Rebar Calculator — bars and total length for a slab grid

How it works

Slab reinforcement is a grid of bars laid at a fixed spacing in both directions across a rectangle, per Wolfram MathWorld's A = length × width. The count in each direction is:

bars = ⌊(span − 2 × edge clearance) ÷ spacing⌋ + 1

The span and clearance are in inches; the "+ 1" is the fencepost rule — a run of gaps needs one more bar than the number of gaps. The edge clearance keeps the outermost bars set in from the slab edge (typically about 3 inches, so the concrete protects the steel).

There are two sets of bars:

• Lengthwise bars run the full length and are spaced across the width.
• Widthwise bars run the full width and are spaced along the length.

The total length is each direction's bar count times the span those bars cross, added together — the number of linear feet of rebar to buy (usually in 20-foot sticks). Add tie wire for the intersections and a little extra for laps where bars overlap.

Worked example

For a 20 × 10 ft slab with 12-inch spacing and 3-inch edge clearance: across the 10 ft (120 in) width, (120 − 6) ÷ 12 = 9.5, so ⌊9.5⌋ + 1 = 10 lengthwise bars (each 20 ft). Along the 20 ft (240 in) length, (240 − 6) ÷ 12 = 19.5, so ⌊19.5⌋ + 1 = 20 widthwise bars (each 10 ft). That is 30 bars in total and 10 × 20 + 20 × 10 = 400 feet of rebar.

Frequently asked questions

How much rebar do I need for a slab?

Count the bars in each direction — ⌊(span − 2 × edge clearance) ÷ spacing⌋ + 1 — then multiply each count by the span its bars cross and add the two totals. A 20 × 10 ft slab at 12-inch spacing needs 30 bars and 400 feet.

What spacing should rebar be?

Residential slabs commonly use 12 to 18 inches on center; heavier loads use closer spacing. Your engineer or local code specifies the exact spacing and bar size for the job.

What is edge clearance (cover)?

It is the distance the outer bars are held in from the slab edge, usually about 3 inches, so the concrete covers and protects the steel from moisture and corrosion.

Why one more bar than the number of gaps?

The bars sit at the ends of each grid space, so a run with n equal gaps has n + 1 bars — the same fencepost counting that gives one more post than the number of fence sections.

Do I need extra for overlaps?

Yes. Where bars are too long for a single stick they overlap (lap) by roughly 40 bar diameters. Add a little extra length for laps, plus tie wire to fasten the intersections.

How is the total length used to buy rebar?

Rebar is sold in standard lengths, often 20-foot sticks. Divide the total length by the stick length and round up to know how many sticks to buy, keeping cut offcuts for shorter runs.