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STEELTOETOOLS

Rebar Calculator

Enter the slab or wall dimensions, the spacing each way, the bar size and the lap length from your drawings, and this calculator returns the bar count, the linear footage, the stock lengths to buy and the weight in pounds and tons. Spacing and bar size are yours to enter — this tool never suggests either.

ft
ft
in
From your drawings. This is the spacing measured across the width.
in
Enter the same value both ways for a square grid, or 0 to count one direction only.
in
From your drawings. It sets where the first and last bar land.
in
From your drawings or your engineer — lap length depends on bar size, concrete strength and the code in force. Leave at 0 to count no splices.
ft
What your supplier delivers. 20 ft is the common mill length in the US.
%
Cut-offs, bends and damaged bar.

Results are estimates for planning purposes. Verify with your supplier, engineer, or a qualified professional before ordering material or performing work.

Spacing is an input, never an output. Bar size, spacing, cover, lap length and where the steel sits in the section are design decisions. They come from your drawings, your engineer or the code your jurisdiction adopted. This page counts and weighs the mat you were handed.

Rebar sizes, areas and weights

BarNominal diameterAreaWeightMetric mark
#30.375 in0.11 in²0.376 lb/ft#10
#40.500 in0.20 in²0.668 lb/ft#13
#50.625 in0.31 in²1.043 lb/ft#16
#60.750 in0.44 in²1.502 lb/ft#19
#70.875 in0.60 in²2.044 lb/ft#22
#81.000 in0.79 in²2.670 lb/ft#25
#91.128 in1.00 in²3.400 lb/ft#29
#101.270 in1.27 in²4.303 lb/ft#32
#111.410 in1.56 in²5.313 lb/ft#36

Diameters, areas and weights: Caltrans Bridge Design Details Table 13.1.1, ASTM Standard Reinforcing Bar Data (September 2025) — the same nominal values specified in ASTM A615 and A706 and tabulated in the CRSI Manual of Standard Practice. The metric mark in the last column is the nominal diameter rounded to the nearest millimetre: a #4 bar is 0.500 in, which is 12.7 mm, so it is marked #13. It is the same bar, not a different one. #14 and #18 bars also exist, at 7.650 and 13.600 lb/ft.

The one number worth memorizing

One square inch of bar weighs 3.4 lb per foot. Steel runs 490 lb/ft³; divide by 144 square inches per square foot and you get 3.403 lb per foot per square inch of section. Every weight in the table above is the nominal area times that number:

  • #4: 0.20 in² × 3.4 = 0.68 lb/ft (published 0.668, because the exact area is 0.196 in²)
  • #9: 1.00 in² × 3.4 = 3.40 lb/ft — exactly the published value, because a #9 is defined by its 1.00 in² area
  • #11: 1.56 in² × 3.4 = 5.31 lb/ft

Reading the marks on the bar

Every bar is rolled with its own identification, and CRSI lists the sequence: the mill mark first, then the bar size, then a letter for the type of steel — S for carbon steel to ASTM A615, W for low-alloy steel to ASTM A706, the one specified where welding or seismic ductility matters. The grade comes last, either as a number (60, 75, 80, 100, 120) or as rolled lines: one line for Grade 60, two for 75, three for 80 and 100, four for 120. If you are handed a bundle with no paperwork, that is where the answer is.

What this count does and does not include

  • Included: straight bars each way on the grid you entered, cut back by your cover at all four edges, one lap per stock length overrun at the lap you entered, a waste percentage, the number of intersections, and the steel area per foot the grid provides.
  • Not included: bends, hooks, corner bars, dowels, hairpins, edge and perimeter bars, extra steel at openings, chairs and supports, tie wire, and anything the detailer adds. On a real job the bar list carries all of that.
  • Two-way versus one-way. Set the crosswise spacing to 0 to count a single direction — useful for a wall with vertical bars only, or for pricing one mat at a time.
  • Walls. Enter the wall length and height. Two curtains of steel means running the calculation twice, or doubling the result.
  • The concrete around it. The slab calculator, the footing calculator and the stairs calculator give the volume the steel goes into.

How it's calculated

Bars in each direction
bars = floor( (dimension across − 2 × cover) / spacing ) + 1
Bars that run lengthwise are counted across the width, and vice versa. The + 1 is the first bar: five spaces means six bars.
Length of one bar
bar length = dimension along − 2 × cover
Splices
pieces per bar = ceiling( bar length / stock length ) ; splices = pieces − 1 ; added length = splices × lap
Total linear feet
total = Σ bars × (bar length + splice allowance), both directions, × (1 + waste % / 100)
Weight
weight (lb) = total linear ft × unit weight (lb/ft)
Unit weights are the ASTM standard reinforcing bar data: 0.376 lb/ft for #3, 0.668 for #4, 1.043 for #5, 1.502 for #6, 2.044 for #7, 2.670 for #8, 3.400 for #9, 4.303 for #10, 5.313 for #11. They follow from the nominal area and the density of steel: 1 in² of bar weighs 3.4 lb per foot (490 lb/ft³ ÷ 144).
Stock bars to buy
stock bars = ceiling( total linear ft / stock length )
A count of full lengths. Real orders come in bundles and often in a bar list with bends and marks — this is the tonnage check, not a shop drawing.
Steel area provided (information only)
As (in²/ft) = bar area (in²) / spacing (in) × 12
Areas: 0.11 in² for #3, 0.20 for #4, 0.31 for #5, 0.44 for #6, 0.60 for #7, 0.79 for #8, 1.00 for #9, 1.27 for #10, 1.56 for #11. Shown so you can check the mat against a drawing that specifies steel area. It is not a design check.

Frequently asked questions

A #3 bar weighs 0.376 lb/ft, #4 is 0.668, #5 is 1.043, #6 is 1.502, #7 is 2.044, #8 is 2.670, #9 is 3.400, #10 is 4.303 and #11 is 5.313. The pattern is simple: one square inch of bar cross-section weighs 3.4 lb per foot, so multiply the bar area by 3.4.

For #3 through #8 it is the nominal diameter in eighths of an inch: a #4 bar is 4/8 = 0.500 in, a #8 is 1.000 in. Above #8 the numbering keeps going but the diameters are set by area instead — a #9 is 1.128 in because its area is exactly 1.00 in².

With 3 in cover and a 12 in grid each way, 20 bars run each direction, 19.5 ft long, for 780 linear feet — 521 lb of #4 or 814 lb of #5 before waste. Change the spacing and the count changes with it; the spacing itself has to come from your drawings.

That is a design decision and this calculator will not make it. Bar size, spacing, cover, lap length and where the mat sits in the slab come from your drawings, your engineer or the code your jurisdiction adopted. This tool prices the mat you were given.

Twenty feet is the common stock length from US mills and yards, with 40 and 60 ft available on order and 10 ft cut lengths common in home centers. Enter what your supplier actually delivers — it drives both the splice count and the number of full lengths you buy.

Enter the lap length your drawings call for. The calculator counts one splice each time a bar is longer than a stock length and adds that lap to the total footage. Lap length is not a number to guess: it depends on bar size, concrete strength, bar coating and the code in force.

The calculator gives the number of bar intersections, which is the upper bound on ties — many crews tie every other intersection on a flat mat. Chairs and supports are spaced by the detailer, not by us.

Sources & references

Content checked against these sources — last reviewed August 29, 2026.

By — Editorial team of SteelToeTools.com (published by LSEA SAS) Updated v1