Bar table from TxDOT Item 440, checked against its own geometry
A bar's size number is its diameter in eighths of an inch, which everybody knows, and which stops being true at #9. A #9 is 1.128 in, not 1.125, because the five largest sizes are the old square bars carried forward and sized to match their areas. This counts the mat, and it shows you the bar table checked against the geometry rather than copied from another calculator.
Rebar mat quantities
ft
ft
in
in
in
ft
in
%
A quantity takeoff only. Bar size, spacing, cover and lap length are design decisions that come from your drawings or your engineer, not from this page.
Specification summary
Generated from on .
Reopen that address to reproduce these figures exactly.
Entered
Result
The mat in plan, at the spacing you entered
Worked example
A 30 ft by 20 ft slab, #4 bar at 18 in on centre both ways, 3 in cover, 20 ft stock, no lap entered, 5% for cuts.
Inside the cover. 3 in off each edge leaves 29.5 ft by 19.5 ft of mat.
Bars the long way. 19.5 ft across at 18 in on centre is 13 spaces plus one, so 14 bars at 29.5 ft.
Bars the short way. 29.5 ft at 18 in is 19 spaces plus one, so 20 bars at 19.5 ft.
Linear feet. 14 × 29.5 + 20 × 19.5 = 803 ft.
Splices. Every 29.5 ft bar exceeds a 20 ft stock length, so 14 of them need one join each: 14 splices.
Weight. 803 ft plus 5% is 844 ft, at 0.668 lb a foot: 564 lb.
The lap material is missing from that figure on purpose, and the calculator says so rather than quietly guessing. Fourteen splices at a 25 in lap would add 30 ft of bar, about 20 lb. At a 40 in lap it is 47 ft. Lap length comes out of ACI 318 and depends on the concrete strength, the bar grade, the cover and the spacing, so it is on your drawings and it is not a number this page will make up.
The formula
Counting a mat is fence-post arithmetic in two directions:
n = ⌊(S − 2c) / p⌋ + 1 LF = n₁ℓ₁ + n₂ℓ₂ W = LF × w
S, c
the slab dimension across which bars are spaced, and the cover taken off each edge, ft
p
spacing on centre, converted to ft
+ 1
the fence-post term. Thirteen spaces need fourteen bars
ℓ
the length of each bar, which is the OTHER dimension less its own cover
w
weight per foot, from the published table, not from the rounded area
The fence-post term is where mat counts most often go wrong: dividing the width by the spacing gives the number of gaps, not the number of bars. On a 19.5 ft run at 18 in that is 13 against 14, an error of 7% on half the mat.
Why this page prints a derivation next to the table
There are a dozen rebar tables on the web and almost all of them are the same eleven rows copied from each other with no source attached. The figures happen to be right, which is lucky rather than reassuring.
The table here is transcribed from the Texas Department of Transportation's 2024 Standard Specifications, Item 440, Table 1. That is a state government publication, it is free, and it names itself. Then the page does something the copies do not: it derives the weight column independently, from the diameter and the density of steel, and prints both side by side.
Every row lands within a tenth of a percent. That agreement is worth more than either number alone, because two independent routes to the same figure is what a check is, and a copied table has no check in it at all.
The derivation also exposes something the table does not announce. Bars #3 to #8 are numbered in eighths of an inch and #9 upward are not, and the reason is a hundred years old.
Bar sizes as published, checked against their own geometry
Bar
Diameter, in
Area, sq in
Weight, lb/ft
Area x 12 x 0.2833
Number in eighths?
Where the size comes from
#3
0.375
0.11
0.376
0.375
3/8 = 0.375
numbered in eighths
#4
0.500
0.2
0.668
0.668
4/8 = 0.500
numbered in eighths
#5
0.625
0.31
1.043
1.043
5/8 = 0.625
numbered in eighths
#6
0.750
0.44
1.502
1.502
6/8 = 0.750
numbered in eighths
#7
0.875
0.6
2.044
2.044
7/8 = 0.875
numbered in eighths
#8
1.000
0.79
2.67
2.670
8/8 = 1.000
numbered in eighths
#9
1.128
1
3.4
3.397
does not apply
1 in square bar
#10
1.270
1.27
4.303
4.307
does not apply
1 1/8 in square bar
#11
1.410
1.56
5.313
5.308
does not apply
1 1/4 in square bar
#14
1.693
2.25
7.65
7.653
does not apply
1 1/2 in square bar
#18
2.257
4
13.6
13.601
does not apply
2 in square bar
Read 2026-09-08. Columns two to four are as printed by TxDOT. Column five is this page deriving the weight from the diameter, and columns six and seven are where the size number comes from.
