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MAWP and minimum thickness calculator for shells and 2:1 heads

Work out the required thickness and the MAWP of a cylindrical shell or a 2:1 ellipsoidal head under internal pressure. Add a corrosion allowance and the static head of a liquid column, and see every step of the arithmetic.

Method checked against ASME Section VIII, Division 1, UG-27(c)(1) and UG-32 by paragraph number (the source of the 0.6P shell and 0.2P head forms), MIT, Roylance, Pressure Vessels (thin-wall hoop stress PR/t only) and the API 510 Body of Knowledge (section pointers only) on 20 Oct 2026. A calculation aid for study and estimating, not a substitute for the governing code edition, the manufacturer's data or your procedure.

Free MAWP and minimum thickness calculator in US or SI units, with a show-the-steps panel. You enter S and E from your own code book; no stress tables are included. No account needed.

Component, code values and corrosion inputs

Results update as you type. S and E come from your own code edition and the vessel's records. The starting values are a fictional worked example, not suggested ratings.

Changing units loads the SI or US example values. Enter P and S in the same pressure unit.

Look up S for your material and design temperature in your own copy of the code. This tool holds no stress tables and does not check the value you enter. E is the efficiency of the joint that governs the stress you are checking (UW-12 for welded joints).

For a design check, CA is the specified allowance. For an in-service check, use the metal loss expected by the next inspection (for example corrosion rate × years), as your procedure directs. Internal corrosion only.

Leave liquid height at 0 for no static head. Use the vertical height from the top reference (where P applies) down to the lowest point of the part you are checking.

Cylindrical shell results

Pressure at this part
154.33 psi
Required thickness, t
0.27498 in
t + corrosion allowance
0.39998 in
MAWP of this part
210.049 psi
MAWP referred to the top
205.719 psi
Show the steps
  1. Static head at this part: ΔP = 0.433 × SG × h
    = SG 1, h = 10 ft = 4.33 psi
  2. Pressure at this part: P = pressure at top + ΔP
    = 150 + 4.33 = 154.33 psi
  3. Corroded inside diameter: D = D(entered) + 2 × CA
    = 60 + 2 × 0.125 = 60.25 in
  4. Inside radius: R = D ÷ 2
    = 60.25 ÷ 2 = 30.125 in
  5. Required thickness (UG-27(c)(1) form): t = PR ÷ (SE − 0.6P)
    = 154.33 × 30.125 ÷ (20000 × 0.85 − 0.6 × 154.33) = 0.27498 in
  6. Required thickness plus corrosion allowance: t + CA
    = 0.27498 + 0.125 = 0.39998 in
  7. Thickness available for pressure: t = t(actual) − CA
    = 0.5 − 0.125 = 0.375 in
  8. MAWP of this part (UG-27(c)(1) form): P = SEt ÷ (R + 0.6t)
    = 20000 × 0.85 × 0.375 ÷ (30.125 + 0.6 × 0.375) = 210.049 psi
  9. MAWP referred to the top: MAWP(top) = MAWP(part) − ΔP
    = 210.049 − 4.33 = 205.719 psi
  10. Remaining corrosion allowance: t(actual) − t(required)
    = 0.5 − 0.27498 = 0.22502 in

Values are shown to six significant digits; the calculation keeps full precision between steps.

Formulas used

For internal pressure only, with P the pressure at the part, S the allowable stress, E the joint efficiency, R the inside radius, D the inside diameter and t the thickness:

  • Cylindrical shell, circumferential stress: t = PR ÷ (SE − 0.6P) and, solved for pressure, P = SEt ÷ (R + 0.6t). ASME Section VIII, Division 1, UG-27(c)(1).
  • 2:1 ellipsoidal head: t = PD ÷ (2SE − 0.2P) and P = 2SEt ÷ (D + 0.2t). ASME Section VIII, Division 1, UG-32 (the paragraph letter differs between editions).

