Instrument Tubing Pressure Rating Calculator
Estimate the working pressure of seamless instrument tubing from its outside diameter, wall thickness and allowable stress, and compare common tube sizes.
Barlow estimate for common sizes at this allowable stress
| Tube | OD (mm) | Wall (mm) | Estimate (bar) |
|---|---|---|---|
| 1/4" × 0.035" | 6.35 | 0.89 | 386.6 |
| 1/4" × 0.049" | 6.35 | 1.24 | 538.6 |
| 3/8" × 0.035" | 9.53 | 0.89 | 257.6 |
| 3/8" × 0.049" | 9.53 | 1.24 | 358.9 |
| 1/2" × 0.049" | 12.7 | 1.24 | 269.3 |
| 1/2" × 0.065" | 12.7 | 1.65 | 358.3 |
| 6 mm × 1 mm | 6 | 1 | 459.7 |
| 8 mm × 1 mm | 8 | 1 | 344.8 |
| 10 mm × 1 mm | 10 | 1 | 275.8 |
| 12 mm × 1.5 mm | 12 | 1.5 | 344.8 |
Barlow Formula
P working pressure, S allowable stress, t wall thickness, D outside diameter, all in consistent units. 1 MPa = 10 bar.
Worked example
12 mm × 1.5 mm SS 316 tube at 137.9 MPa: P = 2 × 137.9 × 1.5 ÷ 12 = 34.5 MPa, about 345 bar.
Frequently Asked Questions
Why do maker ratings differ from Barlow?
Makers use their own equations, tolerances and safety factors, and the fitting may limit the rating.
Does temperature reduce the rating?
Yes, allowable stress falls at high temperature; apply the maker derating factors.
Can I mix 12 mm and 1/2" fittings?
No, they are not interchangeable.
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