The bar number is eighths of an inch, until it suddenly is not
Everyone knows the first half of this: a bar's size number is its diameter in eighths of an inch. A #4 is 4/8, half an inch. A #8 is 8/8, one inch. It is the single most useful fact on a job site.
It stops at #8. A #9 is not 9/8 = 1.125 in. It is 1.128. A #10 is not 1.250, it is 1.270. A #11 is not 1.375, it is 1.410. Those are not rounding artefacts, they are a different rule.
The five big sizes are the old square bars carried forward. Each round bar was sized to have the same cross-sectional area as the square bar it replaced, so the design tables did not have to be rewritten:
Bar
The square bar it replaced
That square's area
Published area
#9
1 in square
1.0000
1
#10
1 1/8 in square
1.2656
1.27
#11
1 1/4 in square
1.5625
1.56
#14
1 1/2 in square
2.2500
2.25
#18
2 in square
4.0000
4
Read the last two columns. A #14 is a 1 1/2 in square bar, and 1.5² is 2.25, which is the published area exactly. A #18 is a 2 in square, and 2² is 4.00, again exact. The awkward diameters are simply what a circle needs to be to match those areas.
Which matters on a takeoff, because if you assume the eighths rule holds all the way up you will order a #11 as though it were 1 3/8 in, and it is not.
And the weight column is geometry, like everything else here
The published weight of a bar is not measured, it is calculated, and you can reproduce every row:
lb per ft = π(d/2)² × 12 × 0.2833
That last figure is the density of steel, about 490 lb per cubic foot. Run a #4 through it: π × 0.25² = 0.19635 sq in, times 12 in, times 0.2833, is 0.668 lb per foot. The table says 0.668.
Texas Department of Transportation, 2024 Standard Specifications, checked against its own geometry
Bar
Diameter, in
Area, sq in
Weight, lb/ft
Area x 12 x 0.2833
Number in eighths?
Where the size comes from
#3
0.375
0.11
0.376
0.375
3/8 = 0.375
numbered in eighths
#4
0.500
0.2
0.668
0.668
4/8 = 0.500
numbered in eighths
#5
0.625
0.31
1.043
1.043
5/8 = 0.625
numbered in eighths
#6
0.750
0.44
1.502
1.502
6/8 = 0.750
numbered in eighths
#7
0.875
0.6
2.044
2.044
7/8 = 0.875
numbered in eighths
#8
1.000
0.79
2.67
2.670
8/8 = 1.000
numbered in eighths
#9
1.128
1
3.4
3.397
does not apply
1 in square bar
#10
1.270
1.27
4.303
4.307
does not apply
1 1/8 in square bar
#11
1.410
1.56
5.313
5.308
does not apply
1 1/4 in square bar
#14
1.693
2.25
7.65
7.653
does not apply
1 1/2 in square bar
#18
2.257
4
13.6
13.601
does not apply
2 in square bar
Every row lands within a tenth of a percent. The fifth column is the derivation and the fourth is what the specification prints, and they are independent of each other, which is the point of showing both.
Note that the area column is rounded and the weight column is not. A #4's real area is 0.19635 sq in and the table calls it 0.20, because designers want a round number to multiply. If you derive weight from the rounded area you get 0.681 lb per foot rather than 0.668, which is a 2% error on a big order. Use the published weight.
Lap length is not on this page, and here is why
Every bar in a mat longer than a stock length has to be spliced, and a lapped splice adds material. This calculator counts the splices and then stops, unless you tell it a lap length.
That is deliberate. Lap length is a structural design value. ACI 318 sets it from the specified concrete strength, the bar grade, the bar size, the cover, the clear spacing between bars, whether the bar is epoxy coated, whether it is a top bar with fresh concrete cast below it, and whether the splice is Class A or Class B. Two slabs with the same #4 bar can have laps that differ by half again.
Publishing a single number for it would be exactly the thing this site refuses to do, because the failure it causes is not a wasted trip to the merchant. So the field defaults to zero, the calculator warns you when there are splices and no lap entered, and the moment you type the figure from your drawings it costs the material properly.
The same applies to cover, spacing and bar size. All three are on the page as inputs and none of them are calculated.
The fence-post error, and what it costs
The commonest mistake in a mat takeoff is dividing and forgetting to add one. A 19.5 ft run at 18 in on centre has 13 spaces and 14 bars, because a bar sits at both ends as well as in between.
On the worked example that error would drop 1 bar of 14 one way and 1 of 20 the other, about 7% of the steel. It is small enough to survive a sanity check and large enough to leave you short on the day.
The calculator uses the floor of the division plus one, which is the correct form, and it takes the edge cover off before dividing so the outermost bars sit where the drawing puts them rather than on the edge of the slab.