Where the shell form comes from: cut a thin cylinder along its length and balance the pressure force on the cut against the wall's tensile force. That gives a hoop stress of PR/t, twice the longitudinal stress of PR/2t, so the circumferential direction usually governs. Source: MIT, Roylance, Pressure Vessels. The 0.6P term in the code form comes from UG-27(c)(1); Roylance does not give it. Open your own copy of UG-27 for the full paragraph, the longitudinal-stress check in UG-27(c)(2) and the range limits.

The shell form is a thin-wall formula. The calculator flags a result outside the range stated in UG-27(c) (t greater than R/2, or P greater than 0.385SE), because the code sends those cases to thick-wall rules this tool does not cover.

Worked example: shell, with corrosion allowance and static head

These are fictional study inputs, not ratings for any real vessel. A shell has an inside diameter of 60 in before corrosion, S = 20,000 psi (entered, not looked up), E = 0.85 and a corrosion allowance of 0.125 in. The pressure at the top is 150 psi and the shell bottom is 10 ft below the liquid surface, with SG = 1.

  1. Static head: 0.433 × 1 × 10 = 4.33 psi. Pressure at the part: 150 + 4.33 = 154.33 psi.
  2. Corroded inside radius: (60 + 2 × 0.125) ÷ 2 = 30.125 in.
  3. Required thickness: 154.33 × 30.125 ÷ (20,000 × 0.85 − 0.6 × 154.33) = 4,649.19 ÷ 16,907.40 = 0.27498 in. With the allowance added: 0.39998 in.
  4. MAWP at a measured 0.500 in: thickness for pressure is 0.500 − 0.125 = 0.375 in, so P = 17,000 × 0.375 ÷ (30.125 + 0.225) = 210.049 psi at the part.
  5. Referred to the top of the vessel: 210.049 − 4.33 = 205.719 psi.

Change the component to a 2:1 head with E = 1 and the same other inputs: t = 154.33 × 60.25 ÷ (40,000 − 30.866) = 0.232639 in, and the MAWP at 0.375 in is 15,000 ÷ 60.325 = 248.653 psi at the part, or 244.323 psi at the top.

Worked example in SI units

A fictional shell with D = 1,500 mm before corrosion, CA = 3 mm, S = 138 MPa, E = 1, 1 MPa at the top and 3 m of water above the part. Static head is 1,000 × 9.80665 × 3 ÷ 10⁶ = 0.0294 MPa, so P = 1.0294 MPa and R = 753 mm. Required thickness is 1.0294 × 753 ÷ (138 − 0.6 × 1.0294) = 5.642 mm, or 8.642 mm with the allowance. At a measured 10 mm, the part MAWP is 138 × 7 ÷ (753 + 4.2) = 1.2758 MPa, or 1.2463 MPa at the top.

Corrosion allowance and corroded dimensions

Internal corrosion removes metal from the inside, so the wall gets thinner and the inside diameter grows. If the diameter you have is the original one, the calculator adds 2 × CA to it. If it is already a corroded or measured inside diameter, choose that option and nothing is added.

In-service work under API 510 uses measured thickness and the metal expected to be lost by the next inspection. API 510 §7.1 covers corrosion rates and the remaining corrosion allowance (t actual − t required), and §7.2 covers remaining life. Paragraph numbers can move between editions; check the effectivity sheet for your exam.

Part MAWP and vessel MAWP

The MAWP of a part is what that part can hold at its own elevation. Subtract the static head above it to refer it to the top of the vessel. The vessel MAWP is the lowest of those values across all parts (see UG-98). Work out the head with the static head calculator.

What this calculator does not do

It covers only internal pressure on a cylindrical shell (circumferential stress) and a 2:1 ellipsoidal head. It does not cover spheres (UG-27(d)), hemispherical or torispherical heads, other ellipse ratios, longitudinal stress, external pressure (UG-28), other loadings (UG-22), minimum thickness rules (UG-16), nozzles and openings, or thick-wall rules. It holds no allowable stress or joint efficiency tables: you look those up in your own code edition.

The governing code edition, the vessel's nameplate and manufacturer's data report, your approved procedure and the responsible inspector or engineer control. This tool does not rerate a vessel or set an operating pressure.

Next, project the years left before the wall reaches the required thickness with the remaining life calculator.