Rebar Size Chart: Diameter, Weight and Spacing for Slabs and Footings

The number on a rebar is its diameter in eighths of an inch. #3 is 3/8 in and weighs 0.376 lb per foot, #4 is 1/2 in at 0.668 lb, and #5 is 5/8 in at 1.043 lb, so a 20 ft stick of #4 weighs 13.36 lb. Patios and walks use #3, driveways and footings #4, retaining walls #5, usually on a 12 to 18 inch grid.

Slab length (ft)

Slab width (ft)

Edge clearance (in)

Grid spacing (in)

Rebar stick length (ft)

Common grid spacing: 18–24 in for a residential patio, 12–18 in for a driveway.

Total bars needed

16

Total linear feet

160.0 ft

Sticks to order (20 ft)

8

Bars needed by direction

Direction	Bars needed
Running the slab's length	8
Running the slab's width	8

Assumes a 18-inch grid with 3 inches of clearance from every edge. Sticks needed rounds up to the nearest whole 20-foot stick.

The chart: #3 to #8 by diameter, metric name, area and weight

Bar	Diameter	Soft metric	Cross-section	Weight per foot	One 20 ft stick
#3	3/8 in (0.375 in, 9.5 mm)	10M	0.11 sq in	0.376 lb	7.52 lb
#4	1/2 in (0.500 in, 12.7 mm)	13M	0.20 sq in	0.668 lb	13.36 lb
#5	5/8 in (0.625 in, 15.9 mm)	16M	0.31 sq in	1.043 lb	20.86 lb
#6	3/4 in (0.750 in, 19.1 mm)	19M	0.44 sq in	1.502 lb	30.04 lb
#7	7/8 in (0.875 in, 22.2 mm)	22M	0.60 sq in	2.044 lb	40.88 lb
#8	1 in (1.000 in, 25.4 mm)	25M	0.79 sq in	2.670 lb	53.40 lb

These are the ASTM A615 nominal dimensions for deformed carbon-steel bar, the product every lumberyard and steel supplier in the US stocks. The diameter is nominal because a deformed bar is not round: the ribs stand proud of the core, and the figure in the chart is the diameter of a plain round bar with the same weight per foot. That is also why the weights are the reliable column. A #4 bar held against a ruler reads a little over 1/2 in across the ribs; a 20 ft length of it on a scale reads 13.36 lb, and 0.668 times 20 is 13.36.

The bar number is the diameter in eighths of an inch. Four eighths is a half, so #4 is a 1/2 in bar; six eighths is three quarters, so #6 is 3/4 in. The rule runs all the way to #8, which is a full inch. Larger bars exist, #9 through #11 and then #14 and #18, and they belong to columns, bridge decks and heavy foundations. Nothing on a residential site calls for them, and most yards do not stock them.

The soft metric names in the third column are the same bars relabelled. In the 1990s US mills began rolling the metric size mark instead of the inch one, and the number is simply the diameter in millimeters rounded off: 9.5 mm became 10M, 12.7 mm became 13M. A bar stamped 13 is a #4, with the same 0.668 lb per foot. The trap is Canada, where 10M, 15M and 20M describe a separate series with their own diameters: a Canadian 15M is 16.0 mm, which sits between a #4 and a #5 and matches neither.

The cross-section column is the one the spacing table further down is built from. A slab’s reinforcement is specified as an area of steel per foot of width, and that depends on both the bar and how far apart the bars sit.

Where each size ends up on a house lot

Bar	Where it is the usual choice
#3	Patios, walkways, shed floors, equipment pads, and the stirrups and ties inside beams and columns
#4	Driveways, garage and shop floors, strip footings, stem walls, the vertical bars in block cores, sonotube piers
#5	Footings under heavier walls, retaining wall stems and heels, grade beams, driveways built for trucks
#6 and up	Commercial floors, columns, beams and anything that arrives with a stamped drawing

The #4 bar is the residential default for a practical reason as much as a structural one. It is the smallest bar that stays straight when a crew walks across the grid during a pour: #3 is springy enough to bow between chairs, which is tolerable in a patio and a nuisance under a garage floor. A 20 ft stick of #4 at 13.36 lb is a comfortable one-person carry, while #5 at 20.86 lb is awkward alone and #6 at 30.04 lb is a two-person bar on anything but a short walk. Price follows weight, so a stick of #5 costs roughly half again what #4 does, which is why nobody puts it in a patio.

