usmle

Published on Jul 28, 2026 by Dr. Leila Haddad

NBME Step 1 & Free 120 Score Conversion Guide [2026]

Master official NBME practice exams (Forms 25-31) and Free 120 in 2026. Includes 95%+ passing score conversion tables and readiness tracking.

NBME score conversion and passing probability chart for Step 1

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Official Comprehensive Basic Science Self-Assessments (CBSSAs)—commonly known as NBME practice exams—and the official Free 120 practice questions are the gold standard for predicting USMLE Step 1 readiness (NBME Portal, 2026). This guide outlines how to interpret your scores, convert raw percentages into passing probabilities, and structure your final practice exam sequence.

Key Takeaways

  • Official NBME Forms 25 through 31 provide an Equated Percent Correct score and an estimated probability of passing Step 1 if taken within 1 week (NBME Portal, 2026).
  • Scoring ≥ 65% on an NBME form corresponds to a >95% probability of passing the real exam.
  • The Free 120 (a 120-question sample set provided by USMLE) is the closest reflection of test-day question stem length and interface style.

What Are NBME Forms 25–31 and How Do They Work?

The NBME currently offers multiple active CBSSA forms online ($60 per form):

  • Forms 25 to 30: Core self-assessment forms covering all organ systems and basic science disciplines.
  • Form 31: The newest official form, reflecting updated question stem lengths and high-yield image items.

To understand how NBME content maps to official exam specifications, review our Complete USMLE Step 1 Guide.

How Does NBME Score Conversion & Probability Tracking Work?

When you complete an online NBME self-assessment, your report displays an Equated Percent Correct (EPC) score alongside a statistical passing probability:

Equated Percent Correct (EPC)Approximate Scaled Score EquivalentEstimated Probability of Passing (within 1 week)
< 55%< 180High Risk of Failure (< 50%)
58% - 61%186 - 192Moderate Risk (65% - 80%)
62% - 64%193 - 195Borderline (85% - 92%)
65% - 69%196 - 205Low Risk (95% - 98%)
≥ 70%≥ 210Very Low Risk (> 99%)

To evaluate overall passing standards, see our USMLE Step 1 Pass Rate & Passing Score Breakdown.

What Is the Role of the Free 120 Practice Exam?

The Free 120 consists of 120 real retired USMLE Step 1 questions (divided into three 40-question blocks):

  1. Interface Familiarity: Uses the exact NBME Prometric software interface (font sizes, calculator, strike-through, and lab value popup windows).
  2. Predictive Value: A score of ≥ 68 - 70% on the Free 120 taken 3 to 5 days before your exam date confirms strong readiness.
  3. Practice at Prometric Option: Candidates can register to take the Free 120 directly at a Prometric test center for a ~$75 fee to experience a complete test-day dress rehearsal.

For registration and Prometric test center logistics, consult our USMLE Step 1 Registration & Prometric Guide.

  1. 4 Weeks Before Exam: NBME Form 27 or 28 (establishing baseline probability).
  2. 3 Weeks Before Exam: NBME Form 29.
  3. 2 Weeks Before Exam: NBME Form 30.
  4. 1 Week Before Exam: NBME Form 31.
  5. 3 to 5 Days Before Exam: Official Free 120.

To practice with high-yield clinical sample questions, visit our USMLE Step 1 Practice Questions Guide.

Do not panic if your score drops slightly on one NBME form. Variations of ± 3% are normal due to form-specific topic density. Look for a consistent trend across 3 consecutive assessments before making final scheduling decisions.

To integrate these practice exams into a complete study schedule, see our USMLE Step 1 Dedicated Study Plan and compare leading question banks in our Best USMLE Step 1 Qbanks Review.

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