What you need to do to swap out an old knob
Replacing an old-style door knob takes about 15 minutes per door and requires only a screwdriver and a new knob. You remove the interior knob by unscrewing the set screw on its shaft, slide both knobs off, unscrew the latch assembly from the edge of the door, and install the new latch and knobs in reverse order. The hardest part is usually finding the set screw — it is often hidden under a small decorative collar or on the underside of the knob handle.
Old knobs fail for two reasons: the internal mechanism wears out and the knob spins without turning the latch, or the finish corrodes and the knob becomes difficult to turn. If your knob still works but looks worn, you can replace it for appearance alone. If it no longer latches the door, replacement is necessary.
Key Takeaways
- Most old knobs are held on by a single set screw on the interior knob shaft, which you unscrew to remove both knobs at once.
- The latch assembly screws into the edge of the door from inside and must be removed before you can install a new one.
- New knobs come in two styles: those that fit your existing latch hole (the easiest swap) and those that require a different hole size (which means drilling).
- Measure your existing latch hole diameter and backset distance before buying a replacement knob to avoid a second trip.
- Interior and exterior knobs are different — the exterior knob has a keyhole and the interior one does not.
Finding and removing the set screw
The set screw is the small bolt that holds the interior knob to its shaft. On many old knobs, it sits on the underside of the handle where the handle meets the knob body. On others, it hides under a small metal or plastic collar that slides up the shaft. Run your finger along the base of the interior knob to find it — you will feel a small indentation or a collar you can move.
Once you locate it, use a screwdriver (usually a Phillips head, sometimes a flathead) to turn the set screw counterclockwise. You do not need to remove it completely — just loosen it enough that the knob spins freely on the shaft. Once it is loose, both the interior and exterior knobs will slide off the shaft together. Set the set screw aside in a small container so you do not lose it.
Removing the latch assembly from the door edge
After both knobs are off, look at the edge of the door where the latch bolt sticks out. You will see a small metal plate (called the strike plate or latch plate) screwed into the edge of the door. This plate holds the latch assembly in place. Unscrew the two or three screws holding this plate — they run horizontally into the edge of the door — and the entire latch assembly will come free.
Keep these screws. If your new knob uses the same latch size, you will reinstall them in the same holes. If the new knob requires a different latch, you may need new screws, but many hardware stores will include them with the new latch assembly.
Measuring your latch hole before buying a replacement
Door knob latches come in two standard sizes: 2⅜ inches and 2¾ inches. This measurement is the backset — the distance from the edge of the door to the center of the knob hole. Measure from the edge of your door to the center of the hole where the knob shaft goes through. Write this number down.
Also measure the diameter of the hole in the edge of the door where the latch bolt sits. Standard sizes are 1⅛ inches and 1¼ inches. If you buy a new knob with a different backset, you will need to drill a new hole, which adds 30 minutes to the job. If you buy one with the same backset, the new latch will fit into your existing hole and you will only need to swap the hardware.
Bring both measurements to the hardware store or use them when ordering online. Most new knobs list their backset and latch size in the product description.
Installing the new latch assembly
Slide the new latch assembly into the hole in the edge of the door. The bolt should face the direction it did before — usually toward the outside of the door. Align the screw holes on the latch plate with the holes in the door edge, then screw the plate in place using the original screws or the new ones that came with the knob.
Tighten the screws firmly but do not overtighten — you only need them snug enough that the latch plate does not move. If the latch assembly sits too far in or too far out, the knobs will not align properly when you install them.
Installing the new knobs and testing the fit
Slide the exterior knob (the one with the keyhole) onto the shaft first, then slide the interior knob on behind it. Both knobs should sit flush against the door. Insert the set screw through the interior knob and into the shaft, then tighten it clockwise until the knob no longer spins freely. Do not overtighten — you only need it tight enough that the knob turns the latch when you turn the handle.
Test the knob by turning the interior handle. The latch bolt should retract smoothly. Turn the exterior knob — it should also retract the bolt. If the knob spins without moving the latch, the set screw is too loose. If the knob is hard to turn, the set screw is too tight or the latch assembly is misaligned.
Close the door and test the latch from both sides. The door should latch when you close it and open when you turn either knob. If the latch does not catch, the latch assembly may be installed backward or the strike plate on the door frame may need adjustment.
Adjusting the strike plate if the door does not latch
The strike plate is the metal plate on the door frame (not the door itself) with a hole for the latch bolt to enter. If your new knob is the same size as the old one, the existing strike plate should work. If the door does not latch smoothly, the strike plate may need to move slightly up, down, or forward.
Loosen the screws on the strike plate and try closing the door gently. If the latch bolt hits the plate instead of entering the hole, move the plate slightly forward (away from the hinge side of the frame). If the latch enters but the door does not close all the way, move the plate back. Once the door latches smoothly, tighten the screws.
If the strike plate hole is too small for your new latch bolt, you can file it slightly larger or replace the strike plate with one that fits your new latch size. Most hardware stores carry universal strike plates that work with standard latch sizes.
Frequently Asked Questions
What if I cannot find the set screw?
Check the underside of the interior knob handle first — most set screws sit there. If you still cannot find it, try gently prying the knob away from the door with a flathead screwdriver. Some old knobs have the set screw on the side of the shaft rather than on the knob itself. If the knob is stuck, spray a small amount of penetrating oil around the shaft and wait 10 minutes before trying again.
Can I replace just the knob without changing the latch?
Yes, if your new knob uses the same backset and latch size as the old one. Most modern knobs are made to fit standard latch assemblies, so you can often swap just the knobs and leave the latch in place. Check the product description to confirm the backset matches your door before buying.
Do I need to drill a new hole if my new knob has a different backset?
Yes. If the new knob's backset is different from your old one, the knob hole will not align with the latch assembly. You will need to drill a new hole in the door and fill the old one with wood filler. This adds about 30 minutes to the job and requires a drill with a spade bit or hole saw the correct diameter for your new knob.
What is the difference between interior and exterior knobs?
The exterior knob has a keyhole and usually a locking mechanism. The interior knob has no keyhole and is smooth. They are not interchangeable — if you install them backward, the lock will not work. Most new knob sets are clearly labeled "interior" and "exterior" on the packaging.
How do I know if my old knob is broken or just stuck?
Turn the interior handle. If the latch bolt retracts smoothly, the knob works. If the handle turns but the bolt does not move, the internal mechanism is broken and the knob must be replaced. If the handle is hard to turn or stuck, try spraying penetrating oil around the shaft and working it back and forth gently before deciding to replace it.