Sizing Pipe Wall Thickness for a Target Design Pressure

“I need this pipe to hold 5,000 psi — what wall thickness do I actually need?” is one of the most common early-design questions we get, and it’s really just our Internal Yield (Burst) Pressure formula run backwards. This post walks through solving Barlow’s equation for wall thickness instead of pressure, and where that number stops being useful.

Starting from the same equation

API TR 5C3 gives internal yield pressure as:

P = factor × 2 × Yp × t ÷ D

Sizing a pipe is the same equation with a different unknown — you know the pressure you need to hold (P), you know the grade you’re planning to use (Yp) and the OD you’re working with (D), and you’re solving for the minimum wall thickness (t) that gets you there. Rearranged:

t = (P × D) ÷ (factor × 2 × Yp)

Same three variables, same wall factor — just isolated differently. The factor question matters more here than it does when you’re checking an existing pipe. If you’re back-checking a pipe you already have in hand, starting from a factor of 1.0 (nominal wall) to get a quick screening number is reasonable. But if you’re specifying a wall thickness to buy against, you want the factor of 0.875 baked in from the start — otherwise the wall you spec is the nominal figure, and the pipe you actually receive (up to −12.5% under API 5CT’s manufacturing tolerance) could fall short of your target pressure. That’s why the wall-thickness calculator defaults to 0.875 instead of 1.0.

A worked example

Say you’re speccing 5-1/2 in. casing in J-55 (minimum yield strength 55,000 psi), and it needs to hold 5,000 psi at the standard minimum-wall design case:

0.286 in. is the theoretical minimum — not a wall thickness anyone actually stocks. In practice you’d round up to the nearest wall your supplier carries (0.304 in. or 0.361 in. for 5-1/2 in. J-55, depending on the weight available) and then run that real wall back through the burst calculator to confirm the margin you actually end up with.

What this number doesn’t tell you

This sizes the pipe body for internal pressure only. It says nothing about whether that same wall survives external (collapse) pressure — a thinner wall that comfortably passes a burst check can fail well short of it in collapse, since collapse resistance falls off sharply as wall thickness drops. If the string will ever see external pressure exceeding internal (evacuation, cementing, trapped annular pressure), check the wall you land on against our Collapse Pressure calculator too — sizing for burst alone and assuming collapse takes care of itself is a common way to under-design a string.

It also doesn’t account for combined loading (axial tension or compression stacked with pressure), temperature derating, or the connection — which, on a made-up string, is very often the weaker link, not the pipe body.

Try the Required Wall Thickness calculator yourself, or get in touch if you’d rather talk it through with an engineer.