← Category B and High Security prisons
Grendon
Category B and High Security prisons · HMPPS functional group: Category B · Public sector · South West & South Central
HMP Grendon is a public sector prison in the Category B and High Security group. It sits in the top quarter of 14 similar prisons, with a likely position of 1st. Its official HMPPS rating is "Outstanding". Based on official data to March 2026.
At a glance
| Band among 14 category b and high security prisons | Top quarter |
|---|---|
| Likely position |
1st of 14
Median 1st. The range is the 5th to 95th percentile over 1,000 re-weightings and measurement uncertainty. |
| Pillars | Safety Top quarter Decency Top quarter Regime Top quarter Resettlement Top quarter |
| Official HMPPS rating (context) | Outstanding, score 91.7%
Not an input to our bands. |
The Ministry of Justice reports this prison together with Spring Hill. Incident counts cannot be separated, so incident outcomes are not assessed for it and its safety figures describe the combined unit.
The HMIP grades on this page come from an inspection in May 2023. They may not reflect the prison today.
The measures behind the bands
Position is where the prison sits among its 14 peers on that measure alone (lower incident rates are better; higher grades and outcomes are better). For incident outcomes the position is after adjusting for who the prison holds.
| Measure | This prison | Group median | Position in group | Data from |
|---|---|---|---|---|
| Safety pillar position: Top quarter | ||||
| Prisoner-on-prisoner assaults | 18.1 per 1,000 prisoners expected 45.9 | 177 | Top quarter | Apr 2025 to Mar 2026 |
| Assaults on staff | 12.1 per 1,000 prisoners expected 22.5 | 148 | Top quarter | Apr 2025 to Mar 2026 |
| Self-harm incidents | 53.2 per 1,000 prisoners expected 89.8 | 801 | Top quarter | Apr 2025 to Mar 2026 |
| HMIP healthy prison test: safety | 4 (good) | 2.0 | Top quarter | Inspected May 2023 |
| Prisoner survey: safety and security | 4 (highest) | 3.0 | Top quarter | Survey from Mar 2024 |
| Decency pillar position: Top quarter | ||||
| HMIP healthy prison test: respect | 4 (good) | 2.5 | Top quarter | Inspected May 2023 |
| Prisoner survey: respect | 4 (highest) | 3.0 | Top quarter | Survey from Mar 2024 |
| Regime pillar position: Top quarter | ||||
| HMIP healthy prison test: purposeful activity | 2 (not sufficiently good) | 1.0 | Top quarter | Inspected May 2023 |
| Prisoner survey: purposeful activity | 4 (highest) | 3.0 | Top quarter | Survey from Mar 2024 |
| Prisoners in at least half-time purposeful activity | 99% | 57% | Top quarter | 2025/26 |
| Resettlement pillar position: Top quarter | ||||
| Housed on the first night after release | – | 89% | – | 2025/26 |
| In employment six weeks after release | – | 9% | – | 2025/26 |
| In employment six months after release | – | 22% | – | 2025/26 |
| HMIP healthy prison test: preparation for release | 4 (good) | 3.0 | Top quarter | Inspected May 2023 |
Incidents compared with what we would expect
Rates are incidents per 1,000 prisoners over April 2025 to March 2026. The expectation is the average of a statistical model of who the prison holds (sentence length, remand, recalls, age, turnover, crowding) and the average of its HMPPS comparator prisons. Neither includes the prison's own result.
| Outcome | Actual | Model expects | Similar prisons | Expected (used) | Actual ÷ expected | Reading |
|---|---|---|---|---|---|---|
| Prisoner-on-prisoner assaults | 18.1 | 14.4 | 142 | 45.9 | 0.40× | Not assessed (combined reporting unit) |
| Assaults on staff | 12.1 | 3.2 | 129 | 22.5 | 0.55× | Not assessed (combined reporting unit) |
| Self-harm incidents | 53.2 | 13.6 | 564 | 89.8 | 0.59× | Not assessed (combined reporting unit) |
A flag appears only when the result is outside statistical limits and in the same direction under the model, the comparators and their average. It is a prompt to look, not a finding of fault. Counts of one or two incidents are suppressed in the source and estimated here.
