Bars, length and weight for a slab

Rebar Calculator

A 12 × 12 ft slab with #4 rebar 18 in on center, 3 in in from the edges, takes 9 bars one way and 9 the other: 207 ft of steel, about 138 lb, or 18 bars of 20 ft. Enter your slab below.

Sources checked Oct 15, 2026

Your estimate

18 × 20 ft bars

9 bars along the length and 9 across, 207 ft in all, about 138 lb of #4.

Slab plan, bars from aboveLive sketch

18 in on center, 3 in from each edge; every bar fits in one stock length, so there are no laps. A sketch of the count, not a structural drawing.

Bars along the lengthSpread across the 12 ft width
9 × 11 ft 6 in
Bars across the widthSpread along the 12 ft length
9 × 11 ft 6 in
Total length with laps207 ft of bar in the slab plus 0 laps of 22.5 in
207 ft
Stock bars to buy (20 ft)Each run cut from its own bar; reusing every offcut cannot go below 11
18
Weight207 ft × 0.668 lb per ft for #4
138 lb

Bars run evenly from one clearance line to the other: 17.25 in apart across the width and 17.25 in along the length, never more than the spacing you chose.

Welded wire mesh instead? A city driveway rule names 6x6-W1.4xW1.4 mesh over the whole slab: 144 sq ft here, before laps. Mesh sheet sizes are not in our data.

How to calculate rebar for a slab

Take the edge clearance off both ends of each side. The 12 × 12 ft example with 3 in of clearance leaves bar runs of 138 in (11.5 ft).
Count the bars across each side: run ÷ spacing, rounded up, plus one. 138 ÷ 18 rounds up to 8, so 9 bars each way, 18 in all, spread evenly 17.25 in apart. Rounding down would leave the bars wider apart than the spacing you chose.
Multiply by the run length and add the laps. Every run here fits one 20 ft bar, so there are no laps: 18 × 11.5 ft = 207 ft.
Multiply the feet by the bar’s weight per foot. #4 weighs 0.668 lb per foot: 207 × 0.668 = 138.3 lb.
Count stock bars. Each run is cut from its own 20 ft bar, so 18 bars. A run longer than one bar takes extra bars, each overlapping the last by the lap.

Spacing and bar size are your design choice; this page counts steel for the choice you make and does not say what a slab needs. The slab planner uses the same count for the concrete slab calculator, with bars laid on the edge. For the concrete, see how many bags of concrete you need.

Rebar spacing: bars each way at 12 to 36 inches

The 12 × 12 ft example with #4 bars and 3 in clearance, at each spacing the form offers.
Spacing on center	Bars each way	Total bars	Linear feet	Weight
12 in	13	26	299 ft	200 lb
16 in	10	20	230 ft	154 lb
18 in	9	18	207 ft	138 lb
24 in	7	14	161 ft	108 lb
36 in	5	10	115 ft	77 lb

Closer spacing means more steel, more tying and more weight, and the choice belongs to whoever designs the slab. Published examples differ by use, and none is a rule for your slab:

Rebar grid spacing in published slab details

24 to 36 in on center (basketball court slab)Greatmats DunkStar court concrete specifications (2025)
12 in on center, both ways (#4 epoxy-coated bars, commercial and industrial driveway approaches)City of North Salt Lake, UT, drive approach requirements

The form takes your spacing. The example uses 18 in, between the two; it is a placeholder, not a recommendation.

Rebar sizes and weight per foot

Nominal diameter and weight per foot from Caltrans Bridge Design Details, table 13.1.1. The lap is 45 bar diameters (Caltrans), as the form starts it.
Bar	Diameter	Weight per foot	Per 20 ft bar	Lap
#3	0.375 in	0.376 lb	7.5 lb	16.9 in
#4	0.5 in	0.668 lb	13.4 lb	22.5 in
#5	0.625 in	1.043 lb	20.9 lb	28.1 in
#6	0.75 in	1.502 lb	30 lb	33.8 in
#7	0.875 in	2.044 lb	40.9 lb	39.4 in
#8	1 in	2.67 lb	53.4 lb	45 in

