How much rebar do I need for a concrete slab?

Count bars in each direction as (clear span / spacing) + 1. A 20 x 20 ft slab with #4 bar at 16 inches on centre and 3 inches of cover takes 16 bars each way — about 620 linear feet, or 31 twenty-foot sticks, weighing roughly 415 lb.

What size rebar for a 4 inch slab?

#3 or #4 bar at 16 to 18 inches on centre, or welded wire mesh, is typical for a residential 4 inch slab. Driveways and vehicle areas usually get #4 at 16 inches. Structural slabs are engineered — follow the drawings.

How far apart should rebar be in a slab?

16 to 18 inches on centre each way is the common residential grid, tightened to 12 inches for heavier loads. Keep 3 inches of cover from soil-facing faces and edges, and support the steel at mid-depth on chairs.

How much does rebar weigh per foot?

#3 is 0.376 lb/ft, #4 is 0.668 lb/ft, #5 is 1.043 lb/ft and #6 is 1.502 lb/ft. Bar number is the diameter in eighths of an inch, so weight follows the square of the number.

How long should a rebar lap splice be?

A common rule of thumb for non-structural residential slabs is 40 bar diameters — about 20 inches for #4 bar. Structural work uses code-specified development lengths that depend on concrete strength, bar coating and cover, so follow the engineer's drawings.

Rebar Calculator

Bar count each way, total linear feet with lap splices, sticks to buy, tie count and weight — for a slab, footing or grade beam grid.

Quick answer

A 20 x 20 ft grid of #4 bar at 16 in on centre needs 30 bars — 585 linear feet including 0 lap splices, which is 30 sticks of 20 ft bar weighing 391 lb ($371).

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Slab and grid

Length(ft)

Width(ft)

Spacing on centre(in)

16–18 in residential

Edge cover(in)

3 in against soil

Bar size

Grid direction

Stock length(ft)

20 ft is standard; 10 ft at big-box stores

Lap splice(bar diameters)

40d rule of thumb

Price($/lb)

bars = floor(clear span / spacing) + 1 | lap = 40 x bar ⌀ = 20 in | weight = length x lb/ft

Steel takeoff

20 ft sticks to buy

585 linear ft including splices

Bars each way

15 x 15

Cut to 19.5 ft and 19.5 ft

Weight

391lb

0.2 tons

Steel cost

$371

Tie intersections

225

Tie the perimeter and alternate inside

Mesh alternative

The same 400 sq ft would take about 9 sheets of 5 x 10 ft welded wire mesh with 10% overlap. Mesh is cheaper and faster for crack control on a 4 inch slab, but it carries far less load than a bar mat.

Published figure

Bar sizes, weights and areas

Bar	Metric	Diameter	Area	Weight	40d lap	Typical use
#3	No. 10	0.375 in	0.11 in²	0.376 lb/ft	15 in	Slabs on grade, sidewalks, small footings
#4	No. 13	0.5 in	0.196 in²	0.668 lb/ft	20 in	Driveways, patios, residential footings
#5	No. 16	0.625 in	0.307 in²	1.044 lb/ft	25 in	Foundation walls, heavier slabs, piers
#6	No. 19	0.75 in	0.442 in²	1.503 lb/ft	30 in	Structural footings, grade beams
#7	No. 22	0.875 in	0.601 in²	2.046 lb/ft	35 in	Columns and beams
#8	No. 25	1 in	0.785 in²	2.673 lb/ft	40 in	Columns, heavy beams
#9	No. 29	1.125 in	0.994 in²	3.383 lb/ft	45 in	Heavy structural work
#10	No. 32	1.25 in	1.227 in²	4.176 lb/ft	50 in	Heavy structural work
#11	No. 36	1.375 in	1.485 in²	5.053 lb/ft	55 in	Heavy structural work

Derived from the bar designation (diameter in eighths of an inch) and the density of steel.

Steel only works where it sits. Rebar laid on the subgrade and pulled up "as you pour" ends up at the bottom of the slab where it does nothing for the tension face — set it on chairs at mid-depth, or in the upper third for a slab on grade in tension from above.

Bar length here is the clear span after cover is deducted from both ends, so add nothing extra for cover. Splices are added automatically whenever a run exceeds your stock length.

Source & method: Bar diameter comes from the US designation (bar number / 8 inches), area from pi x d²/4, and weight per foot from area x 12 in x 0.2836 lb/in³, the density of steel. This reproduces published nominal weights without copying a specification table.

Reinforcement supplies

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Frequently asked questions

How much rebar do I need for a concrete slab?: Count bars in each direction as (clear span / spacing) + 1. A 20 x 20 ft slab with #4 bar at 16 inches on centre and 3 inches of cover takes 16 bars each way — about 620 linear feet, or 31 twenty-foot sticks, weighing roughly 415 lb.
What size rebar for a 4 inch slab?: #3 or #4 bar at 16 to 18 inches on centre, or welded wire mesh, is typical for a residential 4 inch slab. Driveways and vehicle areas usually get #4 at 16 inches. Structural slabs are engineered — follow the drawings.
How far apart should rebar be in a slab?: 16 to 18 inches on centre each way is the common residential grid, tightened to 12 inches for heavier loads. Keep 3 inches of cover from soil-facing faces and edges, and support the steel at mid-depth on chairs.
How much does rebar weigh per foot?: #3 is 0.376 lb/ft, #4 is 0.668 lb/ft, #5 is 1.043 lb/ft and #6 is 1.502 lb/ft. Bar number is the diameter in eighths of an inch, so weight follows the square of the number.
How long should a rebar lap splice be?: A common rule of thumb for non-structural residential slabs is 40 bar diameters — about 20 inches for #4 bar. Structural work uses code-specified development lengths that depend on concrete strength, bar coating and cover, so follow the engineer's drawings.