Rebar Calculator
Lay out a rebar grid for a slab or continuous bars for a footing. The count includes lap splices, edge clearance and waste, plus weight for ordering.
How it's calculated
For a slab grid, the calculator subtracts the edge clearance from each side, then divides the remaining width by your spacing and adds one bar. That gives the number of bars running each direction.
When a run is longer than one stick, bars are spliced by overlapping them. The lap length is the bar diameter times the number you enter (40 diameters by default), so a #4 bar laps 20". Each splice adds that overlap to the lineal footage.
Weight uses the standard ASTM unit weights: #3 is 0.376 lb/ft, #4 is 0.668, #5 is 1.043 and #6 is 1.502.
Worked example
A 24 × 20 ft slab with #4 bar at 18" on center and 3" clearance has 14 bars running the 24 ft way and 16 running the 20 ft way. The 24 ft bars each need one 20" lap. With 5% waste that comes to about 698 lin ft: 35 sticks of 20 ft #4, roughly 466 lb.
Field notes
- Support the grid on chairs or dobies so it sits in the middle third of the slab. Rebar lying on the ground does nothing.
- Stagger lap splices so they do not all fall in the same line.
- Keep 3" of cover where concrete is cast against earth, 1-1/2" to 2" elsewhere.
- Bending and cutting on site eats steel. On jobs with many corner bars, add more waste.
Common questions
- How far apart should rebar be in a slab?
- Residential slabs commonly use #3 or #4 bar at 12" to 24" on center each way. Driveways and garage floors often use 18". Your plans or local code govern.
- How much should rebar overlap?
- A common minimum is 40 bar diameters: 15" for #3, 20" for #4 and 25" for #5. Engineered plans may call for more.
- Is wire mesh as good as rebar?
- Welded wire mesh controls shrinkage cracking but adds less strength than a rebar grid, and it is hard to keep at mid-depth during the pour.