Use this tool to create a blocked randomisation list for your trial. The generated lists are suitable for use with our simple randomisation service. Unsure about block sizes? Use our simulation tool to help you decide.

Create a list

Seed for random number generator

A comma separated list of treatments

A comma separated list of block sizes — must be multiples of the number of treatments

name: category 1, category 2, …

How to cite this tool

Sealed Envelope Ltd. 2022. Create a blocked randomisation list. [Online] Available from: [Accessed 16 Jun 2024].


Pocock SJ. Clinical Trials: A Practical Approach. Wiley; 1983.


This value is used to initialise our pseudo-random number generator. The same list will be always be created provided you specify the same seed and other parameters .
Treatment groups

A comma separated list of treatments. For unequal allocation duplicate the treatment name, e.g.

Group A, Group A, Group B

for a 2:1 allocation ratio.

Block sizes
A comma separated list of block sizes. The sizes must be multiples of the number of treatments. Use our simulation tool to help you decide on suitable sizes.
List length

The minimum number of rows to generate. The generated list may be slightly longer than this because of the need to fill blocks.

Important! If you are using stratification, you must make sure that the list is long enough to cover the maximum number of randomisations you expect to perform within any stratum.

The safest option is to set the list length to sample size x number of strata.

Stratification factors

Must be formatted

name: category 1, category 2, …

For example, to stratify by age you could use

Age group: Under 30, 30 - 50, Over 50

Randomisation code
If you select this option an extra column will be produced containing a unique randomisation code. The code is in the format of two letters followed by 1 or more digits (depending on list length). For instance, AB1, HK6, ZF8 etc. The two letter prefixes are chosen at random and will never be the reverse of any other two letter prefix. For instance, if the prefix TD is chosen, then the prefix DT will not appear.

✽ For lists created before March 2015 please use this list generator to recreate your list.