You might need to replace a doorknob because it keeps falling off. That's not too difficult. If you want to replace the doorknob because it isn't working, you probably need to replace the entire lock, not just the knob. First, you need to know what kind of lock you have. The two common types are mortise and tubular. Second, you need to know the function. The three common functions are passage (hall and closet), privacy (bed and bath), and entry (uses a key). If things go well, you should be able to replace a knob on an interior door with just a screwdriver. The type of lock will determine whether you need a flat blade or philips screwdriver. Replacing knobs on exterior doors can be more complicated and is best left to a professional.
There are two things you should consider carefully. First, where should you buy the parts you need? The most popular door knob lock in America is the Kwikset 400T. Retail price is about $28 but you can usually find it at a big box store for half that price. You'll pay full price at a locksmith shop but you'll also get more expert advice. You could also go to a hardware store and buy a cheap imitation for less than ten dollars the quality of the lock will be poor. You get what you pay for. Second, how far can you go before you decide you need a professional to bail you out? That depends a lot on your skill level but it depends even more on Murphy's Law. Nine out of ten knob replacement jobs are easy but you never know if yours will be that one in ten which is difficult and complicated. Be prepared for the fact that you might have to give up and call a locksmith to come over and finish the job for you. Expect to pay about $50 for a service call plus about $60 per hour for labor.
Antique Mortise Locks
If the doorknob on your hall closet keeps falling off or if it spins and nothing happens, you probably have an antique mortise lock. The knobs are usually solid brass or crystal. If you look at latch on the edge of the door, you'll see a metal rectangle surrounding the latch about 3/4 inches wide and 4 or 5 inches tall. Mortise locks usually have a solid spindle with a square cross-section which runs from one side of the door through to the other side. The spindle is threaded and the two knobs screw onto it. When they are in position, a set screw on the neck of the knob presses against one side of the spindle to keep the knob from unwinding.
The first thing to do is simply try tightening the set screw, with a flat blade screwdriver. Before you tighten the screw, wind the knob down until it is flush with the faces of the door and then back it off half a turn. Now look at the knob and imagine you're seeing a compass, with North pointing up, South down, East to the right, and West to the left. Wind the knob a little farther back until the set screw is either NW or NE or SW or SE. Then tighten the screw. Test the knob. Is it working? You're done. If the screw keeps coming loose, you may need to put some blue threadlocker on it. You can get a bottle from the hardware store for about $5.
If tightening the set screw didn't work or if the knob pulls straight off the spindle, you may have a stripped knob or a stripped spindle or both. Don't worry, they come in a set of two knobs and a spindle together in a box for about $12. You can choose either brass knobs or glass knobs. With the door open, unwind one knob completely off the spindle and then pull the other knob with the spindle attached completely out of the lock. Take it with you to a locksmith shop to buy a new knob/spindle set.
Put the new spindle through the door and try to center it. Now wind a knob onto each end of the spindle. Try to use the same number of turns on each knob. You don't have to count the turns, just do one turn on the inside and one on the outside and one inside and one outside et cetera until the knobs are snug. Now do like before with the set screw. Back off each knob a little bit so there is some play and then line up the set screws either NW or NE or SW or SE and tighten them. Test the knobs. If it still doesn't work, give up and call the locksmith.
Modern Tubular Locks
If the doorknob on your bedroom has a little button you can push to lock it from the inside, you probably have a modern tubular lock. If you look at the latch on the edge of the door, you'll see surrounding the latch either a small metal circle, barely bigger than the latch itself, or a metal rectangle 1 inch wide and just over 2 inches tall. The rectangle has two philips head screws in it, and this is what holds the latch to the edge of the door. Also there are two philips head screws that go through the door. You unscrew them by holding the screwdriver parallel to the knobs.
Before you remove the lock, you need to get a measurement called the backset. 95% of the time it will be 2+3/8 inches, but sometimes it will be 2+3/4 inches. The backset is measured from the edge of the door to the center of the knob. Hold the measuring tape parallel to the door. If you have a metric ruler, the backset will either be 60 millimeters or 70 millimeters. If you have trouble measuring the backset, just be sure to take the latch with you when you buy the new lock. Now you're ready to remove all four screws. The two knobs will come right off the door, each one still attached to a round piece of metal called a rose which sits on the face of the door. The spindle is attached to the outside knob. Once the knobs are removed, the latch should slide out from the edge of the door. If it doesn't slide out easily, make sure the screws were removed from the edge of the door and then try prying gently with the screwdriver.
Take the old lock with you to the store to buy a new one. A passage function tubular lock should cost about $10 to $20. A privacy function lock should cost $15 to $25. An entry function lock should cost $20 to $40. You can choose the same brand that you already have but that's not necessary. Since the 1970s, all tubular locks are designed to fit into the same size hole. The only difference is whether the backset is 2+3/8″ or 2+3/4″. Many locks are adjustable so they can fit both backsets.
When you get the new lock home, the first step is to slide the latch into the edge of the door. If the old latch was surrounded by a metal rectangle with rounded corners and the new one doesn't have rounded corners, you may need to use a chisel to square up the corners first or it won't fit flush. Put in the two short wood screws to hold the latch in place. Now put the outside knob on the correct side of the door with the spindle going through the latch body. Then put the inside knob on the spindle from the other side. You may have to rotate the knob back and forth to get it to slide on. If there's a button on the inside knob, you may have to twist the button back and forth a bit too. Pay attention to where the machine screws go and line up the screw holes on the roses. Then put in the screws and tighten them almost all the way.
Before you finish tightening the screws, try turning the knob and see if the latch retracts. You may notice that it binds up if the knobs are off center. Try to center the knobs with one hand and then tighten the screws with your other hand. Test the knobs again before you close the door. When you're ready to test it with the door closed, stand on the side of the door than has the screws, just in case it binds and you have to loosen the screws again. If the lock works fine with the door open but doesn't work with the door closed, it's time to give up and call the locksmith.
The author is a Certified Professional Locksmith.