For a pipe under internal pressure, the minimum wall thickness is set by the hoop (circumferential) stress staying within the material’s allowable design stress. British practice (BS 3601 for carbon steel, the harmonised BS EN 13480 for metallic industrial piping, and PD 5500 for pressure vessels) uses the thin-wall relationship t = P·D / (2·f·z + P), switching to the thick-walled Lamé equation once the diameter-to-thickness ratio falls to about 20 or below.

From calculated to ordered thickness

• Design stress f is the code allowable for the grade at the design temperature — typically the lower of yield÷1.5 and tensile÷2.35 — reduced by the weld joint factor z for welded pipe.
• Corrosion / erosion allowance is added to the pressure thickness to give the minimum acceptable wall through life.
• Mill under-tolerance (12.5% for seamless pipe) is grossed up so the thinnest point of the delivered pipe still meets the minimum.

This tool is for estimating and pre-selection. Confirm the allowable stress, weld and thinning allowances and external-pressure / combined-load checks against the governing code and the project specification before procurement or fabrication.