Giovanni Valle is a licensed architect and LEED-accredited professional and is certified by the National Council of Architectural Registration Boards (NCARB). He is the author and managing editor of various digital publications, including BuilderSpace, Your Own Architect, and Interiors Place.
Most people end up replacing a door knob at some point. When this time comes, you will need to know what type of door knob to buy. However, this may leave you wondering whether all door knobs are universal or if you need to buy a specific type.
Door knobs are not universal, but there aren’t exponentially increasing options for what you need. You need to consider three things: the backset, the hole diameter (called a cross-bore) where the doorknob goes, and whether it’s for interior or exterior use.
I will discuss the form and function differences among door knobs in this article. Keep reading to learn about the defining features of door knobs and more about choosing the right knob for your door.
What Differentiates Door Knobs?
Door knobs are differentiated by their compatibility with different door backsets, cross-bores, and locations. Furthermore, door knobs made for exterior use have different functions and features than knobs made for interior use.
Backset Length Affects Door Knob Choice
The backset is the distance from the door’s edge to the center of the hole where the knob goes. Most door knobs have a backset of either 2 ⅜ inches or 2 ¾ inches (6.03 cm or 5.08 cm). This means different things depending on your situation.
If you have a pre-existing hole in the door (which is likely if you’re replacing a doorknob), you’ll need to measure the backset to get a suitable replacement. If there’s not a hole in the door yet, you’ll need to know the backset of the doorknob you plan to install so you know the right spot to use your hole saw.
The wrong backset length can cause problems with the latch bolt assembly, which is the part that moves when you turn the knob. Using a knob incompatible with the backset length will mean that the latch bolt assembly will either be too long for the door to latch or too short to reach the doorknob.
Cross-Bore Diameter Affects Door Knob Compatibility
The cross-bore usually measures 2 ⅛ inches (5.08 cm) in diameter, though there are some exceptions. As with the backset, you either need to know the diameter of the hole in your door or you should know the size of the cross-bore your knob requires before you cut into your new door.
Sometimes, you may find a larger cross-bore that requires a wider doorknob and a wider rosette. The rosette is the round metal plate that goes around the doorknob and covers the mechanics of the doorknob.
Interior vs. Exterior Door Knobs
Where your doorknob goes is as important as its measurements. It’s important to understand that different door functions require different knobs.
Interior Door Knobs Are Lighter-Weight
Typically utilizing a lighter-duty door knob, an interior knob usually weighs less and is smaller than an exterior doorknob. They’re meant to hold interior doors closed and aren’t intended to offer any serious security. Since interior doors are often hollow-core doors, they’re lighter in weight. Interior door knobs don’t have to work very hard to hold them closed.
Also, the majority of interior knobs do not feature locking mechanisms. They break down into three categories:
- Passage knobs: these knobs don’t lock and usually get installed on closet doors.
- Dummy knobs: these serve an aesthetic purpose and don’t turn any mechanism (they don’t turn at all).
- Privacy knobs: these knobs have a lock without a key—instead, there is just a hole on the other side of the knob. These usually go on bathroom and bedroom doors.
The lock on a privacy knob isn’t very secure, so you wouldn’t want to use this type on an exterior door. Rather, they supply a lower level of security that preserves privacy in the locked room (as the name suggests).
Exterior Door Knobs Focus on Security
While we typically think of door knobs as tools to open doors, the exterior doorknob’s primary function is to keep the door securely closed. They have keyed locks and sturdier construction than their interior counterparts. While they’re not impenetrable, they are tougher than interior knobs.
Keyed Entry Knobs Provide Additional Security
Most of us grew up with a keyed entry exterior doorknob on the house’s front door. This knob’s lock is harder to pick than a privacy knob, which is easily disengaged with a bobby pin. You can pick an exterior knob with a bobby pin, but it’s more complicated than a privacy knob.
While exterior knobs offer better security than an interior passage knob, they are much more effective when paired with a deadbolt usually installed above it.
Keyless Entry Knobs Work Without Physical Keys
Keyless knobs have grown in popularity of late. They usually operate on battery power, and while they typically have a key and keyhole, their main function is to work without that key. Keyless knobs may have a number pad in which you enter a 4-digit code to open or close the lock, or there may be a key fob or some other method.
Think of a hotel: when you check in, they give you a credit card-sized electronic key. Keyless entry knobs work on a similar principle, making it possible to open the lock without a physical metal key.
Some smart homes offer lock operation through a smartphone app, which follows the same principle. These smart locks offer a way to open your door without a key. If you’re a person who loses keys often, this can offer great convenience and save the cost of a locksmith or a visit from the window replacement guy.
Older Door Knobs Can Complicate Replacement
If your door is nearing antique status, you may be ready to find a more modern and secure doorknob. In this instance, you may find yourself drilling new holes. In some cases, you may even need to replace the door entirely.
Older knobs may have a much narrower spindle. In other words, the spindle is the piece of metal the knob turns to activate the latch. In this case, the cross-bore may be way too small for the new knob. You might also encounter a different backset than one of the common sizes.
The first problem can be addressed with a hole saw. You will use the saw to cut a larger hole in the door. The second problem means you will have to move the hole to the left or right. Keep in mind, you may find yourself with the old hole still in the door after completing this process.
While door knobs aren’t universal, they’re pretty standardized, depending on your specific needs. Knowing the backset measurement allows you to buy the correct replacement or drill the cross-bore in the right spot on the door. Determining whether you need interior or exterior knobs further narrows the field from which you’re choosing.
Know your measurements before you start cutting into your door. “Measure twice, cut once” is a cliche because it’s sound advice.
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