Rebar Calculator

How the bar count works

A slab grid has two sets of bars: one set runs along the length, the other along the width. Each set is counted the same way.

The division gives the number of gaps between bars. Rounding it up means no gap is wider than the spacing you enter; the + 1 adds the bar at the start. When the run is not an exact multiple of the spacing, spread the bars evenly and they end up a little closer than the spacing.

Example: a 20 × 12 ft slab, #4 at 18 in, 3 in from the edges

1. Clear runs: 20 − 2 × 0.25 = 19.5 ft long, 12 − 2 × 0.25 = 11.5 ft wide
2. Bars running the length: 11.5 ÷ 1.5 = 7.67 → 8 gaps + 1 = 9 bars, each 19.5 ft, about 17.25 in apart
3. Bars running the width: 19.5 ÷ 1.5 = 13.00 → 13 gaps + 1 = 14 bars, each 11.5 ft, 18.00 in apart
4. Total length: 9 × 19.5 + 14 × 11.5 = 336.5 ft
5. Weight: 336.5 ft × 0.668 lb/ft = 225 lb
6. 20 ft bars: every run is shorter than 20 ft, so no splices. The calculator fits pieces into bars longest first and reuses offcuts where they fit, but here the 0.5 or 8.5 ft left from each cut is too short for another run: 23 bars

Example: a 24 × 24 ft slab, #4 Grade 40, 20 ft bars

1. Clear run each way: 24 − 0.5 = 23.5 ft, which is longer than a 20 ft bar
2. Bars each way: 23.5 ÷ 1.5 = 15.67 → 16 gaps + 1 = 17; 34 runs in all
3. Each run needs 1 lap splice of 20 in (IRC value for #4, Grade 40): 23.5 + 1.67 = 25.17 ft of bar, made from one whole 20 ft bar and a 5.17 ft piece
4. Whole bars: 34 runs × 1 = 34. The 34 short pieces fit 3 to a bar: 34 ÷ 3 rounded up = 12
5. Total: 855.7 ft, 572 lb, 46 bars of 20 ft (34 whole + 12 cut) with 34 splices

The count assumes perfect cuts. A real cut plan may need a bar or two more for saw kerf and damaged ends.

Rebar sizes and weights

US rebar sizes are numbered in eighths of an inch: a #4 bar is 4/8 = ½ in across. Weight per foot depends only on size, not on grade. The values below are the same in Nucor’s size table and ASTM A615 Table 1.^[1][2][3]

10 ft or 20 ft bars

Home centers stock #3 and #4 rebar in 10 ft lengths and #4 in 20 ft lengths (the 20 ft #4 bar we checked is Grade 40); mills ship 20, 40 and 60 ft lengths.^[4][5][3] Short bars are easier to carry but cost you laps. The 20 × 12 ft slab above needs 23 bars of 20 ft (460 ft bought). With 10 ft bars every run needs splices, and it takes 40 bars (400 ft bought). The laps add 53.3 ft of steel (389.8 ft instead of 336.5 ft) and 32 splices to tie, yet the 10 ft order is shorter here because the 20 ft bars leave long offcuts that fit no other run. Try both lengths in the calculator.

Lap splices

Where one bar ends and the next begins, the two overlap and are tied together. The calculator uses the tension lap lengths in IRC Table R608.5.4(1).^[6] That table is written for concrete walls; we use it as a guide for slabs because no slab table was available to us, so follow your plans if they give a different length.

In bar diameters, Grade 40 laps are 40 diameters and Grade 60 laps about 60. A rule of thumb of 40 to 48 diameters is often quoted; it falls short of the IRC table for Grade 60, so check which grade you are buying.

Edge cover and where the bars sit

Cover is the concrete between the steel and the outside of the slab. IRC R608.5.4.1 asks for 3 in where concrete is cast against earth, 1½ in for #5 and smaller bars where formed concrete is exposed to earth or weather (2 in for #6 and larger), and ¾ in where it is not exposed.^[6] The calculator’s edge cover (3 in by default) is how far the bar ends stop short of the slab edge.

For a slab on ground, IRC R506.2.4 says to support the reinforcement so it stays between the middle and the upper third of the slab while the concrete is placed.^[7] That is what chairs and bar supports are for. IRC R506.1 sets the slab minimum at 3½ in.^[8]

About the 18 in default

The calculator starts at 18 in spacing because that figure is often repeated for residential slabs. It is a common practice, not a code value: we could not find it in the IRC sections we checked or in any industry document we could open. Enter the spacing from your plans or your building department.

Welded wire mesh instead of rebar

Welded wire mesh (also called WWR or remesh) is the other way to put steel in a light slab. The Wire Reinforcement Institute’s guide for light construction lists two common styles: 6x6-W1.4xW1.4 (10 gauge) at about 21 lb per 100 sq ft, and 6x6-W2.9xW2.9 (6 gauge) at about 42 lb.^[9] WRI places mesh in the middle of a 4 in slab and 2 in below the top of a 5 or 6 in slab, on supports 2 to 3 ft apart.^[9] Home centers sell 3.5 × 7 ft sheets and 5 × 150 ft rolls; suppliers also stock larger sheets such as 5 × 10 and 8 × 20 ft.^[10][11]

Rebar rows are our arithmetic: 133.3 ft of bar per 100 sq ft (two directions at 18 in), ignoring edges and laps; area per foot is the bar area × 12 ÷ 18. Mesh rows are from WRI. The table compares amounts of steel only. It does not say which one your slab needs. For a whole slab with concrete, base and mesh or rebar in one list, use the concrete slab calculator.

Before you order

• Count chairs or bar supports too: WRI’s common practice for mesh is one every 2 to 3 ft.^[9]
• Rebar at home centers is sold by the piece, but the weight tells you what you are hauling: 23 bars of 20 ft #4 weigh about 307 lb.
• Dowels into an existing slab, bent corner bars and footing steel are not in this count. Add them from your plans.

Related calculators

• Concrete slab calculatorConcrete, gravel base and mesh or rebar in one list
• Concrete calculatorCubic yards and bags for slabs, footings and posts
• Concrete slab thicknessHow thick to pour a patio, walk or driveway
• Project materials listPrint everything you added from each calculator

Prepared by the Yard & Build editors. The formulas behind this calculator are checked against hand-worked answers every time the site is built. Bar weights and code values checked September 26, 2026.