The patio slab guide covers where a patio moves from mesh to #3 or #4 bar, and the garage slab guide covers the #4 grid and the footing a garage floor is poured inside. Retaining walls are the common place a homeowner meets #5: the stem of a poured wall and the heel of its footing both see bending, and the retaining wall calculator is where the block alternative gets sized. In a block wall the vertical bars drop into the cores and get grouted, and the concrete block calculator covers the grout those cores take.

Spacing from 12 to 18 inches, read as steel per foot of slab

Spacing is quoted on center, the distance from the middle of one bar to the middle of the next, and it is the same in both directions unless the drawing says otherwise. For slabs on grade the usual residential range is 12 to 18 inches: 18 for a patio or shed floor, 16 for a driveway, 12 where a pickup, a trailer or a lift post will sit. Twenty-four inches shows up on patios as the loose end of the range.

What the spacing actually controls is the area of steel crossing every foot of slab width, and that is how an engineer specifies it. The table gives that area in square inches per foot, each direction, for the three bars a residential slab is likely to use.

Bar	12 in on center	16 in on center	18 in on center	24 in on center
#3 (0.11 sq in)	0.110	0.083	0.073	0.055
#4 (0.20 sq in)	0.200	0.150	0.133	0.100
#5 (0.31 sq in)	0.310	0.233	0.207	0.155

Two things fall out of it. Going from #3 to #4 at the same spacing nearly doubles the steel, more than tightening #3 from 18 to 12 inches does, which is why the usual upgrade for a driveway is the heavier bar rather than more of the lighter one. And #3 at 12 inches and #4 at 24 inches carry almost the same steel (0.110 against 0.100) with very different bar counts, so the choice between them is about handling and chair count rather than strength.

For anything structural, a slab that carries a building, a suspended slab, a footing, a wall, the spacing and the bar are set by the engineer or the plan reviewer, and the table above is for understanding the drawing, not replacing it. For the ordinary slab on grade, pick the spacing, put it into the rebar calculator with your slab dimensions, and it returns the bar count each way and the sticks to buy. Because the count rounds up, the bars usually land slightly closer than the nominal spacing, never wider.

Cover: the 3 inches ACI 318 sets against earth

Cover is the concrete between the surface of the bar and the nearest face of the concrete, and it exists to keep water and air off the steel. ACI 318, the structural concrete code that building departments adopt by reference, sets a minimum of 3 inches wherever concrete is cast directly against earth and stays in contact with it. That is the footing case: the bars in a strip footing sit 3 inches up off the soil on chairs or on precast concrete blocks, and 3 inches in from the trench walls. For formed faces that are exposed to weather the same code allows less, 1.5 inches for #5 and smaller bars and 2 inches for #6 and larger, and interior surfaces need less again. Where a set of stamped drawings exists it governs, and where it does not, check with your local building department before assuming a figure.

A 4 inch slab on grade cannot give its steel 3 inches below and anything useful above, and the code is not trying to make it. Slab-on-grade reinforcement is crack control rather than structural steel, so the practice is to hold it at mid-depth, 2 inches from the surface in a 4 inch slab and 2.5 inches in a 5 inch one, where it is covered equally both ways. What does carry over to the slab is the edge: bars stop 3 inches short of every side, which is the clearance input in the calculator above. The slab thickness guide covers why thickness, strength and reinforcement are decided separately.

Lap splices at 40 bar diameters

A bar longer than a stick is made by overlapping two bars and tying them, and the overlap is the lap. The field rule for slabs and footings is 40 bar diameters, with 12 inches as the floor.

Bar	Lap at 40 diameters
#3	15 in
#4	20 in
#5	25 in
#6	30 in
#7	35 in
#8	40 in

Three habits make a lap work. Tie it at both ends with doubled tie wire so the bars cannot slide apart when the concrete pushes on them. Stagger the laps along a run so that adjacent bars do not splice at the same place, which otherwise leaves one weak line across the slab. And at the corner of a footing, do not try to bend the main bar around; cut an L-shaped corner bar with each leg a full lap long and tie it to both runs.

Forty diameters is a planning figure. ACI 318 computes the required lap from the concrete strength, the bar size, whether the bar is coated, how close the bars are, and what share of them are spliced at one location, and an engineered drawing often calls for more than 40 diameters for a bar in tension. Use the table to order steel and to check a drawing against; use the drawing to build.

