- What Is Actually Known About the NBII Pass Rate
- What the National Board Confirms About the Exam
- Why a Single Pass-Rate Number Would Mislead You
- Where Candidates Realistically Lose Points
- The Nine Domains and What Each Demands
- Why Code Calculations Deserve Extra Attention
- Who Sits This Exam and Who Hires for It
- Sequencing Your Preparation by Domain
- Frequently Asked Questions
- The National Board does not publish a pass rate in the sources checked for this article, so any specific percentage you see should be treated skeptically.
- The exam is 85 questions administered in one day; exact timer, current fee and pass mark are unverified here.
- The scope document, NB-331-IS Revision 7, lists nine body-of-knowledge headings without published percentage weights.
- Required references include NBIC Parts 1 and 2, RCI-1/NB-263, ASME Sections I, IV and VIII Division 1, B31.1 and CSD-1.
What Is Actually Known About the NBII Pass Rate
If you searched for the National Board Inservice Inspector Commission pass rate hoping for a tidy percentage, here is the honest answer: the issuer information and scope document reviewed for this article do not publish one. No cohort statistics, no year-over-year trend, no first-attempt versus retake split. Anyone quoting a precise figure without citing the National Board of Boiler and Pressure Vessel Inspectors directly is guessing, or worse, borrowing a number from a different credential that happens to share an acronym.
That matters because the Inservice Inspector Commission is a very specific thing. It is the commission issued through the National Board for inspectors who perform inservice inspection of boilers and pressure vessels, and it is tested against a defined scope document, NB-331-IS Revision 7. Statistics from other certifications with similar-sounding names tell you nothing about this exam. If you are still orienting yourself, the explainers on what NBII is and what NBII stands for clarify exactly which credential this site covers.
This article therefore does something more useful than repeating an unverifiable statistic. It lays out what the issuer does confirm, explains why a raw pass rate would be a poor planning tool even if one existed, and shows where preparation effort realistically separates passing candidates from those who need another attempt. For the broader difficulty picture, see How Hard Is the NBII Exam? Complete Difficulty Guide 2026.
What the National Board Confirms About the Exam
The verified facts are short, and it is worth being precise about them.
- Governing body: The National Board of Boiler and Pressure Vessel Inspectors.
- Scope document: NB-331-IS Revision 7 (the PDF is filed under the URL name NB-331I), organized under nine detailed body-of-knowledge headings.
- Format: Issuer examination information confirms 85 questions administered in one day.
- Weights: The scope document does not publish percentage weights for the nine headings.
What is not verified here: the exact exam timer, the current fee, and the pass mark. Do not assume a time limit from the phrase "one day"; a one-day administration does not tell you how long the exam portion lasts. For the most current numbers, check the National Board directly, and use our pages on the NBII passing score and NBII certification cost as orientation rather than as a substitute for the issuer's own notices.
| Item | Status |
|---|---|
| Number of questions | 85, confirmed by issuer examination information |
| Administration | One day, confirmed |
| Body-of-knowledge headings | Nine, listed in NB-331-IS Revision 7 |
| Percentage weights per heading | Not published |
| Exam timer | Unverified here |
| Current fee | Unverified here |
| Pass mark | Unverified here |
| Published pass rate | Not found in reviewed sources |
Eligibility and prerequisites are a separate gate from the exam itself. Candidates should review NBII requirements and the issuer's own documentation before assuming they can sit the exam on a particular date, and check NBII exam dates for scheduling context.
Why a Single Pass-Rate Number Would Mislead You
Even if the National Board published an aggregate figure tomorrow, it would be a weak guide for your own plan. Here is why.
Candidate Backgrounds Are Wildly Uneven
People arrive at this exam from very different places. Some have years of boiler and pressure vessel field experience, including repair and alteration work under NBIC Part 3. Others come from insurance or jurisdictional roles where they have seen the paperwork but less of the physical equipment. A blended statistic hides that spread. A welder-turned-inspector and a candidate who has never opened a manway are not the same population.
