How to join 21 SAS
Joining 21 SAS starts with contacting the reserve unit covering your area, not with a hillside. There is an application and medical stage, then briefing and assessment events, then selection. The part that decides the outcome is the months of unsupervised training you do before any of it.
Step 1 — contact the unit
The units recruit regionally and run their own briefing events. Go through the Army's official UK Special Forces Reserve route rather than anything you read on a forum, and go to a briefing early — before you are fit enough, not after. You will learn what you are actually training for.
Step 2 — paperwork, medical, references
This stage is unglamorous and slow, and it catches people out purely because they start it late. Deal with it properly and in parallel with your training rather than treating it as an afterthought.
Step 3 — briefing and assessment
Before selection proper there are assessment events, and mine is written up in full: part one covers the application, part two covers the ATC Pirbright weekend, what the staff appeared to be looking for and where people around me came unstuck.
Step 4 — the preparation that decides it
Aerobic base first, for weeks, before any weight. Hills before load. Weight added gradually and late. Back-to-back weekend days in the kit you will actually use, navigating for yourself. Then back off before the event. My free prep guide sets that out as a twelve-week shape.
How long should you give yourself?
Longer than you want to. If you cannot currently run easily for an hour, your first project is eight weeks of consistent mileage, and everything else comes after that. People who compress the timeline arrive injured.
A note on accuracy
Entry criteria, standards and selection dates are set by the Army and change over time. Take anything current from the Army's own UK Special Forces Reserve pages; take everything below as one applicant's experience.
Get my 21 SAS reserve selection prep guide
What the stages are, what I was training to before I applied, and how I'd build the twelve weeks before a selection weekend. Written from my own application, free, no pitch.