If you're considering a kitchen or bathroom remodel that includes a faucet replacement, then you'll need to know some things about your sink. Sinks and faucets come in different configurations. The faucet configuration has to line up with the number of holes in the sink. The two components don't necessarily have to match exactly, as extra holes can be hidden with plugs and baseplates. The good news is that even though holes covered by the existing faucet can be tough to spot—a few tips can help you count them all, and there are workarounds to make some faucets fit when there's not an exact match.
First the basics, bathroom sinks come in two basic configurations, drop-ins and under mounts. If your sink has a lip that lays on the countertop, you have a drop-in. If the sink is attached to the bottom of the counter, you have an under mount.
In many cases you can simply examine your faucet and count the holes. If you have separate handles for hot and cold, you have three holes in the sink. Measure the distance between the handles. Hold the the tape measure above the faucet if it's in the way.
If the distance is six inches or more you need a “widespread" or “eight inch spread" faucet. If the measurement is less than six inches you need a “centerset," “mini-spread" or a “four inch spread faucet." Each of these configurations is for a faucet that will use all three holes in the sink.
But wait. Look closer. The existing faucet may include a mounting plate or base that surrounds the faucet and may be hiding holes you can't see from the top. If you can get a good look at the bottom of the sink by using a flashlight and a tape measure, count the holes and measure the spread.
If you can't see or measure the holes from underneath, you may need to remove the faucet and you will need some tools. Start by looking under the sink for the incoming water lines. There will be two, one for hot water and one for cold water. Look for shut-off valves—small handles that when turned clockwise will shut off the water to the faucet. If you don't have shut-off valves, you'll need to turn the main water supply off. Once the water is off, turn on the faucet to drain whatever water is left in the lines.
To remove the existing faucet get yourself a basin wrench, which is a basic plumbing tool and unscrew the nuts that hold the water lines in place—you'll need to unscrew both nuts. If you have a one handle/one faucet configuration there should only be one nut to unscrew.
Remember “righty-tighty, lefty loosey" when attacking that nut. Clockwise makes it tighter, counter clockwise makes it looser. Once the water lines are off, there may be larger nuts that hold the faucet against the bottom of the sink and countertop. Use your basin wrench to remove those nuts. You may need to lay on your back to do this. If the nuts seem stuck, apply or spray a penetrating oil to whatever surface you can get to and let the oil sink in for awhile.
Ideally, once the nuts are off, the faucet should lift out. Old putty, caulk or hard water can make things more difficult as the faucet may seem to be glued to the vanity top. Use a razor knife or a sharp putty knife to remove old caulk or mineral deposits and then gently pry the faucet loose. Be careful not to damage the vanity top. Slip a towel or block of wood under the surface you're prying against. Once you have the faucet out, count the holes and measure the distance apart.
If there is only one hole in your sink you'll need a faucet designed with the handle as part of the faucet. If you have three holes but want a decorative set of water lines above the sink, select a “bridge faucet" that mixes the hot and cold.
If you want to change from a two handle set to a one handle you'll need to buy a single handle faucet with a base that's wide enough to cover the other two unused holes. Many bases will cover over unused holes that are four inches or less apart. Some single faucets are also available with eight-inch baseplates. Bridge faucets and separate mixing taps can also be installed on a three-hole sink if the new fixture has a base plate wide enough to hide the hole.
When you're finished with this task, while you may not have quite become a plumber you will be qualified to shop for new fixtures and faucets. Congratulations, smart homeowner!