Rebar Weight Calculator
Rebar weight calculator. Estimates the total weight and cost of reinforcing steel for a slab or footing from bar size, spacing and dimensions. Educational only.
Rebar Weight Calculator
How to use the rebar weight calculator
Choose your units
Switch between metric (metres, millimetre bars, kilograms) and US (feet, numbered bars, pounds). The bar-size list and labels update to match.
Enter the slab size
Type the length and width of the slab or footing area to be reinforced. The calculator assumes a single orthogonal mesh covering the area.
Set bar size, spacing and count
Pick the bar diameter and the centre-to-centre spacing called up on the drawing. Then choose how bars are counted: whole spaces only, or with an extra bar at the far edge where the span is not a whole number of spaces. The result says which count it used and what the other one gives.
Read weight, length and cost
You get the total weight, total bar length, the number of bars, and — if you enter a price — an estimated cost. Add an allowance for laps, starter bars, and chairs.
Rebar take-offs — counting the steel in a slab
From a grid to a weight
Concrete is strong in compression but weak in tension, so reinforcing bars — rebar — are cast into it to carry the tensile forces that would otherwise crack and fail it. In slabs and footings the steel is usually arranged as a grid, or mesh, of bars running in two perpendicular directions at a set spacing. Estimating how much steel a slab needs starts from that geometry. The number of bars running in one direction is the span they cross divided by the spacing, plus one (because a grid of, say, five spaces has six bars). Most spans are not a whole number of spaces, and then there are two ways to count: stop at the last whole space, leaving a strip narrower than the spacing at the far edge with no bar, or add one bar at that edge. A 5.1 m span at 200 mm gives 26 bars the first way and 27 the second. This calculator lets you pick either and says which it used; it does not decide which one your job needs. Each of those bars runs the full length of the perpendicular dimension. Do this for both directions and add the totals, and you have the total length of bar. Multiply by the bar's weight per unit length and you have the total weight — the number a supplier prices and a truck carries.
The weight per unit length depends only on the bar's diameter. In metric practice a tidy formula captures it: kilograms per metre equals the diameter in millimetres squared, divided by 162. So a 12 mm bar comes out at 12² / 162 ≈ 0.889 kg per metre and a 16 mm bar at 1.580. Hong Kong's Construction Standard CS2:2012 lists 0.888 and 1.579 kg/m in its Table 2, computed from 0.00785 kg per mm² of section per metre; worked through exactly, that makes the divisor 4 / (π × 0.00785) = 162.2, so the rounded 162 reads about 0.12% high at every size. The calculator uses the table's values, not the shortcut. US bars carry a number that matches the diameter in eighths of an inch up to #8 (a #4 bar is 4/8 = 1/2 inch, a #8 is 1 inch); above that the rule stops holding, and a #14 bar is 1.693 inches across, not 14/8 = 1.75 (Caltrans Bridge Design Details, Table 13.1.1). Each size has a standard weight per foot. The calculator stores these unit weights and applies the right one for the bar you pick, so a slab's reinforcement becomes a single orderable quantity in kilograms or pounds, with an optional cost if you supply a price.
"Rebar weight is geometry times a constant: count the bars each way, total their length, and multiply by kilograms per metre — which for metric bars is close to diameter squared over 162."
A take-off estimate, not a design
This calculator does a quantity take-off — it tells you roughly how much steel a given grid contains — but it does not design the reinforcement, and that distinction is important. The bar size, spacing, number of layers, cover, and detailing of a real slab or footing come from a structural engineer's design to the governing code, based on the loads, span, support conditions, and exposure. A take-off also has to account for things this simple grid ignores: laps where bars overlap and are tied together (their length is set by the bar size and the design code), extra bars at edges and openings, top and bottom mats in two-way or suspended slabs, starter and dowel bars, and the chairs and spacers that hold the steel at the right height. Wastage and standard stock lengths add a little more. So treat the figure here as a planning and budgeting estimate for a single mesh: useful for ordering and costing a simple slab, but always rounded up and supplemented by an allowance for laps and accessories. For the actual reinforcement design and a precise schedule, rely on the engineer's drawings and bar bending schedule, and follow your local code.
d squared over 162, and the laps nobody counts
Rebar carries the tension that concrete can't.
