Prison population and capacity
Isis · August 2026
Isis prison is holding 599 people in the 478 certified places in use — 125% full.
Read from the Ministry of Justice’s August 2026 bulletin, in the vintage we archived on 11 October 2026. The count was taken on 24 August 2026.
125%
full
599 people in accommodation certified for 478, August 2026
That is the 25th-fullest of the 120 establishments in the August 2026 bulletin.
It has held more people than its certified places in use in every month we hold, back to March 2018.
The fullest establishments in August 2026
- … 14 establishments between
- out of 120 establishments with accommodation in use
Population divided by in-use certified normal accommodation, for every establishment in the same bulletin. Establishments with no accommodation in use have no percentage and are left out of the ranking rather than placed at either end of it.
Every month we hold for Isis
One cell per month from Mar 2018 to Aug 2026, shaded from this establishment’s own emptiest month to its fullest. The record starts in March 2018 because that is the first bulletin published as a spreadsheet; earlier months exist only as word-processor files we will not guess at.
The last twelve months
| Month | Population | In-use CNA | Full |
|---|---|---|---|
| Aug 2026 | 599 | 478 | 125% |
| Jul 2026 | 593 | 478 | 124% |
| Jun 2026 | 587 | 478 | 123% |
| May 2026 | 559 | 478 | 117% |
| Apr 2026 | 561 | 478 | 117% |
| Mar 2026 | 594 | 478 | 124% |
| Feb 2026 | 588 | 478 | 123% |
| Jan 2026 | 580 | 478 | 121% |
| Dec 2025 | 589 | 478 | 123% |
| Nov 2025 | 599 | 478 | 125% |
| Oct 2025 | 597 | 478 | 125% |
| Sep 2025 | 606 | 478 | 127% |
What the bulletin publishes, and what it means
- Baseline CNA478
- The sum total of all certified accommodation in an establishment, except normally cells in punishment or segregation units and healthcare cells not used for long-stay patients.
- In-use CNA478
- Baseline CNA less those places not available for immediate use — damaged cells, cells affected by building works.
- Operational capacity628
- The total number of prisoners an establishment can hold without serious risk to good order, security and the proper running of the planned regime. Determined by Prison Group Directors on the basis of operational judgement and experience.
- Population599
- The number of people held. The publisher notes that the prison unlock figure may be lower, as the population includes prisoners on authorised absence.
The publisher’s own definitions, quoted from the bulletin. We divide the population by in-use CNA rather than by operational capacity: operational capacity is what an establishment can be pushed to hold, in-use CNA is what it was certified to hold decently, and dividing by the larger of the two flatters every prison in the estate.