How to Calculate Rebar in a Concrete Wall

Split the wall into two directions. Vertical bar count = wall length ÷ vertical spacing + 1 (the +1 accounts for the bar at each end). Horizontal layer count = wall height ÷ horizontal spacing + 1. Multiply each by the bar area to get total As, then check ρ = As / (gross area) against the ACI minimums: 0.0012 vertical and 0.0020 horizontal for Grade 60 #5-and-smaller bars. For a 20-ft-long, 10-ft-high wall at #5 @ 12 in both ways: 21 vertical bars and 11 horizontal layers per face.

Use a single mat only when the wall is ≤ 8 ft tall, non-seismic, and the backfill is well-drained granular soil with no hydrostatic pressure — and the mat must sit on the tension (earth) face. Anything taller, any basement wall with water, and every shear wall needs a double mat (both faces). The decision is driven by whether the moment reverses: a basement wall bends one way (earth face in tension), so one mat can work; a shear wall moment reverses under seismic load, so both faces must be reinforced.

Vertical bar count = (wall length ÷ vertical spacing) + 1. The +1 adds the starting bar at the wall end. For a 20-ft (240-in) wall at #5 @ 12 in on center: 240 / 12 + 1 = 21 vertical bars per face. Always round up to the next whole bar, and never space the first or last bar more than half the spacing from the wall end. The same formula gives horizontal layer count using wall height ÷ horizontal spacing + 1.

ACI 11.6.1 sets the minimum vertical ratio at 0.0012 of the gross wall area for Grade 60 bars #5 and smaller (0.0015 for #6 and larger), and the minimum horizontal ratio at 0.0020 (0.0025 for #6 and larger). In special structural walls for SDC D–F, both ratios rise to 0.0025. The vertical ratio can be lower than the horizontal because verticals resist bending while horizontals primarily control shrinkage and temperature cracking, which demand more distributed steel.

A basement wall resists out-of-plane earth and water pressure, so vertical bars on the earth face govern and a single mat is often acceptable below 8 ft. A shear wall resists in-plane seismic shear, so horizontal web steel and concentrated vertical boundary bars govern, and a double mat is mandatory. Shear walls in SDC D–F also need confined boundary elements at the ends (closely spaced ties around bundled verticals), which basement walls never require.

ACI 18.10.6 requires boundary elements (confined wall ends) in special structural walls when the compressive stress at the wall edge exceeds 0.2 fc′ under the design load combination, or when the wall is in SDC D–F and the axial load is high. The boundary zone is detailed like a column — 4+ concentrated vertical bars confined by #3 ties at 4 in on center within the confinement length Lo. Basement and ordinary walls never need boundary elements; only special seismic shear walls do.