Metric weight: kg/m ≈ d² / 162 (d in mm); the exact divisor is 162.2.
A 12 mm bar weighs 0.888 kg/m and a 16 mm bar 1.579 (Hong Kong CS2:2012, Table 2).
US bar numbers = diameter in eighths of an inch up to #8 only; a #14 is 1.693 in (Caltrans).
A #4 bar is 1/2 inch and 0.668 lb/ft (Illinois DOT table).
Bars each way = whole spaces + 1, or one more with a far-edge bar.
Closer spacing + bigger bars = more steel.
Laps add length set by the bar size and the design code — take it from the drawings.
Don't forget chairs, starters and edge bars.
Bar size and spacing come from an engineer's design.
Frequently asked questions
Count the bars in each direction (the whole number of spaces in the span, plus one, and one more if you choose a far-edge bar), multiply by the length each bar runs, and add the two directions to get the total bar length. Then multiply by the bar's weight per unit length. Metric weights come from Hong Kong's CS2:2012 Table 2, which the d²/162 shortcut approximates; US weights come from the Illinois DOT reinforcement bar table. The calculator does all of this for the slab size, bar, spacing and counting choice you enter.
It's the standard shortcut for the mass per metre of a round steel bar: weight in kg/m equals the diameter in millimetres squared, divided by 162. It comes from the bar's cross-sectional area times steel's density. For example a 16 mm bar is 16² / 162 = 256 / 162 ≈ 1.580 kg/m, against 1.579 in CS2:2012 Table 2. The constant bundles the area calculation and the density of steel (7,850 kg/m³, which CS2 states as 0.00785 kg per mm² per metre). Worked exactly, the divisor is 4 / (π × 0.00785) = 162.2; rounding it to 162 makes the shortcut read about 0.12% high at every size.
Up to #8, the bar number is the nominal diameter in eighths of an inch: a #3 bar is 3/8 inch, a #4 is 4/8 = 1/2 inch, a #8 is 8/8 = 1 inch. Above #8 that stops holding. Caltrans's Bridge Design Details (Table 13.1.1, ASTM bar data) give a #9 as 1.128 inches, not 1.125, and a #14 as 1.693 inches, not 1.75. Each size has a standard weight per foot — #4 is 0.668 lb/ft, #5 1.043 and #8 2.670, as in the Illinois DOT reinforcement table this calculator uses. The calculator's US mode lists these directly, so you just pick the bar designation from the drawing.
The one you pick, and the result says which. "Whole spaces only" counts floor(span ÷ spacing) + 1 bars each way, starting at one edge, so when the span is not a whole number of spaces the last bar stops short of the far edge by less than one spacing. "Add a far-edge bar" puts one more bar on that edge, so the last gap is shorter than the spacing. On a 5.1 × 4.1 m slab at 200 mm that is 26 + 21 = 47 bars the first way and 27 + 22 = 49 the second. "Whole spaces only" is selected when the page opens. Neither is a rule this page sets: use the count your drawings or bar bending schedule use.
Not exactly. The figures here are nominal masses. Hong Kong's CS2:2012 (Table 3) allows a test specimen to deviate from nominal mass per metre by up to ±4.5% for bars over 8 mm and ±6.0% for bars of 8 mm and under. That is Hong Kong's standard. Singapore's bars are made to SS 560 and Malaysia's to MS 146, which set their own requirements; this page has not seen those tables and does not apply CS2's figures to them.
No. The calculator assumes each bar runs the full slab dimension as a single length. In reality, bars come in stock lengths and must overlap (lap) where they join, with the overlap tied together to transfer force. Laps add length that depends on the bar size and the design code; take it from the drawings. For a real take-off, add a lap allowance on top of the figure here, plus extra for edge bars, starters, and standard-length wastage.
Those come from the structural design, not from a quantity calculator. An engineer sizes the bars and sets the spacing based on the loads, span, slab thickness, concrete strength, and code, and shows them on the drawings and bar bending schedule. This tool takes the bar size and spacing you've been given and works out the quantity. If you don't have a design, you need one before pouring anything structural — guessing reinforcement is unsafe.
