Rebar Spacing Rules — Slabs, Footings & Lap Splices
Rebar spacing is a structural decision that arrives on site as a shorthand — "#4 at 18 each way" — and this guide unpacks the shorthand: where the numbers come from, what changes them, and the details (cover, laps, chairs) that make the spacing actually work.
Quick answer: Common residential slab spec: #4 rebar at 18 inches on center each way, 3 inches of edge cover, laps of 40× bar diameter (20 in for #4), on 2-inch chairs. Heavier loads tighten spacing to 12 inches; light slabs may use 6×6 welded wire mesh instead. Engineer details always outrule rules of thumb.
The common spacings
- 4-in residential slab (patio, shed): #4 at 18 in OC each way — the working default
- Vehicle or heavy-use slab: #4 at 12 in OC each way, 5-6 in of concrete
- Strip footing: 2 continuous #4 bars minimum, 3 in off the bottom
- Light walkway: 6×6 welded wire mesh is the common alternative
Cover, chairs, and laps
- Edge cover: 2-3 in between the outer bar and the form — steel needs concrete around it to fight rust
- Bottom cover: bars sit on 2-3 in chairs, not on the grade — rebar at the bottom of the pour is decoration
- Lap splice: 40 × bar diameter (#4 → 20 in), staggered so laps don't stack in one line
- Ties: wire-tie intersections on the perimeter and every second or third grid crossing
Why spacing matters as much as bar size
The steel grid controls crack width and distributes loads across the slab — closer spacing controls cracks better than a bigger bar farther apart, up to a point. Doubling bar size while doubling spacing is not an equivalent trade; the engineer's detail exists precisely because the interactions aren't linear. What the rules of thumb give you is a sane default and a way to sanity-check a quote.
The weight and logistics reality
#4 weighs 0.668 lb/ft — a 20-foot stick is 13.4 lb, and a 10×12 slab grid at 18 inches is about 111 lb of steel, nine 20-foot sticks in the truck bed. Suppliers price by the hundredweight; the rebar calculator on this site outputs the count, the sticks, and the weight from your dimensions.
When this guide doesn't apply
Structural slabs, suspended decks, bridge work, and anything with a stamped drawing run on engineer's details — bar size, spacing, cover, and laps are specified and inspected. Regional codes amend the residential numbers too. Use the defaults to plan and quote; build to the detail sheet when there is one.