More keyholes does not mean more security
The most common assumption we at Deadbolt Fixers hear from homeowners is that a lock with a key on both sides must be the stronger lock. Usually it is not. Within a product line, the double cylinder version typically shares its bolt, housing and outside cylinder with the single cylinder model. Schlage's commercial B600 series, for instance, lists both its single and double cylinder functions under the same Grade 1 rating.
What changes is the inside. A single cylinder deadbolt has a thumb turn that anyone indoors can twist. A double cylinder deadbolt swaps that turn for a second keyed cylinder, so the bolt only moves with a key from either side.
That one difference matters against exactly one kind of break-in, and it creates exactly one new risk. The rest of the single vs double cylinder deadbolt decision is about weighing those two things for your door and your household.
Text version of this diagram
Single cylinder: a keyed cylinder on the outside and a thumb turn on the inside. Anyone inside can open the lock without a key.
Double cylinder: a keyed cylinder on both the outside and the inside. A key is needed to open the lock from either side.
How each design holds up against real threats
It helps to stop thinking about "secure" in general and look at each way a locked door actually gets opened.
Reaching through glass
This is the only case where the double cylinder clearly wins. If a glass panel, sidelight or window puts the thumb turn within arm's reach, breaking one pane is enough to open a single cylinder lock. With a keyed interior there is nothing to grab.
Kicking or shouldering the door
Neither design helps here. Doors that fail under a kick usually fail at the strike and jamb, where short screws hold the strike to thin trim. Consumer Reports found that adding a box strike with 3-inch screws significantly improved kick-in resistance for most locks it tested. That upgrade is covered on our page on strengthening the strike and jamb.
Picking, bumping or drilling the outside cylinder
The outside cylinder is usually the same on both versions, so the design choice changes nothing. Resistance to these attacks comes from the cylinder itself. If that worries you, look at a high security deadbolt with a UL 437 listed cylinder rather than a second cylinder.
Getting out in an emergency
This is where the single cylinder clearly wins. A thumb turn works by feel, in the dark, in smoke, for a child or a visitor. A double cylinder works only if the right key is in the right place and the person can use it under stress.
Single vs double cylinder deadbolt comparison
This table puts the trade-offs side by side. Most rows favor the single cylinder; the one that does not is the reason double cylinder locks exist.
| Question | Single cylinder | Double cylinder |
|---|---|---|
| Opens from inside without a key | Yes, with the thumb turn | No, a key is needed |
| Glass within reach of the lock | Weak point: the turn can be reached | Protected: nothing to turn |
| Kick-in at the jamb | Depends on the strike, not the lock type | Depends on the strike, not the lock type |
| Picking, bumping, drilling | Depends on cylinder quality | Depends on cylinder quality |
| Fast exit in a fire | Yes | Only if the key is at hand |
| Daily key routine | None indoors | Inside key must live in one fixed spot |
| Code questions on egress doors | Rarely | Common; local adoption decides |
| Visitors, sitters, tenants | No instructions needed | Must be shown where the key is |
If you want to see how both fit among keyless, vertical, rim and mortise designs, our overview of the main deadbolt types lays them out.
Household-by-household advice
The right lock depends as much on who lives behind the door as on the door itself. Here is how the single vs double cylinder deadbolt question usually plays out for different households.
Homes with babies, toddlers and school-age kids
Use a single cylinder lock on the main exit door. Some parents like a double cylinder because a toddler cannot open it, but in a fire it also traps older children who could otherwise get themselves out, and it sends the adults looking for a key.
If wandering is the worry, deal with it separately, with measures that slow a small child without locking the adults in. Revisit the choice as children grow; a ten-year-old can learn a thumb turn in one try.
Older adults and anyone with limited grip or eyesight
Keep the thumb turn. Fishing a key off a hook and turning a cylinder takes fine motor control, good light and a calm head, all of which can be in short supply during an emergency.
The ADA's rule for operable parts in public buildings is a useful benchmark at home too: hardware should work with one hand, without tight grasping, pinching or twisting of the wrist. A large, easy thumb turn meets that idea far better than a small key does.
Rentals and short-term stays
Landlords face a specific problem: tenants, their children and their guests change, and none of them chose the lock. A keyed interior depends on every one of those people learning where the inside key lives.
Many landlords settle on single cylinder deadbolts and put the effort into a stronger strike instead. If you are considering a double cylinder in a rental, confirm the local code and landlord-tenant rules first.
Doors with glass panels or sidelights
This is the household where the double cylinder argument is strongest. Measure the problem honestly: stand outside, imagine one pane gone, and check whether your hand would reach the turn.
If it would, and the door is not the home's main exit, a double cylinder lock is a reasonable choice for a household that can manage the key. If the door is the main exit, look at glass upgrades or security film first and keep the thumb turn.
Vacation homes and shared houses
Guests and housemates are the least likely people to know where an inside key is kept. A house that changes occupants often is a poor fit for a keyed interior, even if the glass would justify one.
What building codes say, in plain terms
Residential codes in many US states are based on the International Residential Code. Its egress door rule, quoted from the 2009 model edition, says egress doors "shall be readily openable from inside the dwelling without the use of a key or special knowledge or effort." A double cylinder deadbolt needs a key, so it is the obvious thing that rule is aimed at.
