The metric rebar weight formula is W (kg/m) = D²/162, where D is the nominal bar diameter in millimetres and 162 is the unit-mass constant for carbon steel at 7,850 kg/m³ density [S1][S2][S3]. The same constant appears in the equivalent form 0.006165 × D², which gives 0.888 kg/m for a 12 mm bar and 1.580 kg/m for a 16 mm bar at standard ambient reference conditions [S2].
The 162 constant is not arbitrary: it is the consolidated term (4 × 10⁶ / (π × 7,850)) that converts a diameter in mm over a 1 m run into kg, and the more precise value is 162.2 or 162.28 depending on which rounding of density is used [S1][S3]. Yield-grade variation (Grade 40, Grade 60 / Fe500, Grade 80) does not change the calculation, because mass density of carbon rebar is held within a tight band regardless of tensile class [S1].
How the D²/162 Constant Falls Out of 7,850 kg/m³
For a 1 m cylindrical bar of diameter D mm, cross-sectional area equals (π/4) × D² in mm², which converts to (π/4) × D² × 10⁻⁶ m²; multiplying by 1 m length gives volume in m³, then by 7,850 kg/m³ gives mass [S1][S3]. With unit conversion D² in mm² to m² carrying the 10⁻⁶ factor, the denominator collapses to roughly 162.28, and rounding to 162 produces results within about 0.17% of the more precise figure for diameters between 6 mm and 40 mm [S1][S2].
Stainless and alloy steels deviate from the 7,850 kg/m³ baseline because chromium, nickel, and molybdenum additions shift the specific weight, and those grades should not be run through the 162 divisor without first verifying the actual density [S3]. For ordinary ASTM A615 / BS 4449 / IS 1786 carbon rebar in the 6–40 mm range, the nominal-mass approach is the field default and is the basis for the rebar reference tables used by estimators [S1].
Standard kg/m Values Across the 6–40 mm Range
Wellco Wholesale's ISO-aligned chart lists 0.222 kg/m at 6 mm, 0.395 kg/m at 8 mm, 0.617 kg/m at 10 mm, 0.888 kg/m at 12 mm, 1.580 kg/m at 16 mm, 2.470 kg/m at 20 mm, 3.850 kg/m at 25 mm, 6.310 kg/m at 32 mm, and 9.860 kg/m at 40 mm [S2]. Cross-checking these against the D²/162 formula: 12²/162 = 144/162 = 0.888, 16²/162 = 256/162 = 1.580, 20²/162 = 400/162 = 2.469, 25²/162 = 625/162 = 3.858, all matching the chart to within 0.3% [S1][S2].
These values assume a plain-round equivalent cross-section; the raised rib deformations rolled into modern deformed rebar are not added on top of the nominal mass, because the standard already accounts for surface geometry inside the rolling-tolerance band [S1]. For cut-and-bend jobs, the same kg/m figure is the basis for procurement, and a separate density check is rarely needed unless the mill cert reports a divergent heat analysis [S3]. When planning cuts, the per-bar mass scales linearly with length: a 12 m stock bar of 16 mm rebar weighs 12 × 1.580 = 18.96 kg, which is the figure estimators carry into the rebar cutter cycle-time and scrap-recovery calculations [S1].
Total Bar Weight: Adding Length and Quantity
For project-level takeoffs, the per-metre figure is extended into Total Weight (kg) = (D²/162) × L × Q, where L is the length of each bar in metres and Q is the number of bars [S1]. On a slab pour requiring 240 pieces of 12 mm rebar at 6 m each, the math runs 0.888 × 6 × 240 = 1,278.72 kg, or roughly 1.28 t of Grade 60 stock before any waste or lap allowance [S1][S2].
Lap splices, hooks, and bends are typically captured as a separate multiplier rather than baked into the linear formula, and a 5–10% waste allowance is normal for rebar-heavy pours because offcuts and bent-bar returns accumulate fast [S1]. For projects priced per hundredweight or per piece, the per-piece mass is the bridge between kg/m and dollar figures, and the Rebar pricing comparison walks through that conversion in detail. Bending-allocation losses are easier to track when the crew is using a calibrated rebar bender and the cut list is referenced against actual bar stock rather than theoretical lengths.
What D²/162 Does Not Capture
The formula assumes a uniform cylindrical bar of standard carbon-steel density, so it does not adjust for epoxy-coated or galvanised rebar, where the coating adds 1–3% mass on top of the steel core depending on coating class [S3]. It also does not adjust for stainless rebar (≈8,000 kg/m³ density band) or for the small mill-tolerance window allowed under most national standards, typically ±5% on individual bar mass and tighter on heat averages [S1][S3].
Coating mass is calculated separately by multiplying the surface area of the bar (π × D × L, with D and L in consistent units) by the coating's areal density in kg/m²; for fusion-bonded epoxy at 175–300 µm thickness this commonly lands in the 0.2–0.4% range for 12–25 mm bars, which is why estimators usually ignore it for residential work and track it explicitly for bridge and coastal jobs [S3]. For splice-heavy designs, the linear formula still applies to the parent bars, but the rebar coupler assembly mass must be added to the takeoff rather than folded into D²/162. Crews laying out bar on a rebar tool bench will notice that the linear formula matches scale weight within the mill-tolerance window, and any larger deviation is usually a counting or length error, not a formula error.
Side-by-Side: D²/162 vs 0.006165 D² vs Chart Lookup
For a 20 mm bar, D²/162 returns 2.469 kg/m, 0.006165 × D² returns 2.466 kg/m, and the published chart value is 2.470 kg/m, a spread under 0.2% [S1][S2]. For a 6 mm bar the figures are 0.222 kg/m (formula), 0.222 kg/m (0.006165 D²), 0.222 kg/m (chart) [S2]. For a 40 mm bar the figures are 9.877 kg/m (formula), 9.864 kg/m (0.006165 D²), 9.860 kg/m (chart) [S1][S2]. The drift widens slightly at the top of the range because the rounded 162 understates the true divisor of 162.28, but the difference remains under 0.2% across the 6–40 mm envelope that covers virtually all commercial rebar.
For procurement or shipping manifests, the chart value is the safest number to quote, because it already reflects the nominal-mass convention used by mills and ISO-aligned distributors [S2]. For field math when a chart is not at hand, the 0.006165 D² form is easier to key into a phone calculator with a single multiplication and no division, and for engineers who want to show the derivation in a method statement, the full (π/4) × D² × 10⁻⁶ × 7,850 form makes the density assumption visible [S1][S3]. Rebar classified as long steel in commodity indices tracks the same per-tonne pricing references that the per-metre formula feeds into at the order stage.
Trackable Signals for the Next 6 Months
Watch the next revision of the long-steel PPI series, which feeds the same per-tonne reference prices that D²/162 multiplies through to give per-bar and per-project mass; the 2026 PPI release cadence is the next data node that ties formula output to dollar values. Watch for any mill-side bulletin on density variations for higher-chromium Grade 80 or stainless-clad rebar, since those would push the constant 162 off-baseline and force a per-grade constant rather than a single 7,850 kg/m³ assumption. Finally, watch for distributor chart refreshes in the 32–40 mm band, where the rounding gap between 162 and 162.28 is largest and most likely to show up as a 0.2% line-item variance on heavy civil takeoffs. [S2]