Pilot & Feasibility Studies · PILOT-SS

Sample Size for Pilot and Feasibility Studies

Plan a sample around the feasibility question the pilot must answer—not around an intervention effect.

1. Planning objective

What must the pilot study establish?

Estimate a feasibility proportionRetention, consent, adherence, fidelity, or another percentage.Proportion precision Estimate a recruitment or feasibility rateRecruitment per month, week, day, site-month, or another exposure unit.Poisson precision Define progression criteriaDistinguish an unacceptable value from an acceptable value.Decision thresholds Detect at least one important problemPlan a high probability of observing a problem if it occurs at a specified rate.Occurrence probability

Need a starting point?Load the worked example for the selected objective.

2. Feasibility parameter and denominator

Precision for a proportionPILOT-PROP-CI

Feasibility parameter

Statistical denominator

Equal-group allocation

Optional. This only rounds the calculated total so every group has the same size; it does not change the statistical method.

3. Planning assumptions

Interval method

Use compatibility mode only to reproduce the BMJ tutorial convention.

Confidence interval direction

Upper is suitable for undesirable outcomes such as dropout; lower for desirable outcomes such as retention.

Anticipated proportion (%)

Enter a percentage, not a decimal.

Target margin / half-width (percentage points)

For a one-sided Wilson limit, this is the distance from the anticipated proportion to the selected limit.

Confidence level (%)

4. Required pilot size

Planning record

Item	Finding

Protocol reporting

Methods

Combine several feasibility objectives0 of 6 requirements

Calculate a requirement, then add it here to compare compatible denominators.

The largest compatible requirement determines each denominator-specific target. Participant counts and site-time remain separate.

Objective	Method	Denominator	Requirement	Action

Planning uptake from approached or screened to randomized (%) optional

About this calculator

Scope

This calculator supports external pilot trials and quantitative feasibility studies. It does not establish intervention efficacy or safety, replace protocol-specific statistical consultation, or determine progression by itself.

Why the denominator matters

Recruitment uptake uses eligible people approached; retention uses randomized participants; fidelity may use the intervention arm; and recruitment rates use site-time. Requirements with different denominators must not be treated as interchangeable.

Implemented methods

PILOT-PROP-CI: two-sided Wilson score intervals, directional one-sided Wilson limits, and a separately labelled BMJ tutorial compatibility convention for a feasibility proportion.
PILOT-RATE-CI: Poisson score precision for a recruitment or other event rate.
PILOT-PROG-PROP: Lewis normal-approximation progression design with continuity correction.
PILOT-PROG-RATE: exact one-sample Poisson progression design.
PILOT-PROBLEM: probability of observing at least one important problem.

Important method detail

The recommended proportion method uses Wilson—not the Wald formula used by the general single-proportion calculator. The directional Wilson option directly constrains the selected upper or lower limit. BMJ tutorial compatibility instead reproduces the companion app's presize Wilson-width convention and is reported separately. For an anticipated 80%, two-sided 95% confidence, and a 10-percentage-point half-width, Wilson requires 60 denominator units. Progression-proportion results are always rounded upward.

When not to use this calculator

Do not use it to test efficacy, derive an intervention effect from a small pilot, formally assess safety, optimize a continuous-outcome variance estimate, or plan a clustered, adaptive, multilevel, or internally piloted design without protocol-specific statistical work.

References

Ying X, et al. Determining sample size for pilot trials: a tutorial. BMJ. 2025;390:e083405.
Eldridge SM, et al. CONSORT extension to randomised pilot and feasibility trials. BMJ. 2016;355:i5239.
Lewis M, et al. Determining sample size for progression criteria for pragmatic pilot RCTs. Pilot and Feasibility Studies. 2021;7:40.
Viechtbauer W, et al. A simple formula for calculation of sample size in pilot studies. J Clin Epidemiol. 2015;68:1375–1379.

Research-planning disclaimer: Review the feasibility objective, denominator, assumptions, progression criteria, and calculation before using a result in a protocol.

Suggested citation

Yaacob, Najib Majdi. “Sample Size for Pilot and Feasibility Studies.” Web calculator. NajibMY, 2026. Accessed September 9, 2026. https://najibmy.netlify.app/tools/sample-size/pilot-feasibility-studies/.