Rebar calculator

A 20 × 12 ft slab with #4 bars 18 in apart each way, kept 3 in from the edges, takes 22 bars of 20 ft: 317 linear feet and 212 lb of steel.

Slab length

Slab width

Bar size

Spacing, each way

Your plans or local code set the spacing.

Keep bars back from the edges

Bars sold in

Prices used for the cost estimate

Price per bar

For #4 bar, 20 ft. Online listings in November 2026: $9.22 to $18.77 at 7 stores.

Sales tax

22 bars of 20 ft

317 linear feet

212 lb of steel

What to order Buy 22 #4 bars of 20 ft and tie wire for 112 crossings.

Cost estimate

Rebar: 22 × $11.43	$251.46

All quantities and the working

Bars along the length	8 bars, each 19.5 ft
Bars across the width	14 bars, each 11.5 ft
Crossings to tie	112
Weight	212 lb at 0.668 lb per ft

How it was worked out

Across 11.5 ft at 18 in: 8 bars; across 19.5 ft: 14 bars

8 × 19.5 ft + 14 × 11.5 ft = 317 linear ft

How the grid is counted

Bars run both ways across the slab. Take the clearance off each edge to get the length of each run, divide the width by the spacing and add one for the bars that run the length, then do the same the other way. Linear feet is the number of bars in each direction times their length. Every place two bars cross is a tie.

Across 11.5 ft at 18 in: 8 bars; across 19.5 ft: 14 bars

8 × 19.5 ft + 14 × 11.5 ft = 317 linear ft

Bars are sold in 20 ft lengths (some stores also sell 10 ft). The calculator cuts as many runs as fit from each bar. When a run is longer than a bar, the bars have to overlap and be tied where they meet; it counts the extra bar but not the overlap, because the lap length comes from your plans.

Weight of rebar

ASTM A615 nominal sizes and weights
Bar	Diameter	Weight per foot	20 ft bar
#3	0.375 in	0.376 lb	7.5 lb
#4	0.5 in	0.668 lb	13.4 lb
#5	0.625 in	1.043 lb	20.9 lb

The bar number is its diameter in eighths of an inch: a #4 bar is 4/8, or ½ in.

#4 rebar for common slabs

#4 bar, 3 in from the edges
Slab	Spacing	20 ft bars	Linear ft	Weight (lb)	Ties
10 × 10 ft	12 in	10	190	127	100
10 × 10 ft	18 in	8	133	89	49
10 × 10 ft	24 in	6	95	63	25
12 × 12 ft	12 in	24	276	184	144
12 × 12 ft	18 in	16	184	123	64
12 × 12 ft	24 in	12	138	92	36
12 × 20 ft	12 in	32	464	310	240
12 × 20 ft	18 in	22	317	212	112
12 × 20 ft	24 in	16	232	155	60
20 × 20 ft	12 in	40	780	521	400
20 × 20 ft	18 in	28	546	365	196
20 × 20 ft	24 in	20	390	261	100
24 × 24 ft	12 in	96	1,128	754	576
24 × 24 ft	18 in	64	752	502	256
24 × 24 ft	24 in	48	564	377	144
20 × 40 ft	12 in	80	1,570	1,049	800
20 × 40 ft	18 in	55	1,080	721	378
20 × 40 ft	24 in	40	785	524	200

Online store prices, November 2026
Item	Typical	Range	Stores
#4 rebar, ½ in × 20 ft	$11.43	$9.22 to $18.77	7
6 × 6 welded wire mesh roll, 5 × 150 ft (750 sq ft)	$171.98	$139.99 to $209.00	6

Spacing, bar size and whether a slab needs rebar at all come from your plans or your building department; this calculator counts the steel once you know them. For welded wire mesh instead, choose it under "Reinforcement, base and forms" in the slab calculator.

Sources

Rebar physical properties (ASTM A615 nominal dimensions and weights), Cemstone, May 2020. Weight per foot: #3 bar 0.376 lb, #4 0.668 lb, #5 1.043 lb; diameters 3/8, 1/2 and 5/8 in.
Online store listings for bagged concrete, mortar, grout, block, rebar, mesh, gravel, sand, soil, mulch and pavers, Google Shopping results (Home Depot, Lowe's, Menards, Ace Hardware, Walmart and others), November 2026. Single-item prices by store; the calculators use the median as the default and show the range.