NIH grants explained: R, K, F, T — what the letters mean

Published April 2, 2026 · Updated Jun 12, 2026

The National Institutes of Health (NIH) issues thousands of grants a year, all organized into "activity codes" that start with a letter. Picking the right activity code is half the battle.

R series — Research project grants

R01 — The flagship. Investigator-initiated, 3–5 years, typically $250K–$500K/year in direct costs. This is what a tenured professor gets.

R03 — Small grant, 2 years, up to $50K/year. Good for pilots, new methods, or secondary data analysis.

R21 — Exploratory/developmental, 2 years, up to $275K total. Higher-risk ideas. Does not require preliminary data.

R15 (AREA) — Academic Research Enhancement Award. Only for primarily undergraduate institutions. 3 years, up to $300K direct.

R41/R42 — Small Business Technology Transfer (STTR). R43/R44 — Small Business Innovation Research (SBIR). For small businesses partnering with research institutions.

K series — Career development awards

K awards pay part of your salary so you can spend 75% of your time on research while being mentored.

K08 — Mentored Clinical Scientist Research Career Development. For clinicians moving into research.

K23 — Mentored Patient-Oriented Research. Clinical research track.

K99/R00 — Pathway to Independence Award. 2 years mentored + 3 years independent. The "get-a-faculty-job" grant.

K24 — Midcareer Mentoring Award. For established clinicians who mentor trainees.

F series — Fellowships

F30 — Predoctoral MD/PhD fellowship. F31 — Predoctoral PhD fellowship. F32 — Postdoctoral fellowship. 2–3 years.

T series — Training grants

T32 — Institutional training grant. Funds a department to support multiple trainees. You apply as a trainee through your program director, not directly.

Which one should you apply for?

  • Graduate student → F31 or F30
  • Postdoc → F32 (or ride a T32)
  • Early-career faculty, clinician-researcher → K08 or K23
  • Postdoc transitioning to faculty → K99/R00
  • Independent faculty → R21 for pilot, then R01
  • Small business with a biomedical idea → SBIR (R43/R44)
  • Undergraduate institution faculty → R15

The NIH paylines reality

NIH funds roughly the top 10–20% of applications per study section. Your score has to beat the "payline" for that Institute in that cycle. Some Institutes are more generous (NIGMS), some are tight (NCI).