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API Inspector Certification · Last updated November 2026

API 653 Aboveground Storage Tank Inspector Exam: free practice test

The API certification exam for aboveground storage tank inspectors, based on API 653, API 650 and the other documents on the effectivity sheet. Exam windows run in March to April, July and November. The 2027 exams use API 653 5th Edition with Addenda 1 to 4 and API 650 14th Edition (2025).

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About the API 653 exam

The API certification exam for aboveground storage tank inspectors, based on API 653, API 650 and the other documents on the effectivity sheet. Exam windows run in March to April, July and November. The 2027 exams use API 653 5th Edition with Addenda 1 to 4 and API 650 14th Edition (2025).

What's on the exam

API 653
Shell Evaluation and Minimum Thickness
6 key facts
Bottoms, Settlement and Foundations
5 key facts
Inspection Intervals and Practices
6 key facts
Repairs, Alterations and Hot Taps
5 key facts
API 650 Design, Materials and Testing
5 key facts
Damage Mechanisms, Cathodic Protection and Linings
6 key facts
Welding and NDE
5 key facts

Each branch is a topic in the official outline. Members see every key fact on the full mind map. Read the free study guide →

How to pass the API 653 exam

  • Shell tmin and maximum fill height problems are the core calculations. The fill height formula is given in the exam, but you must know which H, E and S to use.
  • Bottom plate minimum thickness (Table 4.4) and some joint efficiency tables can be asked closed book. Learn them by heart.
  • Settlement evaluation (Annex B) is open book but slow. Practice reading Figures B.1 to B.13 before exam day so you don't lose time.
  • Only the RP 571 mechanisms on the effectivity sheet are tested, and only Sections 4.3.2 and 6.7 of RP 576.

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What's in the premium API 653 prep

Premium API 653 prep is studied one topic at a time. Each step starts with that topic's key notes and flashcard deck, then its own question sets of 25, so you master one topic before you move to the next. When every topic is strong, the 6 full mock exams show you're ready.

3,000practice questions, every answer explained
6full-length mock exams, 170 questions each, timed
800flashcards with spaced repetition, sorted by topic
60comprehensive key notes across 7 topics

Your step-by-step study path

  1. 1
    Shell Evaluation and Minimum Thickness9 key notes, 115 flashcards, then 450 practice questions in 18 sets of 25
    Notes free with an account
  2. 2
    Bottoms, Settlement and Foundations9 key notes, 115 flashcards, then 425 practice questions in 17 sets of 25
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  3. 3
    Inspection Intervals and Practices9 key notes, 115 flashcards, then 425 practice questions in 17 sets of 25
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  4. 4
    Repairs, Alterations and Hot Taps9 key notes, 115 flashcards, then 425 practice questions in 17 sets of 25
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  5. 5
    API 650 Design, Materials and Testing8 key notes, 115 flashcards, then 425 practice questions in 17 sets of 25
    Premium
  6. 6
    Damage Mechanisms, Cathodic Protection and Linings8 key notes, 115 flashcards, then 425 practice questions in 17 sets of 25
    Premium
  7. 7
    Welding and NDE8 key notes, 110 flashcards, then 425 practice questions in 17 sets of 25
    Premium
  8. Final step: 6 full mock exams170 questions each, timed like test day. Pass them, then book your exam.
    Premium
Free with an account

Read the complete API 653 study notes for “Shell Evaluation and Minimum Thickness”

Our first full set of comprehensive notes, about 15 minutes of reading: The two tmin formulas · Choosing S and E · Evaluating a corroded area · Maximum fill height · Pitting, a worked exam example, the traps examiners use and a one-minute recap. Create a free account to read it and see how premium notes work before you decide.

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Study tools included

3,000 practice questions

3,000 exam-style questions per exam, organized topic by topic in sets of 25. Every answer explained, with the rule it comes from.

6 full mock exams

6 full-length timed mock exams per exam, with real question counts and time limits, so test day feels like practice day.

Exam Readiness Score

One 0 to 100 number that tells you when you're ready to book. Reach 80 and you're covered by our Pass Guarantee.

800 flashcards + spaced repetition

800 flashcards per exam, in a deck for each topic, that come back right before you'd forget them. Five minutes a day keeps every number fresh.

60 comprehensive key notes

60 key notes per exam, split across its topics and each explained in full: the concepts, worked exam examples, the traps examiners use and a quick recap. Your first set is free with an account.

