Calculate the exact weight of Octagonal Bars (Octagon Rods) in Mild Steel, Stainless Steel, and other metals instantly with our free online calculator. Simply enter your across-flats dimension and length in mm or inches, and get accurate results in both kg and lbs.

Weight Calculation for Octagonal Bar

The weight of an Octagonal Bar (also known as an Octagon Rod) is determined by its cross-sectional area, length, and material density. Octagonal bars are commonly used in architectural metalwork, decorative fencing, and specialized machining applications where a unique geometric profile is required. The standard density for Mild Steel is 7.85 g/cm³. Accurate weight calculation is essential for material procurement, cost estimation, and structural planning in fabrication projects.

Octagonal Bar Weight Formula and Manual Calculation

The industry-standard formula for calculating octagonal bar weight uses the "Across Flats" (AF) dimension. The geometric constant for a regular octagon is derived from 2 × (√2 - 1) ≈ 0.8284 relative to the square of the width, or more precisely using the apothem.

Step 1: Calculate Cross-Sectional Area (mm²)

Area = 2 × (1 + √2) × (Across Flats / 2)² ≈ 0.8284 × Across Flats²

Note: The constant 0.8284 is an approximation for the area factor relative to the square of the across-flats dimension.

Step 2: Calculate Weight (kg)

Weight (kg) = Area (mm²) × Length (m) × Density Factor

Density Factors for common materials:
0.00000785 for Mild Steel (MS)
0.00000793 for Stainless Steel (SS 304/316)
0.00000850 for Brass
0.00000270 for Aluminium

Simplified Formula for MS Octagonal Bar:
Weight (kg) = Across Flats² (mm) × Length (m) × 0.00000650

For imperial units (inches and lbs):

Weight (lbs) = Across Flats² (in) × Length (ft) × 2.80 (for Steel)

Step-by-Step Calculation Example

  • Step 1: Identify dimensions: Across Flats (AF) = 20 mm, Length = 1 m (Mild Steel)
  • Step 2: Calculate Area Factor: 0.8284 × 20² = 0.8284 × 400 = 331.36 mm²
  • Step 3: Apply density factor: 331.36 × 1 × 0.00000785
  • Step 4: Result: 331.36 × 0.00000785 = 2.60 kg
  • Step 5: Compare with Aluminium: 2.60 × (2.70/7.85) ≈ 0.89 kg

This calculation aligns exactly with the results provided by the octagonal bar weight calculator in kg.

Metal Octagonal Bar Weight Chart (Quick Reference per Metre)

Size AF (mm) Mild Steel (kg/m) Stainless Steel (kg/m) Aluminium (kg/m)
10 mm0.65 kg0.66 kg0.22 kg
12 mm0.94 kg0.95 kg0.32 kg
16 mm1.66 kg1.68 kg0.57 kg
20 mm2.60 kg2.63 kg0.89 kg
25 mm4.06 kg4.11 kg1.39 kg
30 mm5.85 kg5.92 kg2.00 kg
40 mm10.40 kg10.52 kg3.56 kg

Frequently Asked Questions

What is the formula for Octagonal Bar weight?

The formula for Octagonal Bar weight is: Weight = 0.8284 × Across Flats² (mm) × Length (m) × Density Factor. For Mild Steel, you can use the simplified constant: AF² × Length × 0.00000650 to get the weight in kg.

How much does a 20mm MS Octagonal Bar weigh per meter?

A standard 20mm Mild Steel Octagonal Bar weighs approximately 2.60 kg per metre. This is calculated using the formula: 0.8284 × 20² × 0.00000785 = 2.60 kg. This is slightly lighter than a hex bar of the same across-flats dimension due to the smaller cross-sectional area.

What is the difference between Octagonal and Hexagonal bars?

Octagonal bars have eight sides, while Hexagonal bars have six. For the same "Across Flats" dimension, an octagonal bar has a smaller cross-sectional area and therefore weighs less than a hexagonal bar. Octagonal bars are often used for decorative purposes, while hex bars are preferred for mechanical gripping.