Clinical trial sample size, calculated in your browser
This calculator estimates how many participants a two-arm trial needs, using the two-proportion z-test most early-phase and pilot studies rely on. Set your expected response rates, significance level and power, and it returns a per-arm and total number instantly, adjusted for dropout.
- Two-proportion z-test
- Adjusts for dropout
- No sign-up required
Free sandbox · No credit card · 21 CFR Part 11 aligned
BMI (Body Mass Index)
Auto-derivedHeight (cm)
172
Weight (kg)
68.5
Result
23.2 kg/m²
Reference: WHO · shown read-only during data entry
Free tool
Calculate your sample size
Move the sliders. Nothing here is saved or sent anywhere; it runs entirely in your browser.
You need, per arm
108
91 completers + dropout margin
Total to enroll (2 arms)
216
Two-proportion z-test, equal allocation. A rough planning estimate, not a substitute for a biostatistician on your protocol.
The formula
What this calculator actually computes
This tool uses the standard two-proportion z-test for a two-arm superiority design: n per arm equals (z-alpha/2 plus z-beta) squared, times the sum of each rate’s variance, divided by the squared difference between the two rates. It is the same formula behind most published protocols for a binary outcome, rounded up and then inflated for the dropout rate you expect.
- Works for any two response rates between 1% and 99%.
- Choose a two-sided alpha of 0.10, 0.05 or 0.01.
- Choose 80 to 95% power.
Stage 1
DraftBuild and edit freely
Stage 2
ApprovedLocked for live use
Stage 3
In the casebookOnly approved forms
SDV requirement changed on Systolic BP
Data manager · Reason: critical safety value
Form approved: Vital Signs (VS) v2
Study owner · Locked for live use
Answer changed: Weight 68.0 to 68.5 kg
Site coordinator · Reason: transcription error
Before you lock this in
What this tool does not replace
A biostatistician on your protocol
This is a planning estimate. Share the inputs with a biostatistician before you finalize a statistical analysis plan.
The right formula for a continuous outcome
This tool covers binary outcomes (response or no response). A mean-difference design uses a different calculation.
More than two arms
This calculator assumes one treatment versus one control.
A realistic dropout estimate
A long follow-up with many visits usually loses more participants than a short one. Adjust the slider accordingly.
Next step
Turn this number into a study you can actually run
A sample size is only useful once it sits inside a real study build: a visit schedule, eCRFs, randomization and enrollment tracking. Capture’s AI study builder reads a protocol and drafts the visit schedule and forms for you, so you go from this number to a working study the same day.
- Upload a protocol PDF and get a drafted visit schedule.
- Randomize participants once enrollment opens.
- Track enrollment against this target in real time.
Build the study this number belongs to
Free sandbox with every feature. No credit card, no sales call.
What formula does this calculator use?
The two-proportion z-test for a two-arm superiority design, the standard approach for a binary outcome with equal allocation between arms.
Is this a substitute for a biostatistician?
No. It is a fast planning estimate. Any protocol going to a regulator or an IRB should have its final sample size confirmed by a biostatistician.
What if my outcome is a continuous measure, not a response rate?
This tool does not cover that case. A continuous outcome (a mean difference between arms) uses a different formula; ask a biostatistician for the right one.
Does this tool save or send my inputs anywhere?
No. The calculation runs entirely in your browser. Nothing is saved, logged, or sent to a server.
Can I use this for more than two arms?
Not directly. This calculator assumes one treatment arm compared with one control arm.
Is the calculator really free, no account needed?
Yes. It runs on this page with no sign-up. Creating a free sandbox account is only needed if you want to build the actual study.
Keep exploring
Related pages
Randomization list generator
The next planning tool once you know your enrollment target.
Clinical trial study builder software
Turn a protocol into a drafted study in minutes.
CRF builder for clinical trials
Where the eCRFs for this study get built.
Clinical trial randomization software
How allocation works once enrollment opens.
EDC for Phase 1 Clinical Trials
A common home for small, early-phase enrollment targets.
Turn this number into a working study
Free sandbox with every feature. No credit card, no sales call.