Mind maps

See the whole exam on one screen: every topic and the facts under it.

Sample questions with answers

These are the questions in the free test above, with answers. Try the test first. You'll learn more by getting some wrong.

1. An existing tank has D = 120 ft, a liquid height of 48 ft above the bottom of the course, G = 0.85, S = 26,000 psi and E = 1.0. Using the API 653 formula for an entire shell course, what is tmin?
Answer: 0.479 in.. tmin = 2.6(H − 1)DG ÷ (SE) = 2.6 × 47 × 120 × 0.85 ÷ 26,000 = 12,464 ÷ 26,000 = 0.479 in. Using H instead of H − 1 (0.490 in.) is the formula for a locally thinned area, not a whole course.
2. A shell course has an average thickness, less corrosion to the next inspection, of 0.40 in. S = 26,000 psi, E = 0.85, D = 100 ft and G = 1.0. What is the maximum fill height for the entire course?
Answer: 35 ft. H = (S × E × t) ÷ (2.6 × D × G) + 1 = (26,000 × 0.85 × 0.40) ÷ (2.6 × 100 × 1.0) + 1 = 8,840 ÷ 260 + 1 = 34 + 1 = 35 ft. Forgetting the + 1 gives 34 ft; using E = 1.0 gives 41 ft.
3. Under API 653, widely scattered pits may be ignored if no pit leaves less than one-half the minimum acceptable shell thickness (excluding corrosion allowance) and the sum of pit dimensions along any vertical line does not exceed:
Answer: 2 in. in an 8 in. length. API 653, 4.3.2.2: widely scattered pits may be ignored when both conditions hold: remaining thickness at any pit at least one-half the minimum acceptable shell thickness (exclusive of corrosion allowance), and pit lengths along any vertical line totaling no more than 2 in. (50 mm) in any 8 in. (200 mm) length.
4. A tank bottom and foundation have no means of detecting and containing a bottom leak. Under API 653, what is the minimum bottom plate thickness allowed at the next internal inspection?
Answer: 0.10 in.. Table 4.4: 0.10 in. when there is no means of leak detection and containment; 0.05 in. when the design provides detection and containment, or with an applied reinforced lining more than 0.05 in. thick per RP 652.
5. How many shell settlement survey points does API 653 require around a 200 ft diameter tank?
Answer: 20. N = D ÷ 10 = 200 ÷ 10 = 20 points, with a minimum of 8 and no more than 32 ft between points. π × 200 ÷ 20 = 31.4 ft, so 20 points meets both rules.
6. A tank shell has a known corrosion rate: RCA = 0.120 in. and N = 0.008 in./yr. What is the maximum interval for ultrasonic thickness measurements of the shell under API 653?
Answer: 7.5 years. When the corrosion rate is known, the UT interval is the smaller of RCA ÷ 2N or 15 years: 0.120 ÷ (2 × 0.008) = 7.5 years. The external inspection by an authorized inspector is the smaller of RCA ÷ 4N (here 3.75 years) or 5 years.
7. How often must owner/operator personnel carry out routine in-service inspections of a tank's exterior under API 653?
Answer: At intervals not exceeding one month. API 653, 6.3.1: routine in-service inspections by owner/operator personnel at intervals suited to site conditions but not exceeding one month. They don't replace the external inspection by an authorized inspector.
8. Unless RBI or the safeguards in Table 6.1 apply, what is the maximum interval from initial service to the first internal inspection of a new tank under API 653?
Answer: 10 years. API 653 sets 10 years from initial service to the first internal inspection. Tanks with safeguards listed in Table 6.1, such as cathodic protection, a lining or a release prevention barrier, can earn a longer interval within that table's limits.
9. A hot tap is planned on a tank with a shell radius of 360 in. and a shell plate thickness of 0.50 in. What is the minimum toe-to-toe spacing to an adjacent nozzle under API 653?
Answer: 13.4 in.. Minimum spacing in any direction = √(R × T) = √(360 × 0.50) = √180 = 13.4 in., with R in inches (shell radius) and T in inches (shell plate thickness).
10. What is the minimum dimension of a 1.25 in. thick replacement shell plate under API 653?
Answer: 15 in.. API 653, 9.2.2: the minimum dimension is 12 in. or 12 times the replacement plate thickness, whichever is greater: 12 × 1.25 = 15 in.
11. Under API 650, what is the minimum nominal shell plate thickness for a new tank 150 ft in diameter?
Answer: 5/16 in.. API 650 minimum nominal shell thickness: under 50 ft diameter 3/16 in.; 50 ft to under 120 ft 1/4 in.; 120 ft to 200 ft 5/16 in.; over 200 ft 3/8 in.
12. Under API RP 651, which is an accepted criterion for cathodic protection of a tank bottom?
Answer: A polarized potential of −850 mV or more negative relative to a copper/copper sulfate reference electrode. RP 651 criteria include a polarized (instant-off) potential of −850 mV or more negative versus Cu/CuSO4, or at least 100 mV of cathodic polarization. Current flow alone doesn't show the bottom is protected.
13. Ethanol stress corrosion cracking, covered in API RP 571, mainly affects:
Answer: Carbon steel tanks and piping handling fuel-grade ethanol and its blends. RP 571: surface-breaking cracks in carbon steel exposed to fuel-grade ethanol and ethanol/gasoline blends, most often at or near welds that were not stress relieved, where residual stress is high.
14. Under ASME Section IX, a welder's qualification for a process expires when the welder has not welded with that process for:
Answer: 6 months. QW-322.1(a): performance qualification expires when the welder has not welded with the process for 6 months or more. It can also be revoked when there is a specific reason to question the welder's ability.
15. A PQR tension specimen breaks in the base metal outside the weld at 3% below the base metal's minimum specified tensile strength. Under QW-153, the result is:
Answer: Accepted, because it broke in the base metal outside the weld within 5% of the minimum. QW-153.1: the specimen must reach the base metal's minimum specified tensile strength, but a break in the base metal outside the weld or fusion line is acceptable if the strength is not more than 5% below that minimum.

