How to Choose Plate Thickness: Pressure, Corrosion Allowance & Mill Sizes

Choosing a plate thickness means working through four inputs, not guessing. Design pressure sets the load. The grade's allowable stress determines how much metal carries it. Joint efficiency discounts the weld, and a corrosion allowance protects the design life. You solve for the minimum, add the allowance, then round up to a thickness a mill actually rolls. After that, confirm the nominal thickness survives the mill's under-tolerance. This guide, one of our specification guides, walks through each step, states the governing code, and shows where material choice lets you buy thinner.

Goal: what "the right plate thickness" means

The thinnest standard mill plate whose as-rolled thickness, minus under-tolerance, still clears the minimum required thickness plus the corrosion allowance, that's the right plate thickness. Three thicknesses come into play here, and they shouldn't get mixed up:

Minimum required thickness (t) is the calculated wall your pressure, grade and weld demand. The design code governs it, not the mill.

Nominal thickness is what you write on the purchase order and the mill test certificate, say, 16 mm or 5/8 in.

As-rolled, or actual, thickness is what the plate measures on receipt. A mill may roll slightly under nominal within an allowed tolerance. That's why the under-thickness rule in Step 6 matters.

Get the right plate and the vessel is safe and economical at once. Over-specify and you're hauling dead weight and extra cost; under-specify and the wall fails its code check. For the companion material decision, see how to select a stainless grade. This page handles the dimension, that one handles the metallurgy, and between them the plate is fully specified.

Design-authority note: the formula below is just a starting point for a cylindrical shell under internal pressure. Setting the final stamped thickness is the job of the project's design authority, done to the governing code. That means accounting for heads, nozzles, external pressure, loadings and temperature. Nandini Steel supplies genuine mill plate to the chosen specification. It isn't the vessel designer of record.

Before you start: the five inputs you need

Gather these before you touch the formula. Each one feeds directly into the calculation, and the mill order needs it too:

1. Design pressure (P) and temperature, this is the service condition the wall has to withstand, expressed in psi or bar. Temperature counts for a reason: allowable stress drops as temperature climbs.
2. Grade and its allowable stress (S). The grade sets the allowable stress straight from ASME BPVC Section II Part D. Pick a higher-strength grade and the same pressure can live in a thinner wall. Settle on the grade before anything else, see how to select a stainless grade.
3. Joint efficiency (E), which comes down to your welding and radiography plan. Full, spot or no radiography puts E at 1.0, 0.85 or 0.70 under ASME UW-12.
4. Corrosion allowance (CA), take the design life and multiply it by the expected uniform metal-loss rate. What that rate actually is depends on the medium, and on whether the grade stands up to it. See uniform vs localized corrosion.
5. Available mill thicknesses and the tolerance standard that governs them. Mills roll in real, fixed increments, not whatever number math gives you, and the under-tolerance matters: ASTM A20 for pressure-vessel-quality carbon plate, ASTM A480/A6 for stainless and carbon flat products.

Step-by-step: how to choose plate thickness

The method unfolds in six ordered steps, and each one lays out its rule in plain terms. The worked example takes a single case through every stage, ending at an orderable nominal thickness.

Step 1: Fix the grade and its allowable stress

The grade sets the allowable stress S in the thickness formula, so it's the first input. Pull the 0.2% yield strength and tensile strength for the grade, then check ASME BPVC Section II Part D for the allowable stress at your design temperature. The relationship is simple: raise the allowable stress and the minimum thickness drops for the same pressure. A pressure-vessel plate in SA-516 Grade 70 carries a higher allowable stress than lean austenitic stainless does. Duplex 2205 goes higher still. Choose the material with intent. A stronger or higher-PREN grade can buy you a thinner wall. For the governing minimums, check the grade's yield and tensile strength basis and the A516 Gr 70 plate datasheet.

Step 2: Calculate the minimum pressure-retaining thickness

For a cylindrical shell under internal pressure, ASME BPVC Section VIII Division 1, paragraph UG-27 gives the minimum required thickness:

t = PR / (SE − 0.6P)

Where t stands for minimum required thickness, P for design pressure, R for inside radius, S for allowable stress, and E for joint efficiency. Plugging in the inputs above with full radiography (E = 1.0):

The minimum pressure-retaining wall is 0.454 in before any corrosion allowance is factored in. For the full design-basis context, see how to specify A516 for a pressure vessel; for background on the grade itself, check pressure-vessel steel.

Step 3: Apply the joint efficiency (E)

Joint efficiency discounts the shell based on weld quality and how much radiography was done, per ASME BPVC Section VIII Division 1, paragraph UW-12. A lower E means more metal is needed, less inspection drives up the required thickness. Set E before Step 2; it feeds directly into the formula.