The bar number is the diameter in eighths of an inch, so #4 is 4/8 in, or ½ in. Nucor’s rebar sizes list prints the same weights for #3 to #8 without a unit. A #4 bar weighs 0.668 lb per foot, 13.4 lb for a 20 ft stick. Caltrans Bridge Design Details 13.1, reinforcing bar data (2025); Nucor rebar product page

Rebar lap length and stock bar lengths

Caltrans’ construction manual prints a minimum lap of 45 bar diameters for uncoated Grade 60 bars #8 and smaller, so a #4 lap is 22.5 in. That is a state bridge standard, not a backyard slab rule; type your own lap in the form. Caltrans Reinforced Concrete Construction Manual, ch. 4 (2025)

Nucor lists cut-to-length bars of 20 ft, 40 ft, 60 ft. When a run is longer than one bar, the next bar overlaps the last by the lap, so each added bar covers the stock length minus the lap. Nucor rebar product page Place the steel at mid-depth on bolsters or chairs; the slab planner counts them.

Welded wire mesh instead of rebar

Our data has one rule for mesh: Shaker Heights, Ohio, asks residential driveways for either 6x6-W1.4xW1.4 welded wire mesh placed in the middle of the slab, or synthetic fibers. City of Shaker Heights, OH, driveway requirements (2019) QUIKRETE’s sidewalk guide also hangs mesh at the midpoint of the slab. QUIKRETE project guide: Concrete Sidewalks and Small Slabs

No sheet or roll size is in our data, so the calculator gives only the area to cover: 144 sq ft for the 12 × 12 ft example, plus the laps where sheets meet. Check your own city’s rule before you choose mesh or bars.

Numbers this page uses

#4 bar diameter: 0.5 inCaltrans Bridge Design Details 13.1, reinforcing bar data (2025)
#4 weight per foot: 0.668 lbCaltrans Bridge Design Details 13.1, reinforcing bar data (2025)
Lap splice: 45 bar diametersCaltrans Reinforced Concrete Construction Manual, ch. 4 (2025)
Stock bar lengths: 20 ft, 40 ft, 60 ftNucor rebar product page
Edge clearance: 3 inOur default; change it above
Spacing: 18 inOur default; change it above

Rebar calculator questions

How much rebar do I need for a 12 × 12 slab?

With #4 bars 18 in on center and 3 in in from each edge, a 12 × 12 ft slab takes 9 bars each way, 18 in all. Each bar is 11.5 ft, so the grid is 207 ft of steel and weighs about 138 lb. Spacing and bar size are your choice. For the concrete itself, use the concrete slab calculator.

How do I calculate rebar for a slab?

Subtract the edge clearance from both ends, divide each side by the spacing, round up and add one: (138 ÷ 18, rounded up) + 1 = 9 bars along the length, and the same across the width, 17.25 in apart. Multiply by each bar's length, add any laps, then multiply the feet by the bar's weight per foot (#4 is 0.668 lb).

How many 20 ft rebar bars do I need?

The 12 ft example needs 18 bars, because each 11.5 ft run is cut from its own 20 ft bar. The most the steel can shrink to is 11 bars (207 ft ÷ 20 ft), and only if every offcut is used. Nucor lists 20 ft, 40 ft, 60 ft bars.

What spacing should rebar be on a slab?

There is no one spacing. Our sources show 24 to 36 in on center for a basketball court slab (Greatmats) and 12 in each way for commercial driveway approaches (North Salt Lake, Utah). Neither is a rule for your slab; a designer or your building department sets yours.

How much does #4 rebar weigh?

Caltrans lists #4 at 0.668 lb per foot, so a 20 ft bar weighs 13.4 lb. Nucor prints the same figure without a unit.

What is the rebar lap length?

Caltrans prints a minimum lap of 45 bar diameters for uncoated Grade 60 bars up to #8, which is 22.5 in for #4. That is a state bridge standard, not a rule for a backyard slab. The form lets you type your own lap, and the 12 ft example needs none because each bar fits in one 20 ft length.

Can I use welded wire mesh instead of rebar?

Some cities allow it for light slabs. Shaker Heights, Ohio, asks residential driveways for either 6x6-W1.4xW1.4 welded wire mesh in the middle of the slab or synthetic fibers. Our data has no mesh sheet or roll size, so the mesh figure here is the slab area, 144 sq ft for the example, before laps.

A planning aid, not an engineering calculation: your local code and building inspector have the final word. Terms