Online sample size calculator for pilot studies


One of the goals of a pilot study is to identify unforeseen problems, such as ambiguous eligibility criteria or misinterpretations of questionnaire items. The formula below can be used to calculate the sample size needed to be able to identify, with a chosen level of confidence, problems that may arise with a given probability.



ˇ          Confidence:fill in how certain you want to be that you will detect any existing problem

ˇ          Probability:fill in how prevalent a problem should be in order to be important enough for you to want to detect it

ˇ          Use fraction format (e.g. 0.95) instead of percentage (95%)

ˇ          Press Calculate

ˇ          Round result upwards to nearest integer




Example and interpretation


Confidence: 0.95

Probability: 0.05

Sample size: 58.4


Problems with a prevalence of 5% will almost certainly be identified (with 95% confidence) in a pilot study including 59 participants.




If you have used this formula please refer to the following article:


W Viechtbauer, L Smits, D Kotz, L Budé, M Spigt, J Serroyen, R Crutzen : A simple formula for the calculation of sample size in pilot studies . J Clin Epidemiol 2015 Nov;68(11):1375-9