Short answer: the key formulas
In these formulas D is the tube outside diameter (OD) and t is the wall thickness, both in mm. All of them are estimates. Strip width is given only as a theoretical conversion from the outside perimeter, for a rough idea; the actual strip width is set by the roller design, is usually slightly less, and the first coil of a new size should be trial-run to confirm it.
- Strip width, round tube (theoretical conversion): B ≈ 3.14 × D (mm, the outside circumference)
- Strip width, square or rectangular tube (theoretical conversion): B ≈ sum of the four sides (mm, the outside perimeter)
- Weight per meter, round tube: (D − t) × t × 0.02466 (kg/m); square or rectangular tube: sum of four sides × t × 0.00785 (kg/m)
Quick strip width estimate: convert from the outside perimeter
To give a quick rough idea, this guide estimates strip width from the outside perimeter of the finished tube: B ≈ OD × 3.14 for round tube, and B ≈ the sum of the four sides for square and rectangular tube. Only the OD or the side lengths are used, not the wall thickness.
This is a theoretical value for a rough idea only. The actual strip width is set by the roller design, which also accounts for the wall thickness, the weld squeeze-out and the sizing allowance, so it is usually slightly less than the theoretical value. The first coil of a new size should be trial-run to confirm it. Use the theoretical value for a first idea of the strip width you need and the width class of the entry section; the exact width comes from the roller design.
Worked examples: theoretical strip width
These examples only show the conversion. They are theoretical values, not the cutting width for any project, and do not describe the capacity of any mill model.
- Round tube Φ76: 76 × 3.14 ≈ 239 mm
- Large-diameter round tube Φ219: 219 × 3.14 ≈ 688 mm
- Square tube 40×40: 40 + 40 + 40 + 40 = 160 mm
Square and rectangular tube: round-to-square forming and the mother tube
Square and rectangular tubes are usually made by round-to-square forming: the tube is welded round (the “mother tube”) and then shaped stand by stand in the sizing section. Their strip width can be roughly estimated as the sum of the four sides, as above.
A square or rectangular tube can share the forming rollers of the round tube with the same perimeter (its mother tube) and often needs only one more set of sizing rollers (a “half set”).
How to calculate steel pipe weight per meter
Round tube weight (kg/m) = (D − t) × t × 0.02466. The factor 0.02466 equals π × 7.85 ÷ 1000: it converts the ring cross-section, taken at the mean diameter (D − t), using a steel density of 7.85 g/cm³. For round tube this is the theoretical weight.
Square or rectangular tube weight (kg/m) = sum of four sides × t × 0.00785. This is a quick estimate based on the outside perimeter. It does not deduct the rounded corners and does not use the wall centerline, so it reads high: checked against a section with an outside corner radius of 1.5t, it is about 4% to 17% above the theoretical weight for common sizes, and the gap grows as the wall gets thicker relative to the side length. For an exact theoretical weight, use the section formula in the product standard you follow.
- Φ76×3: (76 − 3) × 3 × 0.02466 ≈ 5.40 kg/m
- Φ89×4: (89 − 4) × 4 × 0.02466 ≈ 8.38 kg/m
- Φ60.3×3 (2-inch pipe OD): (60.3 − 3) × 3 × 0.02466 ≈ 4.24 kg/m
- Square 50×50×2: 200 × 2 × 0.00785 ≈ 3.14 kg/m (quick estimate; about 2.96 kg/m for a section with a 1.5t outside corner radius, so the quick figure is about 6% high)
- Rectangular 80×40×2.5: 240 × 2.5 × 0.00785 ≈ 4.71 kg/m (quick estimate; about 4.43 kg/m with rounded corners)
Estimating theoretical output from weight per meter
Theoretical output (t/h) = line speed (m/min) × 60 × weight per meter (kg/m) ÷ 1000. Real output is this figure times your own effective running time: size changes, roller changes, adjustments, inspection and unplanned stops all take time, so the theoretical value is not a production guarantee.
Example (assumed figures; not the rated or promised speed of any model): Φ76×3 round tube weighs about 5.40 kg/m. At an assumed continuous 40 m/min, theoretical output = 40 × 60 × 5.40 ÷ 1000 ≈ 13.0 t/h. Actual speed depends on pipe size and wall thickness.
Coil OD and uncoiler rated capacity
An uncoiler's rated capacity is what its mandrel can carry. Coil OD is also limited by the uncoiler and the workshop, and whichever limit is reached first applies, so please state the actual coil weight, ID and OD in your inquiry. The standard coil ID is 508 mm.
To reach a high weight, a narrow coil needs a very large OD, which is why narrow strip often cannot use the full rated capacity of an uncoiler.
Tips for using these formulas
- The theoretical strip width is only for a rough idea and for estimating the width class of the entry section; the actual width comes from the roller design, is usually slightly less, and is confirmed by a trial run on a new size.
- The theoretical conversion uses only the OD or side lengths, not the wall thickness; tubes with the same OD but different walls need different actual strip widths.
- The quick weight formula for square and rectangular tube reads high, by about 4% to 17% for common sizes; use the product standard for exact values.
- Check the coil weight against the uncoiler's rated capacity, the coil loading method and your crane capacity.
- The density used is 7.85 g/cm³ for carbon steel; for stainless steel and other materials, use the actual density.
Frequently asked questions
- Q: What strip width do I need for 2-inch pipe? A: 2-inch pipe is about 60.3 mm OD, so the rough theoretical conversion gives 60.3 × 3.14 ≈ 189 mm. This is a theoretical value only; the roller design sets the final width, which is usually slightly less.
- Q: What strip width do I need for 50×50 square tube? A: Converting from the sum of the four sides gives 50 × 4 = 200 mm. This is a theoretical value only; the roller design sets the final width, which is usually slightly less.
- Q: Why is my calculated strip width different from the actual cutting width? A: This guide gives a theoretical conversion from the outside perimeter, using only the OD or side lengths and not the wall thickness. The real width also depends on the wall thickness, the roller design, the weld squeeze-out and the sizing reduction, so it is usually slightly less than the theoretical value, and a new size is confirmed by a trial run.
- Q: Can I use the round tube weight formula for square tube? A: Not directly. Use (D − t) × t × 0.02466 for round tube and the quick estimate sum of four sides × t × 0.00785 for square and rectangular tube.
About this article
Formulas compiled by the InGen Engineering Team. All results are estimates, and strip width is given only as a theoretical conversion. They are for early sizing and inquiries; final values are set by the roller design, the equipment proposal and the technical agreement.