API 510 formula sheet: the calculation methods, in one place

The formulas below are the widely published engineering forms, written in our own notation. The code's wording, tables and limits live in your own copy of API 510 and ASME Section VIII, Division 1, so confirm every constant against the edition on the effectivity sheet before you rely on it. For the full list of calculation areas in the Body of Knowledge, see the API 510 calculations guide.

The four moves

Move	Question it answers	Method in one line	Open
Rate	How fast is the wall thinning?	Metal loss divided by years between readings	API 510, corrosion rate paragraphs
Life	How long until the wall reaches required thickness?	Actual minus required, divided by the rate	API 510, remaining life paragraphs
Interval	When is the internal or on-stream inspection due?	The lesser of half the remaining life and the 10-year maximum	API 510, inspection interval section
Pressure and thickness	What can the wall hold, or what must it be?	The part formula for the shape, with S, E and the corroded dimensions	VIII-1 UG-27, UG-32

Hydrotest pressure, static head and joint efficiency feed the fourth move. They are covered after the first three.

1. Corrosion rate (long term and short term)

• Long-term rate = (first reading minus current reading) / years between them.
• Short-term rate = (previous reading minus current reading) / years between them.
• Variables: thickness in inches or mm, years from the actual reading dates. Rates come out in in./yr or mm/yr. 1 mpy is 0.001 in./yr, or 0.0254 mm/yr.
• Check: the later reading is smaller, so both rates are positive. A negative rate means you subtracted in the wrong order.

Worked example. A drum shell location reads 0.750 in. (March 2010), 0.705 in. (March 2018) and 0.681 in. (March 2026).

• Long term: (0.750 - 0.681) / 16 = 0.069 / 16 = 0.00431 in./yr (4.31 mpy).
• Short term: (0.705 - 0.681) / 8 = 0.024 / 8 = 0.00300 in./yr (3.00 mpy).

Keep both. Which rate to use for life is a judgment API 510 explains in its own words, and a stem usually tells you. With no other information, the higher rate gives the shorter life. Check your readings with the corrosion rate calculator.

2. Remaining life

• Remaining corrosion allowance = actual thickness minus required thickness.
• Remaining life (years) = remaining corrosion allowance / corrosion rate.
• The required thickness comes from the pressure calculation in section 4, not from the nameplate or the nominal wall.

Worked example. Same location, required thickness 0.612 in.

• Remaining corrosion allowance: 0.681 - 0.612 = 0.069 in.
• Life at the long-term rate: 0.069 / 0.00431 = 16.0 years.
• Life at the short-term rate: 0.069 / 0.00300 = 23.0 years.

The remaining life calculator runs the same chain.

3. Internal or on-stream inspection interval

• Internal or on-stream interval = the smaller of half the remaining life and the 10-year maximum for a pressure vessel (see the interval section of API 510).
• External inspection intervals are set separately in the same section; this formula does not cover them.
• Other damage mechanisms, service changes and your inspection program can set a shorter interval.

Worked example. From the 16.0-year life above: half is 8.0 years, the maximum is 10 years, so 8.0 years governs and the next internal or on-stream inspection is due March 2034. A location with 30 years of life would give half life 15 years, and the 10-year maximum would govern instead. The inspection interval calculator shows which limit sets the date.

4. Required thickness and MAWP

These are the thin-wall pressure forms for shells and heads, as they are widely published for ASME Section VIII, Division 1. P is pressure at the part, S allowable stress, E joint efficiency, R inside radius and D inside diameter, both in the corroded condition, and t thickness.

Part	Required thickness	MAWP for a given t	Open
Cylindrical shell	t = P R / (S E - 0.6 P)	P = S E t / (R + 0.6 t)	VIII-1 UG-27(c)(1)
Spherical shell or hemispherical head	t = P R / (2 S E - 0.2 P)	P = 2 S E t / (R + 0.2 t)	VIII-1 UG-27(d), UG-32
2:1 ellipsoidal head	t = P D / (2 S E - 0.2 P)	P = 2 S E t / (D + 0.2 t)	VIII-1 UG-32(c)

The shell form applies to the thin-wall range the code sets in UG-27(c). Outside that range the code sends you to its thick-wall rules. The BOK also allows the Appendix 1 outside-diameter formulas for cylinders.

Worked example (shell). Design pressure 200 psi, new inside diameter 72.000 in., corrosion allowance 0.125 in., S = 17,500 psi, E = 0.85.

1. Corroded inside diameter: 72.000 + 2 x 0.125 = 72.250 in., so R = 36.125 in.
2. Required thickness: 200 x 36.125 / (17,500 x 0.85 - 0.6 x 200) = 7,225 / 14,755 = 0.490 in.
3. Measured thickness 0.620 in. Thickness available for pressure after the allowance: 0.620 - 0.125 = 0.495 in.
4. MAWP: 17,500 x 0.85 x 0.495 / (36.125 + 0.6 x 0.495) = 7,363 / 36.422 = 202 psi.