The Exam Is Reference-Driven
The required references (RCI-1/NB-263; NBIC Parts 1 and 2; ASME Sections I, IV and VIII Division 1; B31.1; and CSD-1) span multiple codes with different scopes. Passing is less about raw intelligence and more about whether you know which document governs which situation. A pass rate cannot capture how many candidates were fluent in code navigation versus guessing from memory.
Unknown Weights Change How You Should Read Any Number
Because the scope document lists nine headings without percentage weights, you cannot assume equal coverage, and you cannot safely skip a heading on the theory that it is lightly tested. A candidate who is strong on four domains and weak on five will not be rescued by an average.
Where Candidates Realistically Lose Points
Without published failure statistics, we can still reason from the structure of the scope document about where the exam is demanding. Three characteristics stand out.
- Breadth across codes. One question may hinge on an NBIC Part 2 inspection requirement, the next on an ASME Section VIII Division 1 calculation, the next on a CSD-1 control requirement.
- Quantitative content. The calculations heading covers minimum thickness, MAWP, static head, corrosion rate, remaining life and inspection intervals. These reward practice with actual formulas, not recognition.
- Applied judgment. Deterioration, relief device performance and pressure test selection require you to reason from a scenario to a conclusion, not recall a definition.
For a sense of how these challenges compare, the difficulty guide linked above goes further, and the complete guide to all 9 content areas walks each heading in detail.
The Nine Domains and What Each Demands
Because no percentage weights are published, the most defensible preparation approach is to cover all nine headings and then invest extra time where your own practice scores are lowest. Here is what the scope document asks of you in each.
Domain 1: Inservice Inspection
The operational core of the commission, covering NBIC Part 2 inspection practice and NBIC Part 3 repair and alteration methods.
- Internal and external inspections
- Safety and confined-space entry
- Venting and fuel supply
- Deterioration and hazards encountered in the field
- Repair and alteration methods, including welding
Domain 2: Boiler and Pressure Vessel Types and Terminology
Vocabulary and identification. You must be able to recognize boiler and vessel types and name their parts correctly, because later questions assume that fluency.
- Identifying boiler types
- Identifying pressure vessel types
- Naming components and parts accurately
Domain 3: Conditions Causing Deterioration or Failures
One of the most detailed headings in the scope document, spanning both damage mechanisms and the quantitative assessment of damage.
- Uniform, galvanic, erosion, crevice, pitting and line corrosion
- Exfoliation, selective leaching, grooving and stress-corrosion cracking
- Chemical attack, creep, fatigue, fracture and thermal aging
- Corrosion rate and averaging, local metal loss, weld joint efficiency
- Corrosion in the central portions of formed heads, and remaining life
Domain 4: Installation
Governed by NBIC Part 1 and jurisdictional requirements, across a wide range of equipment.
- Steam and hot-water heating boilers, hot-water supply boilers and potable-water heaters
- Vessels and piping, welding, venting and fuel systems
- Source and discharge requirements for power and heating boilers
- Operating systems, controls, gages and relief devices
Domain 5: Code Calculations
The numerical heading, covered in more depth in the next section.
- Minimum thickness and MAWP for tubing, piping, drums, shells, headers and heads
- Differences between vessel MAWP and component MAWP
- Static head and total pressure
- Metal loss averaging, corrosion rates, remaining service life and inspection intervals
Domain 6: Nondestructive Examination
Know the requirements and principles of each method well enough to choose and interpret them.
- Radiography
- Ultrasonic examination
- Magnetic-particle examination
- Liquid-penetrant examination
- Visual examination
Domain 7: Pressure Testing
Which test, when, and what it demonstrates.
- Liquid (hydrostatic-style) tests
- Pneumatic tests
- Initial-service leak tests
- Vacuum tests
Domain 8: Pressure Relief Devices
A safety-critical heading with both theory and practical inspection content.
- Application and limitations of devices
- Pressure and capacity sizing
- Set pressures and deviations
- Causes of poor performance
- Reasons for and frequency of inspection, inspection and test procedures, and maintenance safety
Domain 9: Controls and Safety Devices
Where CSD-1 knowledge becomes essential.
