Calculate patient-years (person-years) of exposure from the number of patients and mean follow-up, with an optional incidence rate per 100 patient-years.

Conversions use 365.25 days per year, 12 months per year and 7 days per week; they are not exact calendar durations.

Event rate
Blank omits the rate. Enter 0 for an observed zero; use events from the same at-risk exposure.

Disclaimer: For educational/statistical estimation only (not medical advice). For clinical studies, patient‑years are typically calculated as the sum of each participant’s exposure/follow‑up time and incidence/CIs depend on modeling assumptions (e.g., Poisson); follow your protocol/statistical analysis plan and consult a qualified biostatistician for interpretation.

Related Calculators

Patient Year Exposure Formula

The following formula is used to calculate the patient year exposure for a given number of patients and duration of exposure.

PYE = N × D

Note: In general, patient‑years (person‑time) are calculated as the sum of each participant’s exposure/follow‑up time. The formula PYE = N × D equals that sum when D is the actual arithmetic mean follow-up of those same N patients. Equal follow-up is one special case. Do not use nominal study duration as the mean when entry, censoring, discontinuation or losses shorten observation.

Formula source:  Hazra A, Gogtay N. Biostatistics Series Module 8: Assessing Risk. Indian J Dermatol. 2017;62(2):123–129.

Variables:

  • PYE is the patient year exposure
  • N is the number of patients
  • D is the actual arithmetic mean exposure per patient in years

Multiply the number of patients by their actual mean exposure in years, or sum individual eligible follow-up durations. The optional event rate is events divided by total patient-years, multiplied by100. Blank events omit the rate; zero is an observed count. Zero exposure has no defined event rate. Conversions use365.25 days/year,12 months/year and7 days/week, not exact calendar intervals.

What is Patient Year Exposure?

Patient year exposure is a measure used in clinical studies to quantify the amount of exposure to a treatment or condition across a population over a period of time. It is calculated by summing eligible individual follow-up, equivalently patient count times the actual mean follow-up in years. Exposure totals and event rates alone do not establish individual risk, safety or treatment efficacy.

How to Calculate Patient Year Exposure?

The following steps outline how to calculate the Patient Year Exposure.

  1. First, determine the number of patients (N).
  2. Next, determine the actual mean eligible exposure per patient in years (D), accounting for the study protocol and censoring.
  3. Finally, calculate the patient year exposure using the formula PYE = N * D.
  4. After inserting the values and calculating the result, check your answer with the calculator above.

Example Problem : 

Use the following variables as an example problem to test your knowledge.

Number of patients (N) = 100

Duration of exposure in years (D) = 2