Thumbturn or key both sides: which euro cylinder to order
Updated: October 2026
You can leave without hunting for a key, or you can stop someone reaching in and turning the lock. Those two fears pull in opposite directions. Thumbturn or key both sides is that choice. A thumbturn euro cylinder has a key outside and a turn inside. A double cylinder needs a key on the inside as well. The [knob cylinder comparison](/hub/hot-water/knob-cylinder) tables four listings. This page is the function, from the Door and Hardware Federation and public UK sources on 19 October 2026. A qualifying purchase through our links can earn a commission at no extra cost to you.
You can leave without hunting for a key, or you can stop someone reaching in and turning the lock. Those two fears pull in opposite directions. Thumbturn or key both sides is that choice.
You can leave without hunting for a key, or you can stop someone reaching in and turning the lock. Those two fears pull in opposite directions. Thumbturn or key both sides is that choice. A thumbturn euro cylinder has a key outside and a turn inside. A double cylinder needs a key on the inside as well. The knob cylinder comparison tables four listings. This page is the function, from the Door and Hardware Federation and public UK sources on 19 October 2026. A qualifying purchase through our links can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Size | Inside turn | Security words | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  GANZTECH GANZTECH 30/40T chrome | 30/40T, 70 mm | Key knob | Anti-bump, no standard | 8.6 | about £10.99 | View offer |
 Valorada Valorada 55/45T thumb turn | 55/45T, 100 mm | Listing conflict | Anti-pick, anti-bump | 5.5 | about £13.99 | View offer | |
| Best value |  BGS BGS 85290 rotary knob 45/45 | 45/45, 90 mm | Rotary knob | No standard named | 8.3 | about £6.55 | View offer |
On this page
What each side of the door does
A double cylinder locks and unlocks with a key from either side. If a key is already inside, some cylinders still let the outside key work. Sellers call that an emergency function. It is not a knob. The RUCUDIM title says knob, while the bullets describe that double-open behaviour.
A thumbturn opens the door from inside with nothing in your hand. That matters on a final exit, and it is a problem if the turn can be reached. Fitting any barrel in this table does not make the door BS 8621. British Standard 3621 needs a key both sides where someone might reach in, provided another escape exists. The star system is a separate subject, on TS 007 and a knob cylinder.
Letterbox, cat flap and glass
Stand outside and see whether a hand, a stick or a looped cable through the letterbox can reach the inside turn. The Crime Prevention Website describes that attack and points to a deflector or an external letter box to BS EN 13724. A cat flap is the same reach test.
A higher security rating does not remove a reachable knob. TS 007 3-star and an unrated £7 barrel both turn once a hand is on them. Secured by Design names TS 008 for letter plates, which is a specification for the plate, not a test of a cowl you have not seen. If you cannot put the slot out of reach, order key both sides. The Door and Hardware Federation's split is escape versus reach-through.
When keyless exit is the requirement
Some doors have to open from inside without a removable key. Cheshire Fire and Rescue's guidance for houses in multiple occupation says final exits and bedsit doors should work that way, and it names a thumb-turn. If you are a tenant, or the door is a specified fire door, do not change the cylinder because a listing looked convenient.
Where someone outside can already reach the knob, cage the letterbox, move the slot, or use key both sides on a door that is not the only exit. Leaving a key in a double cylinder overnight gives the same reach-through.
A recommendation with a condition
Choose the thumbturn when you must get out without a key and nobody outside can reach the knob. Choose key both sides when they can reach it, and when another way out exists. If the knob is reachable and the door is also your only exit, fix the reach before you buy the turn.
Size comes after that choice. A thumbturn in the wrong split will not screw in. Measuring from the fixing screw, with the knob left off the tape, is on how to replace a euro cylinder. Only buy a thumbturn whose bullets agree that a knob is fitted, and only in the split you measured.
Common questions
I have a letterbox in the door. Which do I fit?
Fit a thumbturn only if you cannot operate the inside knob through that letterbox, with a hand, a stick or a looped cable. If you can, fit key both sides, or block the route with a proper deflector or an external box and then reconsider the turn. A 3-star cylinder does not stop it, because the attacker is using the knob, not snapping the barrel.
There is glass next to the lock. Does that change it?
Yes, if breaking the glass, or reaching through an existing hole, puts a hand on the knob. A thumbturn behind reachable glass is a handle on the outside. The Winkhaus and RAM leaflet on TS 007 hardware says that if the thumbturn can be worked from outside, you use a double cylinder or you make the glass resistant. That is their instruction for the doors that leaflet covers. Treat reachable glass as a reason for key both sides unless the glass itself is dealt with.
I live in a shared house. What does the fire guidance say?
Cheshire Fire and Rescue says that in a house in multiple occupation, final exits and bedsit doors should open from inside without a key, and it names a thumb-turn. That guidance is for those premises. It does not mean every bedroom door in a private house must have a knob, and it does not authorise a tenant to swap a specified fire door. If you do not manage the building, ask the person who does before you change the cylinder.
Can I leave a key in a double cylinder overnight instead?
You can, and it recreates the risk you were trying to avoid. The key is there for anyone who reaches in, and it can leave with them. An emergency function, where the outside key still works while the inside key is inserted, is useful if you lock yourself in with a key, and it is still not a thumbturn. If you want keyless exit, order a turn that cannot be reached, rather than parking a key in the lock.
Does a 3-star rating fix a reachable thumbturn?
No. TS 007 and Sold Secure Diamond address attacks on the cylinder, including snapping. A hand on the knob does not need to snap anything. A £54 marked thumbturn and a £7 unrated thumbturn fail that test the same way. Deal with the letterbox or the glass, or change to key both sides.
Related pages
Sources
Public UK sources named below. amazon.co.uk prices, stars and review counts on these four listings were read on 19 October 2026. Page updated 21 October 2026.
- West Yorkshire Police euro cylinder advice (retrieved 19 October 2026)
- Secured by Design: locks and associated hardware (retrieved 19 October 2026)
- DHF thief-resistant lock assemblies (retrieved 19 October 2026)
- DHF TS 007 star ratings (retrieved 19 October 2026)
- BSI BS 8621:2017+A1:2024 (retrieved 19 October 2026)
- Cheshire Fire and Rescue: houses in multiple occupation (retrieved 19 October 2026)
- ERA fitting instructions (retrieved 19 October 2026)
- Checkatrade: changing a uPVC door lock (retrieved 19 October 2026)
- amazon.co.uk listings B0D5HY325L, B0CLV6BMNX, B0GS5VFV7S and B0H14R6SPH (retrieved 19 October 2026)