Rebar Calculator
Estimate rebar length, bar count and weight for a slab grid.
About the Rebar Calculator
Bars run both ways inside the edge cover. The number in each direction is the clear width divided by the spacing, plus one. Weight per metre for metric bars is d² ÷ 162 kg (d in mm); US bar weights follow ASTM A615 (#4 = 0.668 lb/ft). Add lap lengths (often 40–60 bar diameters) where bars join.
The formula
Bars = ⌊(width − 2 × cover) ÷ spacing⌋ + 1; kg/m = d² ÷ 162
Worked example
Example: Total bar length = 549 ft
- Inputs: Units: Imperial (ft, in); Slab length: 22 ft; Slab width: 14 ft; Bar spacing: 13 in; Edge cover: 3 in; Bar size: #4 (½ in); Stock bar length: 22 ft.
- Formula: Bars = ⌊(width − 2 × cover) ÷ spacing⌋ + 1; kg/m = d² ÷ 162
- Result: Total bar length = 549 ft
- Bars along length: 13
- Bars across: 20
- Weight: 367 lb
Things to keep in mind
- Laps, hooks, chairs and engineering requirements aren't included; follow your structural drawings.
Questions people ask
What spacing is typical?
200–300 mm (8–12 in) each way for residential slabs.
What cover?
Typically 50–75 mm (2–3 in) for slabs on ground.