Step 4: Add the corrosion allowance

The corrosion allowance covers extra wall thickness sacrificed to uniform (general) corrosion over the design life: expected metal-loss rate multiplied by design life. For carbon steel in a mildly corrosive medium, a common value is 1/16, 1/8 in (1.6–3.2 mm); check it against your own corrosion data. Carbon steel is susceptible to general corrosion, so it almost always needs an allowance. Corrosion-resistant alloys resist uniform attack, though, and often need little or none. The worked case adds 0.125 in:

T_required + CA = 0.454 in + 0.125 in = 0.579 in

Add the allowance only where corrosion is uniform. Localized attack, pitting and crevice corrosion, is not uniform metal loss and is not mitigated by a corrosion allowance; it is solved by choosing a higher-PREN grade, not by a thicker wall. Read uniform vs localized corrosion before applying any allowance to stainless.

Step 5: Round up to the nearest available mill thickness

Never round down. Take t_required + CA and round up to the nearest thickness a mill actually rolls. In our worked case, 0.579 in rounds up to 5/8 in (0.625 in, roughly 16 mm). Rounding up to a stocked increment keeps lead time short and cost predictable. Specify a thickness no mill rolls, though, and an ex-stock order turns into a long-lead indent.

Use the plate weight calculator to price out each increment. Once the nominal thickness is set, calculate plate weight from there.

Step 6: Check the mill under-tolerance

A mill may roll slightly under the ordered nominal within an allowed under-tolerance, ASTM A20 governs pressure-vessel-quality carbon plate, ASTM A480/A6 govern stainless and carbon flat products. Confirm the inequality holds at the worst-case thin corner:

> nominal thickness − under-tolerance ≥ t_required + CA

For the 5/8 in order, 0.625 in minus a typical small under-tolerance still clears the 0.579 in requirement. The plate is in tolerance as ordered. If it doesn't clear, step up one increment.

Common pitfalls & warnings

Avoid these anti-patterns, each one ships a plate that either fails to conform or just wastes money.

Rounding down to a thinner stock size to save weight. Do this and the wall fails its code check.

Forgetting the corrosion allowance on carbon steel. Carbon steel is susceptible to general corrosion, so skipping this step is a real risk.

Adding a corrosion allowance to fight pitting won't work. Pitting and crevice attack are localized, not uniform loss, and a thicker wall does nothing to stop them. Step up the grade instead.

Ignoring the mill under-tolerance. Do this and the as-rolled plate ends up thinner than the calculation assumed.

Specifying a thickness no mill actually rolls. That turns an ex-stock plate into a long-lead indent.

Mixing metric and imperial gauge between the calculation and the order.

Omitting the external-pressure or vacuum case. For thin shells, this can govern over internal pressure.

How to verify the thickness you chose

Verification closes the loop between the calculation and the plate on the ground.

Re-check the inequality. Confirm that t_required + CA is less than or equal to nominal minus under-tolerance, both for the final grade and for the radiography plan.

• Ultrasonic (UT) thickness check on receipt to confirm the as-rolled thickness.

Confirm thickness and heat on the EN 10204 3.1 mill test certificate. Every order ships with an EN 10204 3.1 mill test certificate, which ties nominal thickness and heat/cast number to the plate itself. For more detail, see how to read a mill test certificate and the practice behind UT, PMI and MTC.

• PMI to confirm the grade matches the specification.

Impact testing and normalizing matter where low-temperature toughness is required. For cold-service pressure vessels, specify normalized plate with Charpy values at the design temperature, not as-rolled. See normalizing for low-temperature toughness.

Thickness and grade are coupled: choosing the material that lets you buy thinner

Material and thickness aren't independent decisions. A higher allowable stress thins the pressure-retaining wall; a higher PREN cuts into the corrosion allowance, sometimes erasing it entirely. Run the same 300 psi case across different grades, and the coupling shows up clearly.

Duplex 2205 beats 316L on strength, which means the same pressure case gets a thinner wall. See the trade-off at 904L vs duplex 2205 and the PREN ladder that governs the corrosion-allowance side. In practice, the classic saving on sulfuric-acid and high-chloride duty comes from choosing 904L's copper-bearing ~4.5% Mo chemistry instead of paying for a heavier upgrade. On cold-service vessels, the saving comes from stating the plate thickness up front, so normalizing and impact requirements get set for the correct section.

All grades above are supplied ex-stock for immediate dispatch from genuine mill plate (Aperam, Outokumpu, Jindal, POSCO, Nippon Steel, JFE). Request an RFQ for the current stocked thickness list and live pricing.

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