API 653 Exam Study Guide (2026)
Everything the API 653 Aboveground Storage Tank Inspector Exam tests, topic by topic: what each section covers, the facts that come up most, and how to study it. Pair it with the free 15-question practice test.
The exam at a glance
| Questions | 170 multiple-choice questions (140 scored, 30 unscored pretest items): 110 closed-book and 60 open-book |
|---|---|
| Time | 7.5-hour exam day: short tutorial, 2.75 hours closed book, 45-minute lunch, 3.75 hours open book |
| Passing score | 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 |
| 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 |
| Certifying body | American Petroleum Institute (API) Individual Certification Programs (ICP) |
| Test provider | Prometric test centers, in person only (no remote option for API 653) |
Mind map: the whole exam on one screen
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.
1. Shell Evaluation and Minimum Thickness
API 653 Section 4 for tank shells: minimum acceptable thickness for whole courses and locally thinned areas, allowable stress and joint efficiency for existing tanks, corroded areas (t1, t2 and the critical length L), pitting and maximum fill height.
Key facts
- Whole shell course: tmin = 2.6(H − 1)DG ÷ (SE). Locally thinned area: tmin = 2.6HDG ÷ (SE). D in ft, H in ft, t in inches, S in psi. tmin is never less than 0.1 in.
- S for existing tanks comes from Table 4.1: the lesser of 0.80Y or 0.429T for the bottom and second courses, and the lesser of 0.88Y or 0.472T for upper courses.
- E comes from Tables 4.2 and 4.3 for the original joint type and radiography; E = 1.0 for corroded areas away from welds by at least the greater of 1 in. or twice the plate thickness.
- Corroded area: t2 is the least thickness exclusive of pits; L = 3.7√(Dt2) (max 40 in.) sets the vertical length over which t1, the average thickness, is measured.
- Maximum fill height for a whole course: H = (S × E × t) ÷ (2.6 × D × G) + 1. For a local area, drop the + 1.
- Widely scattered pits can be ignored if no pit leaves less than half of tmin (excluding corrosion allowance) and pit lengths total no more than 2 in. in any 8 in. vertical length.
Flashcards
Exam tip: Check H first: whole-course problems use the height from the bottom of the course, minus 1 ft in the formula; local-area problems use the height from the bottom of the corroded area with no minus 1.
2. Bottoms, Settlement and Foundations
Tank bottom evaluation (minimum remaining thickness at the next inspection, the critical zone, annular plates) and settlement surveys and limits from API 653 Section 4, Section 12 and Annex B.
Key facts
- Table 4.4 minimum bottom thickness at the next inspection: 0.10 in. with no leak detection and containment; 0.05 in. with detection and containment, or with a reinforced lining over 0.05 in. thick.
- In the critical zone next to the shell, unless a stress analysis is done, the minimum is the smaller of 0.118 in. (half the API 650 bottom thickness, excluding corrosion allowance) or 50% of the lower shell course tmin, but not less than 0.10 in.; averaging isn't allowed.
- MRT (minimum remaining thickness at the next inspection) = the lesser of RTbc or RTip (remaining thickness after repairs from soil-side and product-side corrosion) minus the expected corrosion over the operating period Or.
- Settlement survey points: N = D ÷ 10 (D in ft), at least 8, no more than 32 ft apart.
- Annex B separates uniform settlement and rigid tilt (usually harmless to the shell) from out-of-plane settlement, edge settlement and bottom settlement, which can overstress the tank.
Flashcards
Exam tip: Settlement questions are open book and figure-heavy. Practice one example of each type (edge, near-shell bottom, remote bottom) so the figures feel familiar.
3. Inspection Intervals and Practices
API 653 Section 6 intervals (routine in-service, external, ultrasonic thickness and internal inspections) and the inspection practices in API RP 575.
Key facts
- Routine in-service inspection by owner/operator personnel: at intervals that suit the site, not exceeding one month.
- External inspection by an authorized inspector: at least every 5 years or RCA ÷ 4N years, whichever is less.
- UT thickness of the shell: if the corrosion rate is unknown, within 5 years; if known, the smaller of RCA ÷ 2N years or 15 years.
- First internal inspection of a new tank: within 10 years of initial service, unless Table 6.1 safeguards or an RBI assessment justify longer.
- Subsequent internal intervals are based on the bottom corrosion rate so the bottom stays above Table 4.4 minimums.
- RP 575 covers how to inspect tank shells, roofs, bottoms, appurtenances and foundations, including leak testing and records.
Flashcards
Exam tip: RCA ÷ 4N belongs to the external visual inspection; RCA ÷ 2N belongs to UT. Swapping the two is the classic trap.
Test yourself on these topics
Take the free 15-question API 653 test, then save your results to unlock your Readiness Score and the mistakes bank.
4. Repairs, Alterations and Hot Taps
API 653 Sections 9 and 12: replacement shell plates, lap-welded patch plates, shell penetrations, bottom repairs, hot taps, and the NDE and hydrostatic test needed after repairs and alterations.