No — it calculates a single orthogonal mesh (one layer). Many slabs, especially two-way or suspended slabs, have reinforcement in both the top and bottom faces, and some have additional bars over supports or around openings. If your design calls for two mats, run the calculator for each and add them, plus any extra bars. The figure here is for one layer covering the area you enter.
It's only as good as the price you enter, multiplied by the estimated weight. Enter the price in any currency; the cost comes out in the same one. Steel prices fluctuate and suppliers may price by the tonne, by length, or with cutting and bending charges, and there may be minimums. The calculator gives a ballpark material cost from your unit price; it doesn't include delivery, cutting and bending, tying wire, chairs, or labour. Use it for budgeting and confirm with a supplier quotation.
It works for any rectangular area reinforced with a single orthogonal mesh — a slab, a pad footing, a strip footing, or a wall panel — by entering that area's two dimensions, the bar size, and the spacing. Just remember it covers one mesh layer: walls and many footings have steel near both faces, and footings often have additional bars, so run it per layer and add the results. For elements with hooks, bends, or links (such as columns and beams), a bar bending schedule is the right tool rather than a simple mesh take-off.
Use it to estimate and order quantities, not to design reinforcement. The size, spacing, layers, cover, and detailing of structural rebar must be designed by a qualified engineer to the governing code, and built to the approved drawings and bar bending schedule. This calculator is a planning and budgeting aid for a simple mesh; it is not a substitute for engineered reinforcement design and inspection.
No. The dimensions and prices you enter are processed entirely in your browser. Nothing is sent to a server, stored, or shared, and no account is required. The calculation runs on your device only.
Method & sources
How it computes
Bars each way = floor(span / spacing) + 1 (whole spaces only, the default) or one more where the span leaves a strip at the far edge (far-edge bar); each result states which. Length = bars along L x W + bars along W x L; weight = length x unit weight. Metric kg/m from Hong Kong CS2:2012 Table 2 (0.00785 kg/mm2 per metre; exactly d^2/162.2). US lb/ft from the Illinois DOT reinforcement table. RecaBuild.rebar, tested.
What this tool implements
- US bar sizes and weights (#3 0.376 to #9 3.400 lb/ft) per Illinois DOT's reinforcement bar table. The page says bar numbers match the diameter in eighths of an inch only up to #8; the #9 (1.128 in) and #14 (1.693 in) diameters it quotes are from Caltrans Bridge Design Details Table 13.1.1.
- Bar counting is a stated choice, not a rule: "whole spaces only" counts floor(span / spacing) + 1; "add a far-edge bar" adds one bar where the span is not a whole number of spaces. Each result names the convention it used and gives the other one's figure. No primary source we opened says which a take-off must use.
- Mass tolerance is quoted only as Hong Kong CS2:2012 Table 3 (a test specimen may deviate +/-4.5% for bars over 8 mm, +/-6.0% for 8 mm and under). It is not applied to Singapore's SS 560 or Malaysia's MS 146, whose tables were not seen.
- Corrected 2026-10-14: the record called d^2/162 equivalent to CS2's 0.00785 kg/mm2 (it is d^2/162.2); the page printed 12^2/162 as 0.888 (it is 0.889); the "eighths of an inch" rule was stated for all sizes; the count used floor only without saying so in the result. Earlier fixes kept: the floating-point bar loss (0.7 / 0.1) and the 16 and 25 mm masses.
Sources
- Civil Engineering and Development Department, Hong Kong. Construction Standard CS2:2012, Steel Reinforcing Bars for the Reinforcement of Concrete, Table 2 (nominal mass per metre) and Table 3 (permissible deviation from nominal mass). https://www.cedd.gov.hk/filemanager/eng/content_77/cs2_2012.pdf
- Illinois Department of Transportation. Reinforcement bars - English (metric): size, diameter, area and weight per foot (approved 1 January 2009). https://public.powerdms.com/IDOT/documents/2744073
- California Department of Transportation. Bridge Design Details, Chapter 13 Reinforcement (September 2025), Table 13.1.1 ASTM Standard Reinforcing Bar Data. https://dot.ca.gov/-/media/dot-media/programs/engineering/documen…
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