How to Calculate Rebar
Rebar (reinforcing steel bar) gives concrete tensile strength. Estimating correctly requires knowing the grid spacing, slab or wall dimensions, lap length for splices, and waste factor for cutting. Engineered drawings specify exact bar sizes; this calculator covers the estimation math.
Rebar Linear Feet Formula
Barseach way=Side LengthinSpacingin+1
Total LF=(BarsEW×Length)+(BarsNS×Width)
Standard Rebar Sizes (US)
| Size | Diameter | Weight / ft | Typical Use |
|---|---|---|---|
| #3 | 3/8 in | 0.376 lb | Stirrups, ties, small slabs |
| #4 | 1/2 in | 0.668 lb | Residential slabs, walls |
| #5 | 5/8 in | 1.043 lb | Heavy residential, light commercial |
| #6 | 3/4 in | 1.502 lb | Commercial slabs, retaining walls |
| #8 | 1 in | 2.670 lb | Large foundations, bridges |
Lap Splice Length
When two rebar pieces meet end-to-end, they must overlap by a minimum length called a lap splice. Rule of thumb for non-seismic residential: 40 times the bar diameter.
Lap=40×Bar Diameter
#3
15 in lap
#4
20 in lap
#5
25 in lap
#6
30 in lap
Seismic Zones Require Longer Laps
In seismic design categories D, E, and F, lap splices must be 50 to 60 times the bar diameter. Some high-seismic details require mechanical splices. Always follow the structural engineer's specifications.Rebar Placement Rules
- Cover - 3 in minimum against earth, 2 in exposed to weather, 3/4 in interior dry
- Chair height - use rebar chairs or dobies; never let bars touch earth
- Tie wire - 16 or 18 gauge annealed wire at every intersection
- No oil or mud - clean bars before placing; light rust is acceptable
Grade 60 is the Modern Standard
Grade 60 rebar (60,000 psi yield) is the modern US standard. All sizes #4 and above are Grade 60 by default. Verify the yield grade stamped on each bar matches the structural plans.