Chairs: what holds the bar in the middle of the slab

Steel on the ground does nothing, because the bar needs concrete around it on every side and the cracks it is meant to hold run through the body of the slab, not along the bottom. The bar is held up on chairs, and the chair height sets the position: a 2 inch chair puts #4 bar at mid-depth in a 4 inch slab. Plastic chairs clip onto the bar and are what most crews use on a vapor barrier; precast concrete blocks, called dobies, carry a tie wire and are the choice in footings and on gravel, where a plastic leg would punch in. Continuous wire bolsters do the same job for mesh.

Spacing the chairs every 3 to 4 feet along the bar is typical, closer for #3 and for mesh, because the grid has to carry the weight of the people placing the concrete. A grid that sinks under a boot during the pour stays sunk.

Mesh or bar in a 4 inch slab

Welded wire mesh for residential slabs is the 6x6-W1.4xW1.4 sheet, still sold under its old name 6x6-10/10: wires 6 inches apart each way, each with a cross-section of 0.014 sq in. That is 0.028 sq in of steel per foot of width. Compare the spacing table: #3 at 18 inches gives 0.073, about two and a half times as much, and #4 at 18 inches gives 0.133. Mesh is not an equivalent to a rebar grid; it is a lighter one.

For a patio, a walkway or a shed floor on a well-compacted base, lighter is enough. Those slabs crack from shrinkage rather than load, and mesh at mid-depth holds a shrinkage crack tight. Mesh is cheaper per square foot, goes down faster, and a 4 inch slab under foot traffic has nothing for a #4 grid to do. The rules for it are the same as for bar: flat sheets rather than rolls, because a roll never lies flat and ends up on the bottom of the slab, chairs underneath, and a lap of one full square between sheets. Sheet sizes vary by supplier; 5 by 10 ft is a common one.

Rebar takes over when the slab carries something: a driveway, a garage floor, a pad for a hot tub, a strip a vehicle crosses, or any slab on expansive clay or poorly compacted fill where the ground will move under it. There the steel has to hold two pieces of a cracked slab at the same level under a wheel, and 0.028 sq in per foot is not enough to do it.

Estimating the weight: a 20x20 slab at 18 inches

Suppliers quote rebar by the stick, by the pound or by the ton, and a quote in pounds has to be turned into sticks before you can compare it with the lumberyard’s price. The route is linear feet times weight per foot.

Set the calculator above to a 20 ft by 20 ft slab, 3 inch clearance, 18 inch spacing and 20 ft sticks. The usable width is 240 minus 6, or 234 inches, which divides into 13 gaps of 18 inches, so 14 bars run each way: 28 bars, counted at the full 20 ft, which is 560 linear feet and 28 sticks, each trimmed by 6 inches on site to leave the clearance at both ends. In #4 that is 560 times 0.668, or 374.08 lb of steel. The same grid in #3 is 560 times 0.376, which is 210.56 lb, and in #5 it is 584.08 lb. A quote of so many cents per pound times 374 lb is the number to hold against 28 sticks at the retail price, which in 2026 typically sits around $10 to $15 a stick for #4 and lower by the bundle from a steel supplier.

Two items ride on that figure. The 14 by 14 grid has 196 intersections to tie, which a single roll of tie wire covers with plenty to spare, and the bars need chairs every 3 to 4 ft along each one. A chair at an intersection carries the bars in both directions, so a checkerboard of every other intersection, 98 chairs, supports the whole 20x20 at 3 ft. Neither is expensive; both get forgotten until the morning of the pour. The concrete itself, 5.43 cubic yards at 4 inches with the usual waste allowance, comes from the concrete calculator, and the two orders go to different suppliers on different days.

Footings estimate the same way with one more step. A strip footing commonly carries two or three continuous bars, so the linear feet are the footing’s perimeter times the bar count, plus a lap at every splice and a corner bar at every corner. On a 96 ft perimeter with two #4 bars that is 192 ft before laps, and the 20 inch laps add up quickly enough that the sticks should be counted on a sketch rather than guessed.

Grade 60, Grade 40 and the marks rolled into the bar

Every bar carries a row of raised marks: a letter or symbol for the mill that rolled it, the size number, a letter for the steel type (S for the carbon steel of ASTM A615), and a grade mark. Grade is yield strength in thousands of psi. Grade 60 is the standard today and is marked either with a 60 or with a single raised line running the length of the bar; Grade 40 carries no grade mark at all and is the softer bar that older residential work used. For slabs, footings and walls the drawings will say Grade 60, and the price difference is small enough that there is no reason to hunt for anything else.

Epoxy-coated bar, the green one, is for concrete that will see de-icing salt or a marine exposure. It needs longer laps and a touch-up wherever it is cut or scraped, so on a residential slab the money usually goes further in an extra inch of cover.