🔗 Rebar Sizing

Written and maintained by the PhDino author · Last reviewed 21 September 2026 · Checked against 2 independent reference calculations · how PhDino checks its numbers

Related standards: ASTM A615

Estimating bar count and total steel weight for a two-way reinforcing grid in a slab.

Concrete is strong in compression but weak in tension — a slab or footing that only had plain concrete would crack under the tensile stresses that bending and shrinkage create. Steel reinforcing bars (rebar) are embedded to carry that tension, since steel is strong in tension where concrete is not.

US rebar sizes are numbered by their diameter in eighths of an inch: a #4 bar is 4/8 = 1/2 inch in diameter, a #6 bar is 6/8 = 3/4 inch, and so on — a naming convention set by ASTM A615, the standard governing steel reinforcing bar. Slabs typically get a two-way grid: bars running in both directions at a set spacing, tied where they cross.

Key formula

Bars needed in one direction ≈ (span ÷ spacing) + 1
Total bar length = (bars one way × length) + (bars other way × width)
Total weight = total length × unit weight per foot (a property of the bar size)

Variables

#size

bar diameter in eighths of an inch, e.g. #4 = 1/2 in

Notes & limitations

• This estimates straight bar count and weight for a simple two-way grid — it doesn't size the bar diameter or spacing itself, which depends on the slab's span, thickness, and expected load (a structural design question, typically governed by ACI 318 for concrete).
• Real placement also needs concrete cover (clear distance from the bar to the surface) and lap splices where bars overlap end-to-end, both of which add a bit more steel than the raw grid length alone.