Single-Cylinder or Double-Cylinder Deadbolt: Choosing by the Door You Have

A single-cylinder deadbolt takes a key on the outside and has a thumb turn on the inside, so you can lock and unlock it from indoors by hand. A double-cylinder deadbolt needs a key on both sides. Neither type is better on its own. The choice depends on what is near the lock, such as glass, and on who uses the door and how they would get out in a hurry.

Single-cylinder: a key outside, a thumb turn inside

A single-cylinder deadbolt has one cylinder, the part with the keyhole, and it faces outside. On the inside there is a thumb turn: a small lever you twist by hand to throw the bolt into the frame or pull it back. Nobody inside needs a key to lock up or to leave.

Keyless exit is the main strength. Anyone in the home or office can open the door quickly, including someone half awake or someone who never carries the keys. On a solid door that opens onto a hallway or a solid wall, the thumb turn costs you very little.

The limitation of a single-cylinder deadbolt sits right next to the thumb turn. If a glass pane, a sidelight (the narrow window beside some doors), or a thin panel is within arm's reach of the lever, someone who breaks or gets through that opening could reach in and turn it. Whether that matters depends entirely on what surrounds your lock.

Double-cylinder: a key on both sides

A double-cylinder deadbolt has a keyed cylinder on the inside as well as the outside. The thumb turn is gone, so locking or unlocking from indoors takes a key.

People usually look at this type for a door with glass close to the lock. A hand reaching through a broken pane finds a keyhole instead of a lever.

The trade-off is leaving. When a double-cylinder deadbolt is locked, nobody gets out through that door without the key. Think about each person who uses it: children, older relatives, overnight guests, staff locking up at closing time. Could each of them find and use a key in the dark or in a hurry? If the honest answer is no for anyone, settle that before choosing this type, and raise it plainly with the locksmith you talk to.

Questions to answer while standing at the door

Start with what is already installed. On the inside face of the deadbolt, a thumb turn means single-cylinder and a keyhole means double-cylinder. Then work through these questions with the door in front of you:

• Is there glass, a sidelight, or a thin panel close enough that an arm through it could reach the inside of the lock?
• Who uses this door, and could any of them need to get out quickly without hunting for a key?
• If the inside needs a key, where would that key be kept, and would everyone know where to find it?
• Is this the main entry, a secondary door such as a back or side door, or a business door with staff and visitors passing through?

The answers usually lean one way. A solid door and people who need a fast exit point toward a thumb turn. Glass near the lock, plus a household or staff who can always manage an inside key, makes a double-cylinder worth asking about. When the answers pull against each other, such as glass beside a door that young children use, the decision is about the whole door and what sits around the lock, and the cylinder is only one part of it.

To talk through a specific door at home or at a business, call Amsterdam Locksmith on the Upper West Side at (917) 200-2978. Describe what is near the lock and who uses the door. If the deadbolt you have is not the type that door needs, the conversation turns to a lock change.

Frequently asked questions

How can I tell from the inside whether my deadbolt is single-cylinder or double-cylinder?

Look at the inside face of the deadbolt, which is usually the lock mounted separately from the handle. A single-cylinder deadbolt has a thumb turn, a small lever you twist by hand. A double-cylinder deadbolt has a keyhole in that spot. If you see a keyhole and are unsure which lock it belongs to, check whether turning a key in it moves the bolt that goes into the frame. A keyed knob or lever is a separate lock.

If I choose a double-cylinder deadbolt, what should I think about regarding where the inside key is kept?

With a double-cylinder deadbolt, the inside key is the only way out through that door, so where it lives matters as much as the lock. It should be somewhere everyone who uses the door knows about and can reach quickly, even in the dark. It also should not hang so close to a glass pane that someone reaching through could grab it, which would undo the reason for choosing this type.

Can a deadbolt be changed from one type to the other, or does the whole lock need replacing?

Switching a deadbolt between single-cylinder and double-cylinder usually means replacing the deadbolt, because the thumb turn or inside cylinder is part of the lock's assembly. Rekeying alone changes which key works, not whether the inside has a thumb turn. Whether a new deadbolt fits the existing holes depends on the door and the hardware, so have a locksmith look at what is installed before you buy anything.

Does the choice of cylinder type change how a deadbolt should be installed on the door?

The bolt, the hole through the door, and the way the bolt meets the frame work the same way for single-cylinder and double-cylinder deadbolts. The difference is the inside face, where a keyed cylinder takes the place of the thumb turn. Door thickness and the existing holes still decide which deadbolt fits, so match either type to the actual door rather than picking a lock first and hoping it suits.