Steel bar unit weight:
formula and worked example

Why reinforcement is estimated by weight

Reinforcing steel is priced, transported and often paid for by weight, while drawings and bar schedules specify it by diameter and length. The formula below translates between the two. It is also used to check delivery notes and to assess the load on lifting equipment.

The shortcut comes from the geometry of a solid round bar and the density of steel. It is a convenient approximation that engineers use in the field, not a replacement for the mill certificate.

Weight also matters for handling and safety. A bundle of 12 mm bars can reach hundreds of kilograms, which sets what crane, trolley or manual lifting is appropriate. Knowing the weight of each bundle before it arrives lets you plan unloading and storage on a site where space is often tight.

The formula and where D²/162 comes from

• Diameter mm: the nominal bar diameter, for example 12 for a 12 mm bar.
• Length m: the total length of bar of that diameter, in metres, including laps, hooks and bends.

The cross-section of a round bar is π ÷ 4 times the diameter squared. Multiplying by the density of steel, about 7,850 kg/m³, and converting units gives roughly 0.00617 × D² kg per metre, which is the same as D² ÷ 162.2. The shortcut rounds the divisor to 162.

Worked example for 12 mm bar over 18 m

A bar of 12 mm diameter is cut to a total of 18 m. Illustrative values.

1. 1Square the diameter: 12² = 144.
2. 2Divide by 162 to get kilograms per metre: 144 ÷ 162 ≈ 0.889 kg/m.
3. 3Multiply by the length: 0.889 × 18 = 16 kg.

What 16 kg tells you

The unit weight for a 12 mm bar, about 0.89 kg per metre, is a figure many site engineers remember. The 18 m total is not a single bar, since bars are commonly supplied in lengths around 12 m, so this quantity would be a bundle of bars or several cut pieces.

Compare calculated weight with what the supplier delivers. Rolling tolerance means actual bar can be a little lighter or heavier than nominal, and standards allow a stated tolerance on mass. If the difference between the delivery note and the calculation is outside normal tolerance, ask for the mill test certificate.

Think about wastage in the steel account. Cutting from standard lengths leaves offcuts, and even a good bar-bending schedule does not eliminate them. Estimates of purchased steel are therefore higher than the scheduled weight by an allowance that depends on how well the lengths nest. Track offcuts on site and reuse them for short pieces such as spacers or stirrups.

Why diameter dominates the tonnage

Length is a linear driver, but diameter is squared, so a small size change is amplified.

Increasing the diameter by 10% from 12 to 13.2 mm moves the weight from 16 to 19.36 kg, about 21% heavier. Switching a whole schedule from 12 mm to 16 mm bars multiplies unit weight by 1.78, which is why designers think carefully about bar sizes before finalising reinforcement.

Errors in bar weight estimates

• Quoting a bar size from memory and using the wrong nominal diameter, for example squaring 10 when the bar delivered is 12.
• Entering length in centimetres or feet, which makes the weight off by a constant factor.
• Leaving out laps, hooks and bends, so the schedule length is shorter than the bar actually required.
• Applying the shortcut to hollow or flat sections, which have different cross-sections from a solid round bar.

Checking a delivery against the schedule

1. 1Read the nominal diameter stamped or rolled on the bar.
2. 2Compute kilograms per metre with D² ÷ 162.
3. 3Multiply by the scheduled length for each size and add up the sizes.
4. 4Compare with the delivery note and flag any difference beyond tolerance.

Keep the heat or batch number on file for each delivery, since it ties the steel to its test certificate.

Store bar off the ground, sorted by diameter and heat number, and keep it covered. Rust on surface is acceptable in many specifications if it can be removed by wire brush, but heavy scaling or pitting reduces the section and is a reason to reject. Weighing a sample length is a quick way to confirm that the nominal size is correct.