Rebar Calculator for Slabs
Slab size in, shopping list out: bars each direction, total linear feet with 5% for laps, and how many 20-foot sticks to put on the truck.
Which bar size?
| Bar | Diameter | Typical use |
|---|---|---|
| #3 | 3/8" | Patios, walkways, light slabs |
| #4 | 1/2" | Driveways, garage slabs, footings — the residential default |
| #5 | 5/8" | Heavy equipment pads, engineered work |
Assumes 3" edge cover and bars at mid-slab height on chairs. Add tie wire (one tie per intersection: rows × cols) and chairs every 3–4 ft along bars.
Common questions
What rebar spacing should I use for a slab?
Residential slabs typically use #3 or #4 rebar on a 16 or 18 inch grid. Driveways and garage slabs lean 12–16", light-duty patios 18–24". Local code and soil conditions govern — when in doubt, tighter spacing and #4 bar.
How much does rebar overlap at splices?
Lap splices should overlap at least 40 bar diameters — about 20 inches for #4 (½") bar. The calculator adds 5% to total length to cover laps and cut waste.
Rebar or wire mesh for a slab?
Mesh (6x6 W1.4) is fine for 4" slabs with foot traffic. Anything carrying vehicles, point loads, or on questionable soil benefits from a rebar grid — it holds position better during the pour and adds real flexural strength.
How far from the edge does rebar start?
Keep 3 inches of concrete cover from every edge, and support bars on chairs at mid-slab height — rebar lying on the gravel does nothing.