welded wire
- Garage floor (4–6"): #4 at 16" o.c.
- Driveway (4–6"): #4 at 18" o.c.
- Suspended slab: engineer-designed; typically #5 or #6 at 12" o.c. with top and bottom mats
- Footings: 2 longitudinal #5 bars minimum (IRC R403); for spread footings, transverse #4 at 12" o.c.
Rebar weight per foot
| Size | Diameter | lb/ft | kg/m | Area (in²) |
|---|---|---|---|---|
| #3 | 3/8" | 0.376 | 0.559 | 0.11 |
| #4 | 1/2" | 0.668 | 0.994 | 0.20 |
| #5 | 5/8" | 1.043 | 1.552 | 0.31 |
| #6 | 3/4" | 1.502 | 2.235 | 0.44 |
| #7 | 7/8" | 2.044 | 3.041 | 0.60 |
| #8 | 1" | 2.670 | 3.972 | 0.79 |
| #9 | 1-1/8" | 3.400 | 5.060 | 1.00 |
| #10 | 1-1/4" | 4.303 | 6.404 | 1.27 |
| #11 | 1-3/8" | 5.313 | 7.907 | 1.56 |
Lap splice length (ACI 318)
Where bars overlap to extend a run, the typical Class B tension lap is 40 bar diameters for f'c = 4,000 psi concrete. For #4 bar (1/2" dia), that's 40 × 0.5 = 20" of overlap. The calculator above will add lap length to total weight when bars exceed 20 ft stock length.
Frequently asked questions
What size rebar should I use for a residential slab-on-grade?
#3 (3/8") or #4 (1/2") rebar at 16–24" on-center each way is standard for residential slabs-on-grade. #4 at 18" o.c. is the most common spec for 4" garage and driveway slabs. For 5"–6" slabs under heavier loads, #4 at 12" o.c. or #5 at 16" o.c. is typical. Always check local building codes — some jurisdictions allow welded wire mesh in place of rebar for residential flatwork.
What is rebar cover and why does it matter?
Cover is the distance from the outer surface of the rebar to the face of the concrete. It protects the steel from moisture, chlorides, and the carbonation front that causes corrosion. ACI 318 minimums: 3" for concrete cast against earth, 1.5" for interior slabs, 2" for exterior slabs exposed to weather. The calculator uses edge cover — don't confuse it with vertical cover (how high the bar is placed above the subbase).
What is a rebar lap splice?
A lap splice is where two bars overlap to transfer force from one bar to the next when a single bar isn't long enough to span the full run. ACI 318 Class B tension lap splice length is typically 40 bar diameters for normal-weight concrete at f'c = 4,000 psi. For #4 bar (1/2" dia), that's 40 × 0.5" = 20". The calculator automatically adds lap length when a bar run exceeds the standard 20-ft stock length.
Do I need rebar in a residential garage floor?
It depends on the code jurisdiction and slab thickness. Many local codes require either rebar or welded wire mesh in garage slabs — check with your building department. IRC does not mandate rebar in garage slabs specifically, but most contractors and engineers include #4 at 16–18" o.c. as a matter of practice, particularly in cold climates where frost heave is a risk. Fiber reinforcement is not a substitute for rebar in structural slabs.
How much does rebar weigh per foot?
#3 rebar: 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. These values come from ASTM A615 and are based on the nominal cross-section of a perfect circle with the bar's nominal diameter. A 20-ft stick of #4 rebar weighs 0.668 × 20 = 13.36 lb. The calculator applies the exact lb/ft for the selected bar size to compute total mat weight.