This tool builds a block-randomized allocation list for two to four arms. Pick your block size, total enrollment and a seed, and it generates a reproducible list instantly. It is a planning tool for checking a design, not your locked, concealed study randomization list.
Free sandbox · No credit card · 21 CFR Part 11 aligned
Day 28 is an Arm B only visit, so Arm A participants never see it.
Free tool
Pick your arms, block size and sample size. Nothing here is saved or sent anywhere; it runs entirely in your browser.
| Allocation # | Arm |
|---|---|
| 1 | A |
| 2 | A |
| 3 | B |
| 4 | B |
| 5 | A |
| 6 | B |
| 7 | B |
| 8 | A |
| 9 | A |
| 10 | B |
| 11 | A |
| 12 | B |
| 13 | A |
| 14 | B |
| 15 | B |
| 16 | A |
| 17 | B |
| 18 | A |
| 19 | A |
| 20 | B |
Why block randomization
Simple randomization can drift: by chance, one arm ends up with noticeably more participants than another, especially in a small trial. Block randomization fixes that by shuffling a balanced block, say two of arm A and two of arm B, then repeating it, so the arms stay close to even the whole way through enrollment.
Day 28 is an Arm B only visit, so Arm A participants never see it.
Before you lock this in
Generate your locked list inside the actual study once the protocol is final.
A real study keeps the upcoming assignment hidden from whoever enrolls the next participant.
Some studies randomize within a stratum, such as site or age group, rather than one list for everyone. That needs a separate list per stratum.
Who was randomized, when, and to which arm, kept permanently.
Next step
Capture randomizes a participant the moment enrollment criteria are confirmed, using the allocation list and blocking you set up in the study build, not a spreadsheet passed around by email.
Site risk
Site 03 needs attention
Action items
2 overdue
Free sandbox with every feature. No credit card, no sales call.
Block randomization: a balanced block of arms is shuffled and repeated until the target enrollment is reached, keeping the arms close to even throughout.
No. The seed makes the list reproducible for planning purposes; it is not a cryptographically secure random number generator. Do not use this list as your locked, concealed study allocation.
Not with this tool today. It supports two to four arms.
No. The list is generated entirely in your browser and is not saved or sent to a server.
Allocation concealment means whoever enrolls the next participant cannot see the upcoming assignment in advance. This planning tool does not provide that; a live study needs the assignment generated and hidden inside the actual system.
Yes. The free sandbox includes full randomization functionality with sample data, no credit card required.
Keep exploring
Clinical trial sample size calculator
The planning tool that usually comes before this one.
Clinical trial randomization software
How randomization works inside a live study.
Block randomization software
More on the blocking method this tool demonstrates.
Randomization without a CRO
Running your own randomization instead of outsourcing it.
Clinical trial study builder software
Where the study this list belongs to gets built.
Free sandbox with every feature. No credit card, no sales call.