Rebar Calculator — Grid for a Slab
Lay out a rebar grid for a slab or footing. Enter the slab size and your bar spacing, and this calculator gives the number of bars each way, the total linear feet, the standard 20 ft pieces, and the tie intersections.
How to use this calculator
- Enter the slab length and width in feet.
- Set the grid spacing on center — 12 to 18 inches is typical for residential slabs.
- Edge clearance keeps bars about 3 inches inside the slab edges so they stay covered.
- Buy a roll of tie wire; you need one tie at each intersection (plus extras).
The formula
A rebar grid has bars running both directions, evenly spaced.
Bars one way = (span − 2 × edge clearance) ÷ spacing, rounded down, + 1. Do this for each direction using the perpendicular span.
Total length = bars running lengthwise × slab length + bars running widthwise × slab width. Pieces = total length ÷ bar length, rounded up. Ties = bars one way × bars the other way.
Worked example
A 20 ft × 10 ft slab, 12 in grid, 3 in clearance, 20 ft bars.
Lengthwise bars = (120 − 6) ÷ 12 + 1 = 10Widthwise bars = (240 − 6) ÷ 12 + 1 = 20Total = 10 × 20 + 20 × 10 = 400 lin ftPieces = 400 ÷ 20 = 20 bars; ties = 10 × 20 = 200
A 20×10 slab on a 12 in grid needs about 400 ft of rebar (20 × 20 ft bars) and ~200 ties.
Frequently asked questions
What rebar spacing for a concrete slab?
Residential slabs commonly use #3 or #4 rebar on a 12–18 inch grid. Closer spacing and larger bars are used for heavier loads; check local code or an engineer for structural slabs.
How much rebar do I need for a slab?
Lay out a grid: count the bars each direction, multiply by the run length, and add them. A 20×10 slab on a 12 in grid is about 400 linear feet.
How far should rebar be from the slab edge?
Keep about 3 inches of clearance from edges and the bottom so concrete fully surrounds the steel and protects it from corrosion. Support bars on chairs at mid-slab height.