Similar prisons
HMPPS names these prisons as the closest comparators, matching on age, sentence length, turnover, sex-offender share, complexity, building age and setting. Rates are per 1,000 prisoners; reoffending is the share of people released in April 2023 to March 2024 who reoffended within a year.
| Prison | Prisoner assaults | Staff assaults | Self-harm | Reoffending | Official rating |
|---|---|---|---|---|---|
| Grendon (this prison) | 18.1 | 12.0 | 53.2 | – | Outstanding |
| Garth | 305 | 254 | 1,102 | 28% | Serious concern |
| Gartree | 136 | 144 | 811 | – | Good |
| Swaleside | 347 | 418 | 804 | 19% | Serious concern |
| Lowdham Grange | 237 | 388 | 1,488 | 14% | Serious concern |
| Isle of Wight | 177 | 104 | 1,139 | – | Serious concern |
| Dovegate | 271 | 103 | 757 | 53% | Good |
| Woodhill | 360 | 432 | 1,421 | – | Serious concern |
| Manchester | 322 | 203 | 1,400 | 17% | Of concern |
| Average of similar prisons | 269 | 256 | 1,115 | 26% |
Who the prison holds
These describe the population, not the prison's performance. They are what the expectations above adjust for. Averages over the four quarter-ends of 2025/26.
| Share of population | This prison | Group median |
|---|---|---|
| Average population | 175 | 611 |
| On remand (untried or convicted, unsentenced) | 0% | 0% |
| Sentence under 12 months | 0% | 0% |
| Sentence 12 months to under 4 years | 0% | 0% |
| Sentence 4 years or more (determinate) | 29% | 39% |
| Life or IPP sentence | 65% | 45% |
| Recalled to custody | 6% | 5% |
| Aged under 25 | 0% | 7% |
| Aged 50 or over | 28% | 18% |
| Releases in the year, per prisoner held | 0.01 | 0.03 |
| Prisoners held in crowded accommodation | 0% | 0% |
Remand is decided by courts and recall by probation and the Parole Board. They are context here, not something a prison controls.
Also shown, not scored
Staff sickness and resignations
Sickness: 9.6 days per full-time employee (HMPPS target 9; median for public prisons in its group 13.9).
Resignation rate: 6.4%.
Position among public-sector prisons in its group: sickness lowest quarter, resignations lower middle. Related to quality in our tests, but the direction of cause is unclear, so it is not scored.
Disorder incidents and finds
| Per 1,000 prisoners, 2025/26 | This prison | Group median |
|---|---|---|
| Incidents at height | 0.0 | 69.6 |
| Barricades | 0.0 | 22.1 |
| Drug finds | 22.8 | 242 |
| Weapon finds | 17.1 | 249 |
| Mobile phone finds | 45.7 | 107 |
Not scored. We cannot separate real disorder from differences in how incidents are recorded, and more finds can mean better searching rather than more contraband.
Healthcare (Care Quality Commission)
Provider: Practice Plus Group Health and Rehabilitation Services Limited; Time for Teeth Limited.
Latest compliance judgement: regulations met (published May 2025).
The CQC does not give prison healthcare a graded rating, so this is context only.
How current is this?
| HMIP grades | Inspected May 2023 |
|---|---|
| Prisoner survey grades | Survey from Mar 2024 |
| Incident rates and workforce | April 2025 to March 2026 |
| Resettlement outcomes | 2025/26 performance year |
| Reoffending | People released April 2023 to March 2024, followed for a year plus a six-month wait for court outcomes |
| Population mix | Quarter-ends June 2025 to March 2026 |
Sources: data downloads and methodology. Think a figure is wrong? Tell us.