How to Calculate Rebar
Rebar is laid in a grid pattern inside concrete slabs and footings. The spacing (distance between bars) determines how many bars you need. Standard residential slab reinforcement uses #4 rebar at 12 inches on center in both directions — meaning bars run every 12 inches across the length AND every 12 inches across the width, forming a grid.
FormulaBars along length = (Width ÷ Spacing) + 1
Bars along width = (Length ÷ Spacing) + 1
Total linear feet = (Bars along length × Length) + (Bars along width × Width)
20-ft pieces = Total linear feet ÷ 20 (round up)
Bars along width = (Length ÷ Spacing) + 1
Total linear feet = (Bars along length × Length) + (Bars along width × Width)
20-ft pieces = Total linear feet ÷ 20 (round up)
Rebar Sizes and Weight
- #3 (3/8") — 0.376 lb/ft. Light reinforcement, temperature/shrinkage steel.
- #4 (1/2") — 0.668 lb/ft. Most common for residential slabs and footings.
- #5 (5/8") — 1.043 lb/ft. Structural slabs, foundation walls.
- #6 (3/4") — 1.502 lb/ft. Heavy structural applications.