The model code is not the law until a state or city adopts it, often with amendments. North Carolina's adopted version, for example, applies the keyless rule to interior egress doors and at least one exterior egress door. Minnesota adopts the 2018 IRC with its own amendments. The result is that two neighboring towns can treat the same back door differently.
Businesses follow separate commercial provisions in the International Building Code and NFPA 101, which allow key-operated locks on some main doors only under strict conditions such as signage. None of this makes double cylinder locks "illegal" across the board, but it does mean the local building department, the authority having jurisdiction, decides what is allowed on your door.
Mixing both types in one house
You do not have to settle the single vs double cylinder deadbolt question once for the whole house. A common and sensible layout looks like this:
- Main exit door: single cylinder deadbolt with a reinforced strike, so everyone has one guaranteed keyless way out.
- Secondary door with glass beside the lock: double cylinder, if the local code allows it and the household passes the key test.
- Other exterior doors without nearby glass: single cylinder.
Whatever the mix, put every cylinder on one key. A double cylinder lock is much easier to live with when the inside key is the same key everyone already carries, which our page on keyed alike deadbolts explains.
Grade matters more than cylinder count for overall strength. If you are comparing models, our explanation of ANSI/BHMA deadbolt grades shows how to read the ratings on the box, and good locks come in both versions.
Questions to answer before you buy
Run through these honestly. If you cannot tick every box, the single cylinder lock is the right answer for that door.
A double cylinder lock fits this door only if
- Glass near the door puts the thumb turn within arm's reach from outside
- The home has another exterior door that opens from inside without a key
- Your local building department has confirmed a keyed interior is acceptable on this door
- Every person who sleeps in the home can find and use the inside key in the dark
- You will keep the inside key in one fixed spot, away from the glass, every time the bolt is locked
The two-question decision rule
When the single vs double cylinder deadbolt choice still feels close, reduce it to two questions: whether someone outside could reach the thumb turn through glass, and whether this is the door your family would use to get out in a fire.
If the first answer is no, or the second is yes, choose a single cylinder. Only a reachable turn on a secondary door makes a double cylinder worth the trade. When you get there, our double cylinder installation service starts with a local code check and ends with a key storage plan, so the lock you choose is one your household can live with.
Sources
- Decoded: Key-Operated Locks(iDigHardware)
- 2018 NC Residential Code Section R311 Means of Egress (formal interpretation, Oct 18, 2023)(NC Office of State Fire Marshal)
- Emergency Escapes and Rescue Openings fact sheet(Minnesota Department of Labor and Industry)
- Decoded: Key-Operated Locking Devices (July 2025)(iDigHardware)
- How to Choose the Best Door Lock for Your Home(Consumer Reports)
- B600 Series Grade 1 Deadbolts(Schlage Commercial)
- ADA Accessibility Standards (2010), section 309.4(U.S. Access Board)
Next step
Talk to a Deadbolt Fixers specialist
Tell us about the door, the lock and what is going wrong. We will tell you what the job involves before any work starts.
Frequently asked questions
Is a single or double cylinder deadbolt more secure?
A double cylinder is more secure only against someone reaching through broken glass to the inside of the lock. Against kicking, prying, picking or drilling, two locks of the same quality perform about the same.
Are double cylinder deadbolts illegal?
Not as a blanket rule. Codes based on the IRC require egress doors to open from inside without a key, so the question is how your local code treats the specific door, and your building department decides.
Which deadbolt is better for a home with young children?
A single cylinder on the main exit door, in nearly every case. Children cannot reliably find and use a key in a fire, and the adults would be hunting for one too.
Is a double cylinder deadbolt a good idea for an older adult living alone?
Usually not. Finding a key and gripping it to turn a cylinder in smoke or darkness is harder with limited eyesight, grip or mobility, so an easy thumb turn plus a stronger strike is the safer pairing.
Can a landlord put double cylinder deadbolts on rental units?
It depends on the local code and landlord-tenant rules, and many landlords avoid them because tenants and their guests do not always know where the inside key is. Check with the local building department before installing one in a rental.
Does a door with a glass window need a double cylinder deadbolt?
Not automatically. If the glass is far enough from the lock that nobody can reach the thumb turn through it, a single cylinder is fine; if it is close, compare a double cylinder with glass upgrades before deciding.
Are double cylinder deadbolts made to the same grades as single cylinder ones?
They can be. Schlage's B600 series, for example, carries an ANSI/BHMA A156.36 Grade 1 rating and includes both a single cylinder model (B660) and a double cylinder model (B662).
Why do people worry about single cylinder deadbolts next to glass?
Because a thumb turn opens the bolt without a key, so if a broken pane puts it within arm's reach, the lock can be opened from outside without touching the cylinder.
What is the biggest downside of a double cylinder deadbolt?
Anyone inside needs a key to get out, so in a fire or other emergency the exit is only as fast as the search for that key. Every other drawback, from code questions to briefing guests, follows from that one fact.
How can I tell if my deadbolt is single or double cylinder?
Look at the inside of the door. A turn piece you twist by hand means single cylinder; a second keyhole means double cylinder.
Should every exterior door have the same type of deadbolt?
No. Many homes are best served by single cylinder locks on the main exit and other easy-exit doors, with a double cylinder only on a secondary door that has glass beside the lock, if local code allows it.
Does a double cylinder deadbolt make sense on a vacation home?
The egress risk exists whenever someone is inside, and guests are the least likely people to know where an inside key is kept. If the home is rented out or lent to guests, a single cylinder lock is the safer choice.