How to Install a Bathroom Vanity (Plumbing)
Learning how to install bathroom vanity plumbing yourself can save hundreds of dollars in labor and give you a bathroom that looks professionally finished, as long as you follow the right order and test every connection before you close up. A vanity install is mostly about three connections: the drain, the hot water supply, and the cold water supply. If those three are aligned, sealed, and leak-free, the rest is cabinet work, leveling, and caulking. This guide walks through the full process for a standard freestanding or floating vanity with a single sink, including what to check in the wall before you start, how to set the cabinet so the sink drain lines up, and how to connect a P-trap and supply lines without cross-threading or over-tightening fittings.
Tools and Materials You Need
Gather everything before you disconnect the old vanity so you are not making a hardware store run with the water off. You will need an adjustable wrench, a basin wrench for tight faucet nuts, a level, a tape measure, a stud finder, a drill and driver, a bucket, towels, and a flashlight. For plumbing parts, have a new P-trap kit, slip-joint washers, plumbers tape, silicone caulk, and flexible braided supply lines in the correct length. If your new vanity uses a different drain height than the old one, you may also need a tailpiece extension, a trap adapter, or a short length of drain pipe. Confirm whether your faucet includes supply lines and a drain assembly, and note the drain type: pop-up, grid, or push-button.
Measure twice before you buy. Record the distance from the finished floor to the center of the drain outlet in the wall, the distance between the hot and cold supply stops, the width of the vanity opening, and the location of wall studs. Most bathroom vanities are 30 to 32 inches high to the countertop, with the drain outlet roughly 18 to 20 inches from the floor, but older homes vary widely. Also check the water supply valves: if the old angle stops are corroded, stiff, or leaking, replace them now while the vanity is out. It is far easier to swap a valve with open access than to do it later behind a finished cabinet. For planning context, see our bathroom plumbing hub and bathroom rough-in plumbing costs.
Shut Off Water and Remove the Old Vanity
Start by turning off the hot and cold water at the angle stops under the sink. Turn each valve clockwise until it stops, then open the faucet to relieve pressure and confirm the water is fully off. If a valve will not close completely or drips, shut off the main house supply and plan to replace that valve. Place a bucket under the P-trap and supply connections, because water will remain in the trap and lines even after shutoff. Disconnect the supply lines from the faucet or from the valves, then loosen the slip-joint nuts on the P-trap by hand or with channel-lock pliers and drain the trap into the bucket. Keep towels handy, since the trap holds a surprising amount of water.
Next, disconnect the faucet if it is attached to the old vanity top, and remove any caulk sealing the vanity to the wall. Cut through caulk with a utility knife to avoid tearing paint or drywall. Unscrew the vanity from the wall studs, usually through the back rail or side panels, and lift it out carefully. If the old vanity was glued or heavily caulked, work slowly to avoid damaging the wall. Once it is out, inspect the wall and floor for water damage, mold, or soft drywall around the drain and valves. Repair any damage now, before the new cabinet covers it. Clean the drain opening and valve threads, and cap the drain temporarily with a rag if you will not install the new vanity the same day, to keep sewer gas out of the room.
Check the Plumbing Rough-In
The rough-in is the position of the drain and supply pipes in the wall, and it determines whether the new vanity will fit without major rework. The drain stub-out should be centered behind the sink basin, or close enough that the P-trap can reach it without sharp offsets. The supply stops should be a few inches to either side of the drain, high enough to clear drawers but low enough to hide inside the cabinet. Hold the new vanity in place, or measure from its spec sheet, and compare the sink drain center to the wall drain center. A difference of an inch or two can usually be handled with a trap adapter or a slightly longer trap arm. A difference of several inches may require moving the drain in the wall, which is a job for a plumber if it involves opening drywall and cutting into the stack.
Also check the drain pipe material and size. Most bathroom sink drains are 1.25-inch or 1.5-inch pipe, and the trap kit must match. Verify that the vent is intact and that the drain has a proper trap: every fixture needs a trap to block sewer gas, and the trap must stay full of water. If you are changing from a pedestal sink to a vanity, or from a single sink to a double sink, the drain and vent layout may need professional review. For related drain work, see how to install a P-trap and shower drain installation guide.
