HMP vs YOI: What’s the Difference?
Last reviewed: June 2026
If someone you care about is around the age of 18 to 21, you may be unsure whether they will be held in an adult prison or a young offender institution. The two are run by the same service but are designed for different age groups and different needs. This guide explains who goes where, how the regimes differ, and what happens when a young person turns 18 or 21.
What an HMP is
“HMP” simply stands for His Majesty’s Prison and is the standard prefix for adult prisons in England and Wales — HMP Wandsworth, HMP Ford, and so on. Adult prisons are organised into the four security categories, from Category A through to open Category D conditions. They hold both remand prisoners (awaiting trial or sentence) and sentenced prisoners.
What a YOI is
A young offender institution holds people who are too young for the mainstream adult estate. There are two broad types. Under-18 YOIs hold 15 to 17-year-olds and sit within the youth secure estate alongside secure training centres and secure children’s homes, overseen by the Youth Custody Service. Young adult YOIs hold 18 to 20-year-olds. The regime is built around education, training and personal development, with stronger safeguarding duties because of the age of those held.
| Feature | HMP (adult prison) | YOI |
|---|---|---|
| Typical age | 21 and over | 15–17 (under-18 YOI) or 18–20 (young adult YOI) |
| Run by | HMPPS | HMPPS / Youth Custody Service (under-18s) |
| Main focus | Security category, work, resettlement | Education, development, safeguarding |
| Education | Available but optional for many | Central; under-18s must access education |
| Security categories | A–D | Not categorised A–D in the same way |
What happens when a young person turns 18 or 21
Age does not trigger an automatic, instant move. Transition planning for a young person likely to move into the adult estate usually begins at around 17 years and six months. Some 18-year-olds remain in the youth estate for a period to complete a sentence or to avoid disrupting education, before transferring to a young adult YOI or adult prison. Similarly, a 20-year-old approaching 21 will be prepared for a move to an adult prison rather than being transferred overnight. Decisions weigh maturity, vulnerability, sentence stage and the availability of places.
Other parts of the youth estate
YOIs are not the only option for under-18s. Secure training centres hold children aged 12 to 17 in smaller units, and secure children’s homes — run by local councils — hold some of the youngest and most vulnerable children, aged 10 to 17. Which setting a child is placed in depends on their age, vulnerability and needs. Our guide to secure training centres versus YOIs explains this in more detail.
- “YOI is just a junior prison.” The regime is genuinely different, with education and safeguarding at its centre, not simply a smaller version of an adult jail.
- “They move the day someone turns 18.” Moves are planned around maturity and sentence stage, not the birthday itself.
- “All under-18s go to a YOI.” Many go to secure training centres or secure children’s homes instead, depending on their needs.
A note from experience
Prison Guide’s founder, Daniel Hockey, served his own sentence in the adult estate — from HMP Wandsworth to open conditions at HMP Ford — and writes about that experience in The Secret Prisoner (2021). If your relative is moving between the youth and adult estates, the practical guidance here is grounded in lived experience. More about Daniel.
Frequently asked questions
Can an 18-year-old be sent straight to an adult prison?
Yes. Many 18 to 20-year-olds are held in young adult YOIs, but placement depends on the individual, the available estate and the offence.
Are visits different in a YOI?
The rules are broadly similar to adult prisons, but YOIs often place extra emphasis on family contact because of the age of those held. See our guide on how to visit a prisoner.
Is education compulsory in a YOI?
For under-18s, access to education is a central part of the regime. For young adults it is strongly encouraged and widely available.
Do YOIs have security categories like adult prisons?
Not in the same A–D way. The youth and young adult estate is organised around age, vulnerability and need rather than the adult categorisation system.
Official sources: Children in custody (GOV.UK), Youth Custody Service, HM Inspectorate of Prisons.
