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 and spacing
Pick the bar diameter and the centre-to-centre spacing called up on the drawing. Closer spacing and bigger bars mean more steel.
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). 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 weighs 12² / 162 ≈ 0.888 kg per metre, a 16 mm bar 1.58, and so on. US bars are designated by a number that is the diameter in eighths of an inch (a #4 bar is 4/8 = 1/2 inch) and have standard weights 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 pure geometry times a constant: count the bars each way, total their length, and multiply by kilograms-per-metre — which for metric bars is just 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 (often adding 10–15% or more to the length), 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.
10 Facts About Rebar
Rebar carries the tension that concrete can't.
Metric weight: kg/m = d² / 162 (d in mm).
A 12 mm bar weighs about 0.888 kg/m.
US bar numbers = diameter in eighths of an inch.
A #4 bar is 1/2 inch, ~0.668 lb/ft.
Bars each way = span ÷ spacing + 1.
Closer spacing + bigger bars = more steel.
Laps can add 10–15%+ to the bar length.
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 (span divided by spacing, plus one), 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. For metric bars, kilograms per metre equals the diameter in millimetres squared divided by 162; US bars have standard pounds-per-foot values. The calculator does all of this for the slab size, bar, and spacing 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 ≈ 1.58 kg/m. The constant 162 already bundles the area calculation and the density of steel (about 7850 kg/m³), so you only need the diameter.
In the US system the bar number is the nominal diameter in eighths of an inch. So a #3 bar is 3/8 inch, a #4 is 4/8 = 1/2 inch, a #8 is 8/8 = 1 inch, and so on. Each size has a standard weight per foot — #4 is about 0.668 lb/ft, #5 about 1.043, #8 about 2.670. The calculator's US mode lists these directly, so you just pick the bar designation from the drawing.
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 commonly add 10–15% or more to the total bar length depending on the bar size and code requirements. 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. 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.
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