Set and Level the New Vanity
With the rough-in confirmed, move the new vanity into position without the countertop or sink if they are separate pieces, since the cabinet alone is lighter and easier to level. Use shims under the base to level the cabinet front to back and side to side, checking with a level on the top rails. A vanity even slightly out of level will show in the countertop, cause drawers to slide, and stress the drain connection. Once level, mark the stud locations on the back rail and pre-drill pilot holes. Anchor the cabinet to at least two studs with screws long enough to hold securely, typically 2.5-inch or 3-inch cabinet screws. For a floating vanity, anchor only into studs or solid blocking, never drywall alone.
After anchoring, set the countertop and sink, following the manufacturer instructions for clips, adhesive, or sealant. Set vessel or drop-in sinks after the countertop is secured and seal the rim with silicone. Check that the sink drain hole lines up over the cabinet opening and that there is enough clearance for the P-trap and supply lines. Do not fully caulk the countertop to the wall yet if you still need to adjust the drain, since you may need to shift the cabinet a fraction of an inch to align the trap. Leave final caulking until after the plumbing connections are tested and proven leak-free.
Connect the Drain and P-Trap
Start the drain connection at the sink. Install the drain assembly through the sink basin with plumbers putty or silicone sealant under the flange, as specified by the manufacturer, and tighten the locknut underneath without cracking the basin. Attach the tailpiece, the vertical pipe that drops from the drain into the trap. Cut the tailpiece to length so the trap can sit level and align with the wall drain arm without forcing. Assemble the P-trap dry first to check alignment: the J-bend should connect to the tailpiece on one side and to the trap arm on the other, with the trap arm entering the wall fitting horizontally and with a slight downward slope toward the wall, about one-quarter inch per foot.
Once alignment looks right, disassemble, add the slip-joint washers with the beveled side facing the fitting, and reassemble hand-tight, then snug each nut about a quarter turn with pliers. Over-tightening plastic slip joints can crack the nut or deform the washer and cause the very leak you are trying to prevent. Make sure every washer is seated and no threads are cross-threaded. If the trap arm is too short to reach the wall fitting, use an extension; if it is too long, cut it squarely and deburr the edge. After assembly, run water and watch every joint with a flashlight and a dry paper towel, since slow drips are easier to see on a towel than on a shiny pipe. A properly installed P-trap holds water in its bend to block sewer gas while letting wastewater flow freely.
Connect the Water Supply Lines
Supply lines connect the angle stops to the faucet inlets, and flexible braided stainless lines are the standard choice for most vanities because they are easy to route and resist kinking. Measure the distance from each stop to its faucet inlet and choose lines slightly longer than needed so they curve gently without sharp bends. Connect the line to the angle stop first, then to the faucet, hand-tightening the nut and then snugging it with a wrench. Do not use plumbers tape on compression fittings with a built-in rubber washer, since the washer does the sealing and tape can interfere with the seat. Use tape only on tapered pipe threads where the manufacturer calls for it. Keep hot on the left and cold on the right as viewed from the front, which is the standard US arrangement and prevents scalding surprises.
If your faucet has separate hot and cold inlets, double-check that each line goes to the correct side before tightening fully. Route lines away from the P-trap and drawer slides so they do not rub or get pinched when drawers close. Turn the angle stops on slowly, one at a time, and check each connection for drips while the faucet is off, since supply leaks show up under pressure even when the faucet is closed. Then open the faucet and check again under flow. If a compression nut weeps slightly, tighten it an eighth of a turn at a time until it stops, but do not keep cranking if it does not seal, since the washer or ferrule may be damaged and need replacement. For faucet options and costs, see bathroom faucet installation costs.
Seal, Test, and Finish
Final testing is what separates a good install from a callback. With the faucet running, fill the sink to the overflow level, then release the drain and watch the full flow through the trap and into the wall. Check every slip joint, supply nut, and faucet base connection during and after the test. Leave the cabinet doors open and place a dry towel or paper under the trap overnight if possible, then recheck in the morning for slow leaks that only appear after the pipes warm up and expand. Also test the pop-up drain: it should open fully, close to hold water, and not leak around the flange when the sink is full. If the drain is slow, check for a misaligned tailpiece, a trap installed backward, or debris left in the drain during installation.
Once testing passes, apply a clean bead of silicone caulk where the countertop meets the wall and where the vanity meets the floor if needed, smoothing it with a wet finger for a neat finish. Reinstall drawers, shelves, and door fronts, and confirm they clear the P-trap and supply lines through their full travel. Install the faucet aerator, run both hot and cold water for a minute to flush any debris, and wipe down all chrome or finished surfaces. A careful how to install bathroom vanity plumbing job should last for years without drips, odors, or wobble, and knowing how each connection works makes future faucet swaps and trap cleanings far simpler.