How to use this calculator
- Enter Slab length in ft.
- Enter Slab width in ft.
- Enter Spacing in in — or tap a preset badge to auto-fill a typical value.
- Enter Bar size.
- Press Calculate. The highlighted row is your order quantity; the note beneath it translates the number into real-world terms.
Formula & constants
A slab grid needs (width ÷ spacing + 1) bars each way. #4 rebar weighs 0.668 lb/ft. A 20 × 20 ft slab at 18 in centers uses about 590 linear ft — roughly 395 lb of steel. Every default in the fields above is editable — when your supplier’s spec sheet lists a different density, coverage rate or price, enter that number instead. The waste and compaction allowances follow common trade practice; see our methodology page for the derivation of each formula and the standards it references.
Where this fits in the project
Concrete work starts below grade: excavation, then a compacted gravel base for drainage, formwork, rebar where the slab spec requires it, then the pour. Order by cubic yard and round up — a short load fee costs more than surplus material.
How this connects to your project
No quantity stands alone — each material in the chain depends on the ones around it. These are the planning steps that pair with the rebar calculator:
- Concrete Calculator — Rebar provides tensile strength to concrete.
- Concrete Slab Calculator — Slabs require rebar or mesh reinforcement.
- Concrete Footing Calculator — Footings contain rebar for structural strength.
- Concrete Column Calculator — Columns have vertical rebar cages.
- Concrete Block Calculator — Reinforced block walls need vertical rebar.
- Retaining Wall Calculator — Large retaining walls need rebar reinforcement.
- Deck Calculator — Deck footings may use rebar reinforcement.
Frequently asked questions
Rebar or wire mesh for a slab?
Rebar for structural slabs and vehicle loads; 6×6 mesh controls shrinkage cracking in light flatwork.
Where does rebar sit in the slab?
At mid-depth or slightly above — on chairs, never on the ground. Steel at the bottom does nothing.
Last verified: — prices reflect current national averages. See our methodology.
Estimates are for planning. Material densities, coverage rates and prices vary by region and supplier — confirm final quantities before ordering, and check local code where structural or electrical work is involved.