What a mat weighs, and why it is worth knowing before delivery
Reinforcement is bought by weight and handled by hand. The worked example is 564 lb of #4 in 844 linear feet, which is 43 lengths of 20 ft bar: a bundle two people can move.
Tighten the same slab to 12 in on centre in #5 and it becomes about 2,000 lb in 50 bars of 20 ft. That is a different delivery, a different unloading and a different day, and it is worth discovering at the drawing stage rather than when the lorry arrives.
The weight figure here uses the published pounds per foot rather than the rounded area, and the difference is not trivial: a #4's tabulated area of 0.20 sq in is rounded up from 0.19635, so weight derived from it comes out 2% high.
Where every figure on this page came from
Read 2026-09-08. Bag yields are from the manufacturer's own data sheet. The code table is transcribed from a government publication that prints it in full, and the two amendments are from the states' and cities' own rule documents.
Texas Department of Transportation, 2024 Standard Specifications
A state government publication reproducing the ASTM bar table in full, which is why it is the transcription source here rather than any of the calculator sites that reprint the same figures unsourced.
Seattle Department of Construction and Inspections, page 168
Carries IRC Section R506 in full: minimum thickness, fill depth limits, the 4 in base course and its Group I exception, the vapor retarder and its four exceptions, and where reinforcement must sit.
Why the code table is quoted from Seattle rather than from the ICC.codes.iccsafe.org returns HTTP 403 and cannot be read from here. The 2018 Seattle Residential Code is a municipal government publication that reproduces IRC Table R402.2 in full, names its edition and its jurisdiction on the page, and marks its own amendments separately from the model text. That makes it checkable, which a paywalled document is not.
Frequently asked questions
How much rebar do I need for a 30x20 slab?
At #4 bar 18 in on centre both ways with 3 in cover, 34 bars: 14 running the length at 29.5 ft and 20 running the width at 19.5 ft. That is 803 linear feet, or 844 with a 5% allowance for cuts, and about 564 lb. Bar size and spacing are design decisions and should come from your drawings.
What size is a #4 rebar?
Half an inch. Bar size numbers are the diameter in eighths of an inch, so #4 is 4/8 = 0.500 in, with a nominal area of 0.20 sq in and a weight of 0.668 lb per foot. The rule holds from #3 to #8 and then stops.
Why is a #9 rebar not 1.125 inches?
Because the eighths rule only covers #3 to #8. A #9 is 1.128 in, and the five largest sizes are the old square bars carried forward: a #9 has the same cross-sectional area as a 1 in square bar, a #14 the same as a 1 1/2 in square, and a #18 the same as a 2 in square. Those last two are exact: 1.5 squared is 2.25 sq in and 2 squared is 4.00, which are the published areas.
How much does rebar weigh per foot?
0.376 lb for #3, 0.668 for #4, 1.043 for #5, 1.502 for #6 and 2.670 for #8, per TxDOT Item 440 Table 1. Those are not measured figures, they are the cross-sectional area times the density of steel, and you can reproduce any of them: a #4 is pi times 0.25 squared, times 12 in, times 0.2833 lb per cubic inch, which is 0.668.
How long should a rebar lap splice be?
This page will not give you a number, because there is not one. ACI 318 sets lap length from the concrete strength, the bar grade and size, the cover, the clear spacing, whether the bar is epoxy coated or a top bar, and whether the splice is Class A or Class B. It is on your drawings. Enter it in the field and the calculator adds the material.
How many bars do I need at 18 inch spacing?
Divide the run by the spacing and add one. A 19.5 ft run at 18 in on centre is 13 spaces, so 14 bars, because there is a bar at each end as well as between. Forgetting the plus one is the commonest error in a mat takeoff and it costs about 7% of the steel.
Should I use the area or the weight to work out my order?
The weight. The published area is rounded for design convenience: a #4's real area is 0.19635 sq in and the table calls it 0.20. Deriving a weight from the rounded figure gives 0.681 lb per foot instead of 0.668, which is 2% high across the whole order.
Check these numbers yourself
Item 440, Reinforcement for Concrete, Table 1Texas Department of Transportation, 2024 Standard Specifications. A state government publication reproducing the ASTM bar table in full, which is why it is the transcription source here rather than any of the calculator sites that reprint the same figures unsourced.
How every figure here is verified: Sources & Method. Who builds this: About. Found something wrong? Tell us and it gets fixed or removed.
Figures on this page last checked against the source documents on 2026-09-08. Codes are amended locally; confirm against the edition your jurisdiction enforces.
This is a quantity takeoff, not a design. Bar size, spacing, cover and lap length are structural decisions set by ACI 318 and by your engineer, and none of them are calculated here. The bar table was read from TxDOT Item 440 on 8 September 2026. Lap length in particular cannot be published as a single figure and this page does not try.