The MAWP and thickness calculator shows each step for shells and 2:1 heads.

5. Joint efficiency changes the answer

E comes from the joint type and the radiography level on the nameplate (VIII-1 UW-11 and Table UW-12, with UW-12(d) for seamless parts). A lower E means a thicker wall for the same pressure. The joint efficiency and radiography explainer walks through why.

Worked example. The shell above with three different E values (made up for the comparison; read yours from the table):

E	Required thickness (in.)
1.00	0.416
0.85	0.490
0.70	0.596

Dropping E from 1.00 to 0.70 adds 0.18 in. of required wall. A wrong E carries through remaining life and the interval, so find the category and RT marking first, then substitute.

6. Static head

• Static head (psi) = 0.433 x specific gravity x height of liquid in feet. API 510 study problems use a specific gravity of 1.0 unless told otherwise.
• Pressure at a lower point = pressure at the top + static head.
• A part's allowed pressure at the top = part MAWP minus the static head at that part's lowest point (VIII-1 UG-98).

Worked example. A shell course has a part MAWP of 220 psi and its lowest point sits 26 ft below the top. Static head: 0.433 x 26 = 11.26 psi. Allowed at the top: 220 - 11.26 = 208.74 psi. The vessel MAWP is the lowest of these values across all parts, so a strong part low down can still govern. Use the static head calculator for the elevation step.

7. Hydrostatic test pressure

• Stress ratio = allowable stress at test temperature / allowable stress at design temperature, taking the lowest ratio among the materials.
• Minimum test pressure at the top = 1.3 x MAWP x stress ratio for a hydrostatic test (VIII-1 UG-99(b)). A pneumatic test uses 1.1 in place of 1.3 (UG-100).
• Pressure at the bottom = top pressure + static head of the test water.

Worked example. MAWP 250 psi, allowable stress 20,000 psi at test temperature and 18,000 psi at design temperature, vessel 40 ft tall.

1. Ratio: 20,000 / 18,000 = 1.111.
2. Top: 1.3 x 250 x 1.111 = 361.1 psi.
3. Head of water over 40 ft: 0.433 x 40 = 17.32 psi.
4. Bottom: 361.1 + 17.3 = 378.4 psi.

The hydrotest pressure calculator does this chain for hydrostatic and pneumatic tests.

Unit traps and common slips

• Wrong rate. Read whether the stem wants the long-term or the short-term rate before you divide.
• Mixing in. and mm. 1 in. is 25.4 mm and 1 psi is 6.895 kPa. Do the whole problem in one system, then convert once.
• New dimensions in a corroded formula. Add the corrosion allowance to the inside diameter before you find R, and subtract it from the measured thickness before you find MAWP.
• Forgetting static head. Pressure at the part is not pressure at the top.
• Years from the wrong dates. Count from the reading dates, not from the install year.
• Answering the wrong quantity. Required thickness, MAWP, rate, life and interval all look alike on the screen. Underline what the last sentence asks for.
• Top or bottom. A test pressure is stated at the top. The bottom is higher by the head.

Which formula for this stem?

1. Does the stem give two or more thickness readings and dates? That is a rate problem.
2. Does it give a rate and a required thickness? That is remaining life.
3. Does it ask when the next internal or on-stream inspection is due? Take the lesser of half the life and 10 years.
4. Does it give pressure, S, E and a diameter? Pick the shell, sphere or head form by shape.
5. Does it give a liquid height or elevations? Add static head before you use P.
6. Does it say test, MAWP and stress at two temperatures? Use the stress ratio and 1.3 (hydro) or 1.1 (pneumatic).

Practice the moves under time

Part of this material may be closed-book, so the methods need to be in your head, not just in your binder. Work each example above by hand, then again in SI units. Write your own one-line method for each move and test it on fresh numbers. The closed-book guide covers how that part of the exam runs, and the open-book guide covers the lookups.

Find your weak move

The free 10-question check takes a few minutes, needs no signup and explains every answer, so you see straight away whether the arithmetic or the lookup is costing you points. The Long Seam course then adds a lesson for each Body of Knowledge domain, timed practice exams and a study plan built from your exam date.

Sources

Show the list

• API 510 Body of Knowledge, September 2026 to May 2027, American Petroleum Institute, Section I.A
• API 510 Exam Publications Effectivity Sheet, September 2026 to May 2027, American Petroleum Institute
• Pressure Vessels, David Roylance, MIT (thin-wall stress background)