A rapid drip from a leaking faucet can waste as much as 50 to 75 gallons of water a day -- and if it is the hot water that is leaking, the amount of waste really climbs: energy plus water. Fortunately, fixing a leaking faucet is a fairly simple do-it-yourself chore that can be accomplished with simple hand tools.

Today's faucets fall into three broad categories: compression faucets with regular washers, washerless faucets with diaphragms or disk-type inserts instead of a washer, and single-handle faucets that do not have separate hot- and cold-water handles.

Although external shapes and styling may differ, all washer-type compression faucets are basically similar in design: The handle is connected to a stem that has a rubber or plastic washer on the bottom end. When the handle is turned one way the washer presses down against the valve seat to stop the flow of water. When the handle is turned the other way, the stem raises the washer to permit water to flow out through the spout.

If the washer is badly worn, the faucet drips because the washer does not make a watertight seal when pressed down against its seal; the cure lies in replacing the washer. A new washer may also be called for when a faucet "chatters" or whistles every time the water is turned on -- especially if the noise disappears when the faucet is fully open.

Before taking a faucet apart, shut off the water supply by closing the valve under the sink, or by shutting off the main valve in the basement. Then, as can be seen from the detailed view of a typical washer-type bathroom faucet, remove the handle by taking out the screw that holds it onto the top of the stem. In most cases this screw will be covered by a decorative button-like that snaps into the recess. You will have to pry this out first, using the point of a knife or similar tool.

Some handles are held on by a small threaded collar on the underside, instead of a screw that goes through the top. With these, unscrew the collar, then pry off the handle.

The next step is removing the packing nut, using an adjustable wrench. The packing nut may be in the form of a decorative cap nut that covers the whole assembly, or there may be a separate cap nut that covers the packing nut and is either threaded on over the outside of the faucet body, or threaded onto the outside of the packing nut.

On some faucets, after the handle is removed there will be a decorative bonnet that must be removed before you can get at the packing nut. This decorative bonnet will be held in place with a thin, flat nut that must be unscrewed first. With the nut loose, the bonnet can be slid up and off the stem.

After the packing nut has been removed, the stem can be unscrewed from the faucet body to get at the old washer. Take out the screw that holds it in place, then replace with a new one of the proper size and shape. Some are beveled and some are flat; make sure the new one is the same style if you don't want the faucet to leak again. If you can't be sure of the original shape, take apart another faucet of the same type that you know is working properly, and use the washer from that faucet as a sample. If the brass screw that holds the washer in place is badly chewed up, replace it at the same time.

Reassemble the faucet by screwing the stem back into place, then the packing nut and bonnet (if the faucet has one). If the faucet has been leaking around the stem, replace the packing under the packing nut. This material comes in string form and is impregnated with a special graphite-grease compound. Dig the old material out from under the packing nut and replace it with new material by winding several turns of packing around the stem before tightening the packing nut down.

The last step is replacing the handle and turning the water back on. If it still leaks, or if it starts to leak again in a short time, either you have put in the wrong type of washer, or the faucet seat is damaged to such an extent that even a new washer cannot form a tight seal.

The cure for this is to use a special valve seat grinding tool -- an inexpensive tool available in most hardware stores. It threads into the body in place of the stem and has a burred steel pad at the end that refaces or regrinds the seat when used according to directions.

Kitchen or bathroom faucets with a mixing spout are basically similar in design, and are repaired in much the same way.

In some kitchen faucets, the handle also acts as a decorative bonnet that covers the packing nut, but in some models there is a separate bonnet under the handle that must be taken off before you can get at the packing nut. As explained above, the bonnet will be held down with a flat nut that has to be taken off first.

Shower and tub faucets that protrude from the wall are also similar -- the only difference is that most of the stem and the working parts are inside the wall (below the surface of the tile). To change the washer on one of these, take the handle off, then slide the bonnet off (again there will be a nut to be loosened first).

When the bonnet comes off you may find that the packing nut is below the surface and you cannot get at it with a flat wrench. This means you will need a special deep-socket wrench to take it out. These can be purchased from most hardware stores, or even borrowed in many cases. Slide the wrench over the stem, then use it to unscrew the packing nut so the stem can be taken out of the fixture.

If you cannot see or reach the packing nut because it is covered with cement or plaster, you will have to use a small chisel or an old screwdriver to dig the cement out until the top of the cap nut is exposed and you can slide the socket wrench on over it.

Washerless faucets differ from those with washers in two respects: Instead of a washer they have a small diaphragm or a pair of disks on the end of the stem; and instead of packing they have rubber O-rings around the shank of the stem, up near the top end. When a leak develops, take the faucet apart as described above, then remove the diaphragm or disks and replace with new ones of matching size and shape. Since parts from different brands are not always interchangeable, it is best to take the whole stem assembly with you when shopping for new parts.

Single-control or "one-arm" faucets normally do not develop leaks until after years of use, but when they do there are special cartridges or ball-assembly inserts you can buy to make the needed repairs.