- Fuel controls
- Waterside and fireside controls
- Limit switches and device types
- Level indicators
Why Code Calculations Deserve Extra Attention
The calculations heading is where the exam stops rewarding familiarity and starts demanding execution. The scope document explicitly lists minimum thickness and MAWP across a range of components (tubing, piping, drums, shells, headers and heads), plus the relationship between vessel MAWP and the MAWP of individual components. Add static head and total pressure, and you have a cluster of problems where a unit slip or a wrong formula costs the whole question.
The same heading links directly to Domain 3. Corrosion rate, metal-loss averaging, remaining service life and inspection intervals appear in both. A candidate who understands that a measured thickness loss over a known time becomes a rate, which becomes a remaining life, which sets the next inspection interval, has mastered a chain of reasoning that earns points in two domains at once.
Key Takeaway
Practice calculations by working the full chain: measured thickness, required minimum thickness, corrosion rate, remaining life, inspection interval. Doing the chain once end to end teaches more than twenty isolated formula drills, and it covers material from both Domain 3 and Domain 5.
Because the exam is built around referenced codes, learn where each formula lives (ASME Section I for power boilers, Section IV for heating boilers, Section VIII Division 1 for pressure vessels, B31.1 for power piping) rather than memorizing results. A one-page recap of must-know items is available in the NBII cheat sheet.
Who Sits This Exam and Who Hires for It
The Inservice Inspector Commission is oriented toward people whose daily work involves boilers and pressure vessels after they are installed and operating. Typical employers and settings include jurisdictions that enforce boiler and pressure vessel law, insurance companies that inspect insured equipment, and owner-user organizations that maintain their own inspection programs. Repair organizations also value the knowledge, particularly the NBIC Part 3 repair and alteration content.
We are deliberately not quoting salary figures here, since none are verified in the issuer materials. For earning context, read the NBII salary guide, the ROI analysis, and the overview of NBII jobs. For a career-level view of what the commission represents, see NBII certification.
Sequencing Your Preparation by Domain
Generic study advice is abundant. What is useful for this exam is ordering, because several domains build on others. The sequence below reflects dependencies in the scope document, not a magic number of weeks; stretch or compress it to fit your schedule.
Vocabulary and Equipment
- Domain 2 first: types, parts, terminology
- Skim Domain 4 installation layouts so the equipment becomes concrete
Damage and Inspection
- Domain 3 deterioration mechanisms
- Domain 1 inspection practice and NBIC Parts 2 and 3
- Domain 6 NDE methods, tied to the damage they detect
Numbers
- Domain 5 calculations, using the full thickness-to-interval chain
- Revisit corrosion rate material from Domain 3
Safety Systems and Testing
- Domain 8 relief devices
- Domain 9 controls and CSD-1
- Domain 7 pressure tests
Integration
- Timed mixed-domain practice across all nine headings
- Targeted review of whichever domains scored lowest
For a fuller plan, see the NBII study guide, and consider structured NBII training options if you want instructor-led coverage of the code books. When you are ready to test yourself across all nine domains, start with the practice test platform.
Frequently Asked Questions
None was found in the issuer information and scope document reviewed for this article. Treat any specific percentage without a named, dated source as unreliable.
Issuer examination information confirms 85 questions administered in one day. The exact timer, current fee and pass mark are not verified here, so confirm them with the National Board.
The scope document, NB-331-IS Revision 7, lists nine body-of-knowledge headings but publishes no percentage weights. Plan to cover all nine and let your practice scores guide extra study time.
The required references include RCI-1/NB-263; NBIC Parts 1 and 2; ASME Sections I, IV and VIII Division 1; B31.1; and CSD-1. Knowing which document governs which situation is a core skill.
See our page on the NBII passing score for what is and is not known, and check the National Board directly for the current official requirement.
The takeaway on pass rates is simple: the public data is thin, so your best predictor is your own performance across all nine domains. For the latest on this topic as we update it, bookmark the NBII pass rate page.