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Certification guide

How to become an API certified inspector (510, 570 or 653): API 510 (pressure vessels), API 570 (piping) and API 653 (aboveground storage tanks) are the main inspector certifications in refining and petrochemicals. Each is run by the [American Petroleum Institute's Individual Certification Programs](https://www.api.org/products-and-services/individual-certification-programs) and needs documented experience, an approved application and a pass in a 7.5-hour exam. Here's the path.

API 653 exam FAQ

How many questions are on the API 653 exam?

170 multiple-choice questions (140 scored, 30 unscored pretest items): 110 closed-book and 60 open-book, with a time limit of 7.5-hour exam day: short tutorial, 2.75 hours closed book, 45-minute lunch, 3.75 hours open book. Format: closed-book part, then an open-book part with the effectivity-sheet documents as on-screen pdfs. no paper books or notes; on-screen calculator.

What is the passing score for API 653?

Passing scaled score is 400 on API's 200 to 500 scale. The number of correct answers needed isn't published and varies by form; practice to 75% or higher.

How much does the API 653 exam cost?

US$875 API member / US$1,125 non-member (2026 fee table); US$890 / US$1,140 (2027 fee table). Reschedule US$350, late application US$200. API publishes a fee table for each year, so confirm the current fee on API's Schedules & Fees page before you apply.

Which languages is the API 653 exam offered in?

English; Spanish and Chinese translations available on request.

Where do I take the API 653 exam?

Prometric test centers, in person only (no remote option for API 653).

Is this API 653 practice test free?

Yes. The 15-question practice test on this page is free, with an explanation for every answer and no signup. A free account also unlocks the complete study notes for the first topic. The full API 653 prep is Premium at $147 a month for this exam.

What's in the premium API 653 prep?

3,000 practice questions organized topic by topic (Shell Evaluation and Minimum Thickness: 450; Bottoms, Settlement and Foundations: 425; Inspection Intervals and Practices: 425; Repairs, Alterations and Hot Taps: 425; API 650 Design, Materials and Testing: 425; Damage Mechanisms, Cathodic Protection and Linings: 425; Welding and NDE: 425), 6 full-length timed mock exams of 170 questions, 800 flashcards and 60 comprehensive key notes split across all 7 topics, plus a mistakes bank and the Readiness Score.

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Exam details are summarized from the official candidate information available in November 2026. API changes formats and fees, so confirm with American Petroleum Institute (API) Individual Certification Programs (ICP) before you book. APIInspectorExam is not affiliated with the American Petroleum Institute (API), ASME or Prometric.

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