Key facts
- Replacement shell plates: minimum dimension 12 in. or 12 times the replacement plate thickness, whichever is greater; not thinner than the adjoining plates in the course.
- Lap-welded patch plates on the shell are limited in thickness and size and need minimum fillet weld sizes set out in API 653.
Flashcards
Exam tip: Repair questions are often 'minimum dimension' or 'minimum spacing'. Keep the numbers in one table in your notes and drill them.
5. API 650 Design, Materials and Testing
The API 650 rules API 653 inspectors use: one-foot method design and hydrotest shell thickness, minimum nominal shell thickness, design metal temperature, material groups and impact testing, opening details and weld spacing.
Key facts
- One-foot method (tanks up to 200 ft diameter): td = 4.9D(H − 1)G ÷ Sd + CA and tt = 4.9D(H − 1) ÷ St, in US units.
- Minimum nominal shell thickness: under 50 ft diameter 3/16 in.; 50 to under 120 ft 1/4 in.; 120 to 200 ft 5/16 in.; over 200 ft 3/8 in.
Flashcards
Exam tip: Don't mix API 650 and API 653 formulas: 650 uses 4.9 for new or reconstructed design; 653 uses 2.6 with existing-tank stresses for continued service.
6. Damage Mechanisms, Cathodic Protection and Linings
The RP 571 mechanisms on the API 653 effectivity sheet (atmospheric corrosion, CUI, concentration cell corrosion, MIC, soil corrosion, ethanol SCC, sulfuric acid corrosion and others), cathodic protection of tank bottoms (RP 651) and bottom linings (RP 652).
Key facts
- MIC and soil corrosion attack tank bottoms from the soil side; water bottoms and sludge promote MIC on the product side.
- Ethanol SCC affects carbon steel in fuel-grade ethanol and blends, mostly near welds that were not stress relieved.
Flashcards
Exam tip: CP questions are short definitions. Learn the two criteria and the difference between galvanic and impressed-current systems and you have most of RP 651.
7. Welding and NDE
ASME Section IX qualification of procedures and welders, API 577 welding inspection and metallurgy, ASME Section V NDE methods, and the NDE that API 653 and API 650 require on tank welds.
Key facts
- PQR tension tests pass at the base metal's minimum specified tensile strength, or within 5% below it when the break is in the base metal outside the weld (QW-153).
- Bend tests fail with any open discontinuity over 1/8 in. (3 mm) on the convex surface (QW-163).
Flashcards
Exam tip: Section IX questions are the same across API 510, 570 and 653. Time spent on QW-150 to QW-160 tests and QW-300 performance rules pays off on every exam.
How to pass
- 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.
Frequently asked questions
How long should I study for the API 653 exam?
Most working candidates need 4 to 8 weeks at 45 to 60 minutes a day. Book your exam once your Readiness Score has stayed at 80 or higher for about a week.
Can I take the API 653 exam online?
No. The API 510, 570 and 653 exams are taken in person at Prometric test centers only; API does not offer them by remote proctoring. 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. Check API's exam pages for the current rules.
What score do I need to pass the API 653 exam?
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. The exam has 170 multiple-choice questions (140 scored, 30 unscored pretest items): 110 closed-book and 60 open-book and a time limit of 7.5-hour exam day: short tutorial, 2.75 hours closed book, 45-minute lunch, 3.75 hours open book.
What are the most tested topics?
The outline covers shell evaluation and minimum thickness, bottoms, settlement and foundations, inspection intervals and practices, repairs, alterations and hot taps, api 650 design, materials and testing, damage mechanisms, cathodic protection and linings, welding and nde. Use the free practice test to see which ones you need most.
The premium API 653 study path
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.
Your step-by-step study path
- 1Shell Evaluation and Minimum Thickness9 key notes, 115 flashcards, then 450 practice questions in 18 sets of 25Notes free with an account
- 2Bottoms, Settlement and Foundations9 key notes, 115 flashcards, then 425 practice questions in 17 sets of 25Premium
- 3Inspection Intervals and Practices9 key notes, 115 flashcards, then 425 practice questions in 17 sets of 25Premium
- 4Repairs, Alterations and Hot Taps9 key notes, 115 flashcards, then 425 practice questions in 17 sets of 25Premium
- 5API 650 Design, Materials and Testing8 key notes, 115 flashcards, then 425 practice questions in 17 sets of 25Premium
- 6Damage Mechanisms, Cathodic Protection and Linings8 key notes, 115 flashcards, then 425 practice questions in 17 sets of 25Premium
- 7Welding and NDE8 key notes, 110 flashcards, then 425 practice questions in 17 sets of 25Premium
- Final step: 6 full mock exams170 questions each, timed like test day. Pass them, then book your exam.Premium
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Summarized from API's official candidate information and the rules cited, checked November 2026. Confirm current requirements with American Petroleum Institute (API) Individual Certification Programs (ICP).