The rebar weight per foot formula is weight (lb/ft) = (bar number ÷ 8)² × 2.67. For a #5 bar: (5/8)² × 2.67 = 0.3906 × 2.67 = 1.043 lb/ft. In metric, the formula is weight (kg/m) = d² ÷ 162.2, where d is the bar diameter in millimetres — so a 16 mm bar weighs 1.578 kg/m. Both formulas give the theoretical unit weight of reinforcing steel.
Rather than doing the arithmetic by hand, you can use the Rebar Weight Calculator to get the weight of any bar size, length, and quantity instantly.
The formulas explained: why they work
Both formulas are just shortcuts for a single physical idea: weight per unit length = cross-sectional area × steel density.
In the imperial system, a rebar’s bar number tells you its nominal diameter in eighths of an inch — a #5 bar is 5/8″ in diameter, a #8 bar is 8/8″ (1″) in diameter. The cross-sectional area is therefore π/4 × (bar number ÷ 8)² square inches. Multiply by the density of steel, 490 lb/ft³, and convert square inches to square feet (÷ 144), and you get: π/4 × (N/8)² ÷ 144 × 490 = 2.67 × (N/8)². The constant 2.67 isn’t arbitrary — it’s steel density compressed into the formula.
The metric version follows the same logic. Steel’s density is 7,850 kg/m³. Area = π/4 × d² (with d in mm), converted to square metres (× 10⁻⁶): 7,850 × π/4 × 10⁻⁶ × d² = d² / 162.2. Same physics, cleaner numbers.
Worked examples
Example 1 — #5 bar (imperial). The bar number is 5, so (5 ÷ 8)² × 2.67 = (0.625)² × 2.67 = 0.3906 × 2.67 = 1.043 lb/ft. A 20-ft stick of #5 weighs 20 × 1.043 = 20.86 lb.
Example 2 — #4 bar (imperial). (4 ÷ 8)² × 2.67 = (0.5)² × 2.67 = 0.25 × 2.67 = 0.6675 ≈ 0.668 lb/ft. This is the value you’ll see quoted for #4 bar on nearly every supplier cut sheet.
Example 3 — #8 bar (imperial). (8 ÷ 8)² × 2.67 = (1)² × 2.67 = 2.670 lb/ft. A 40-ft stick of #8 weighs 40 × 2.67 = 106.8 lb — heavy enough that you’ll feel it.
Example 4 — 16 mm bar (metric). d = 16, so 16² ÷ 162.2 = 256 ÷ 162.2 = 1.578 kg/m. A 12-metre bar weighs 12 × 1.578 = 18.94 kg.
Quick reference chart
The table below shows the theoretical unit weight for the most common bar sizes. Values are rounded to three decimal places.
| Bar size | Nominal diameter | Weight (lb/ft) | Weight (kg/m) |
|---|---|---|---|
| #3 | 3/8″ (10 mm) | 0.376 | 0.561 |
| #4 | 1/2″ (13 mm) | 0.668 | 0.994 |
| #5 | 5/8″ (16 mm) | 1.043 | 1.578 |
| #6 | 3/4″ (19 mm) | 1.502 | 2.235 |
| #8 | 1″ (25 mm) | 2.670 | 3.973 |
Key variables that change the answer
Bar number (diameter). Because the formula squares the diameter term, weight grows quickly — a #8 bar weighs four times a #4 bar (2.670 vs 0.668 lb/ft), not twice. Jumping one bar size up is a bigger weight change than most people expect.
Actual vs nominal diameter. The formula gives theoretical weight based on the nominal diameter. Real rolled bars have small mill tolerances, and deformations (the ribs on rebar) add a little mass. Published unit weights are rounded averages, so expect real bars to vary by a percent or two from the table above.
Units. Keep inches in the imperial formula and millimetres in the metric one. Feeding a diameter in inches into the metric formula (or vice versa) is the single most common arithmetic error.
Common mistakes to avoid
Confusing the bar number with the diameter in inches. A #5 bar is 5/8″ (0.625″) in diameter — not 5 inches. The formula divides the bar number by 8 precisely because the number counts eighths of an inch.
Treating theoretical weight as exact weight. The formula gives the nominal, theoretical weight. Mill tolerance, deformation patterns, and scale buildup mean individual bars vary slightly. For ordering steel by tonnage, the theoretical values are the industry standard and are what suppliers quote.
Rounding too early. Rounding the 2.67 constant or squaring a rounded diameter compounds the error when you scale up to thousands of feet. Keep three decimals until your final total, then round.
Does the formula give weight per foot or per bar?
Per foot (or per metre in metric). Multiply the result by the bar’s length in feet to get the weight of one bar, and by the number of bars for the total order weight.
Why do some charts show slightly different numbers?
Different standards round the nominal diameters and the steel density slightly differently. The values from the formulas above — (N/8)² × 2.67 and d² ÷ 162.2 — are the widely published theoretical unit weights, and they match standard ASTM A615 / ISO 6935 tables.
Can I use the metric formula with centimetres?
Only if you convert to millimetres first. The constant 162.2 assumes d is in mm. A 16 mm bar entered as “1.6 cm” would give 1.6² ÷ 162.2 = 0.0158 kg/m — wrong by a factor of 100.
For quick answers on any bar size, open the Rebar Weight Calculator. To go deeper, read the complete guide and the formula explained walkthrough.
