Rebar Weight & Area Calculator

What this calculator does

Two related jobs that get asked in the same breath on every reinforced concrete job.

Weight, for the schedule, the order and the crane: the cross-sectional area of one bar, its mass per metre, and the total tonnage for a given number of bars of a given length.

Area, for the design check: bars at a stated centre-to-centre spacing give you an area of steel per metre width in mm²/m, and that is the number a requirement like A_s ≥ 1005 mm²/m is actually written against. Nobody specifies “twenty-four bars”; they specify an area, and you pick a diameter and a spacing that beats it.

Both come out of the same πd²/4, which is why they belong on one page.

The formula

A_bar     = π d² / 4                    mm², d in mm
m_per_m   = A_bar × 10⁻⁶ × ρ            kg/m, ρ in kg/m³
L_total   = n × L_bar                   m
M_total   = L_total × m_per_m           kg

A_s/m     = A_bar × 1000 / s            mm²/m, s = centre spacing in mm
bars      = ⌊W / s⌋ + 1                 across a width W, a bar at each end

Put ρ = 7850 kg/m³ — the conventional density of carbon steel — into the mass expression and it collapses to a single constant:

m_per_m = (π/4) × 10⁻⁶ × 7850 × d² = 0.0061654 × d²   kg/m, d in mm

That 0.006165·d² is the theoretical mass per metre every mill quotes, and it is the number that appears on the certificate and on the invoice. It is also the origin of the d²/162 rule of thumb used on site in South Asia: 1/162.28 = 0.006162, the same constant to three significant figures. A 16 mm bar: 0.0061654 × 256 = 1.578 kg/m. A 12 mm bar: 0.0061654 × 144 = 0.888 kg/m. Learn those two and you can estimate a slab mat in your head.

Note that mills sell against theoretical mass, not weighed mass. Bars are rolled to a permitted mass tolerance — commonly around ±4.5–6% on a single bar depending on size, and tighter on a bulk consignment. A delivery that is a couple of percent light is still compliant; a delivery that is systematically light is a commercial conversation.

The bar size mapping

The tool takes a diameter in millimetres, which makes it work everywhere, and the flag tells you the nearest US designation. The mapping is arithmetic, not a lookup:

A US #N bar is nominally N eighths of an inch in diameter. So N = round(d / 25.4 × 8), and going the other way, d = N × 3.175 mm. #4 is four eighths — half an inch — 12.7 mm. #5 is 15.875 mm, which is what a specifier means when they write 16 mm. #8 is exactly one inch. The rule holds cleanly from #3 to #8; above that the designations continue (#9, #10, #11, #14, #18) but the bars are sized by area rather than by literal eighths, so #9 is 28.65 mm rather than 28.575, and #18 is 57.3 mm.

Metric N-series bars (written N16, H16, T16, Y16 or 16Ø depending on where you are) run 6, 8, 10, 12, 16, 20, 25, 32 and 40 mm, with 50 mm in some markets. The prefix letter carries the grade and ductility class, not the geometry. Nothing is hardcoded here — enter any diameter, standard or not, and the geometry still holds.

Reading the result

Area per metre is spacing-driven, not width-driven. A_bar × 1000 / s has no width term in it. Halve the spacing and you double the steel; that is the only lever besides diameter. The width input exists purely to tell you how many bars to schedule.

The bar count includes both ends. Twenty-four bars at 200 mm centres span 4.6 m between the outermost bars, not 4.8 m — there are 23 gaps, not 24. Off-by-one here is one of the most common quantity errors on a schedule. In practice the end bars sit inside the cover, so the run available is the member width less two covers; enter that width, not the overall dimension.

Total mass excludes everything that is not straight bar — no hooks, bends, cranks, laps, chairs, spacers or tie wire. Those add 3–8% to the delivered tonnage on a typical slab and considerably more on a heavily detailed column or pile cap. Add your own allowance, or enter the scheduled cut length rather than the clear span.

Typical values

The metric column is πd²/4 computed exactly; the US column is the nominal area published in ASTM A615, which is rounded to a convenient fraction of a square inch and so sits a percent or two off the pure circle.

Spacing in slabs and walls typically falls between 100 and 300 mm. The lower bound is placeability — minimum clear spacing is usually the greatest of the bar diameter, 25 mm, and about 1.33× the maximum aggregate size, so the concrete can get around the steel. The upper bound is crack control, commonly capped at 200–300 mm or a multiple of the member thickness. Both limits belong to the standard applicable to you — ISO 6935-2, EN 1992-1-1 with EN 10080, AS 3600 with AS/NZS 4671, ACI 318 with ASTM A615/A706, or IS 456 with IS 1786.

Worked example

A slab bottom mat: 16 mm bars at 200 mm centres, running 6.0 m in one direction, across a 4.6 m width. Steel at 7850 kg/m³.

A_bar     = π × 16² / 4 = π × 64        = 201.06 mm²
m_per_m   = 201.06 × 10⁻⁶ × 7850        = 1.5783 kg/m
A_s/m     = 201.06 × 1000 / 200         = 1005.3 mm²/m
bars      = ⌊4600 / 200⌋ + 1 = 23 + 1   = 24 bars
L_total   = 24 × 6.0                    = 144.00 m
M_total   = 144.00 × 1.5783             = 227.28 kg = 0.2273 t
A_s total = 24 × 201.06                 = 4825.5 mm²

So one direction of the mat is 24 bars, 144 m of 16 mm bar, 227 kg. If the design called for 1000 mm²/m you have it, with 0.5% to spare — but if it called for 1100 mm²/m you do not, and the fix is 16 mm at 175 centres (1149 mm²/m) rather than jumping to 20 mm bar.

Cross-check the mass against the mill constant: 0.0061654 × 16² = 1.578 kg/m. Same number.

FAQ

Why does my supplier’s kg/m differ slightly from this? Because deformed bar is not a smooth cylinder. The quoted mass per metre is based on the nominal diameter — the diameter of a plain round bar of the same mass per unit length — and the ribs are already accounted for in that equivalence. Small differences also come from the mass tolerance the bar is rolled to, and from a density other than 7850 kg/m³.

Should I use nominal or effective diameter? Nominal, always. Every published area and mass table, and every design equation, is written against the nominal diameter. The rib profile affects bond, not area.

Does the area per metre change if my slab is 3 m or 30 m wide? No. That is the point of expressing it per metre. Only the diameter and the spacing move it. The width only changes how many bars you order.

How do I allow for laps? Add the lap length to the cut length before you enter it, or add a percentage to the total. Lap lengths depend on concrete grade, bar diameter, cover and whether the bars are in tension or compression — typically 30–50 bar diameters — and are set by your design code, not by this calculator.

Can I use this for mesh or fabric reinforcement? For the area arithmetic, yes — enter the wire diameter and the pitch and you get the mm²/m of the sheet in that direction. For weight, no: sheet mass includes both directions plus the overhang, and the manufacturer’s kg/m² figure is easier to use.

This tool provides indicative quantities and areas for scheduling and checking. Bar sizes, spacing limits, cover, lap lengths and minimum reinforcement are governed by the design standard applicable in your jurisdiction; confirm against the issued-for-construction bar schedule before ordering.