How the rebar math works
Clear run each way = slab dimension − 2 × edge clearance (20' − 6" = 19' 6")Bars = clear run ÷ spacing, rounded down, + 1 (234" ÷ 18" = 13 → 14 bars)Linear feet = bars × bar length, both directions (546 LF)Add a 20" lap (40 bar diameters for #4) for every splice when a bar is longer than one stickSticks = LF × (1 + waste) ÷ stick length → 29 × 20' or 63 × 10'Tie points = bars one way × bars the other way (14 × 14 = 196)
Slab reinforcement is not structural design. Footings, thickened edges and anything carrying a building need an engineer’s or the local code’s specification.
Trade pro tips
- Rebar must sit in the middle third of the slab — use 2"–3" chairs or dobies, never "hook it up" during the pour.
- Keep 2"–3" of concrete cover at the edges; exposed ends rust and spall.
- 10 ft sticks fit in a pickup or SUV but need roughly twice the lap splices.
- Wire mesh is faster on thin patio slabs, but it ends up on the ground unless it is chaired too.
Frequently asked questions
How much rebar do I need for a 20x20 slab?
At 18" on center both ways with 3" edge clearance: 14 bars each way, about 546 linear feet — 29 twenty-foot sticks or 63 ten-foot sticks including laps and 5% waste.
What spacing should rebar be in a concrete slab?
Residential slabs commonly use #3 or #4 bars at 12"–18" on center each way; 24" is used on lightly loaded patios. Follow your plans or local code where they specify.
How many rebar ties do I need?
One tie per intersection if you tie every crossing — 196 for a 20x20 slab at 18" OC. Many crews tie every other intersection on the interior and every one on the perimeter.
How long should rebar laps be?
A common rule is 40 bar diameters: 15" for #3, 20" for #4, 25" for #5. Your plans or code may require more.