What You're Replacing and Why It Fails

The toilet water supply line is the flexible hose or rigid tube that carries water from the wall shutoff valve into the tank. Over time—usually five to ten years—these lines develop cracks, kinks, or internal corrosion that cause leaks under the tank, pooling water on your floor, or a slow drip you notice only when the water bill arrives.

Most supply lines fail because of mineral buildup inside the tube, repeated pressure cycles, or physical damage from being bent too sharply during installation. Replacing one is a straightforward job that takes 15 to 30 minutes and costs $15 to $40 for the line itself. You do not need a plumber unless the shutoff valve itself is broken or stuck.

Key Takeaways

  • Turn off the water at the shutoff valve behind or below the toilet, then flush to empty the tank before disconnecting anything.
  • Measure the distance from shutoff valve to tank inlet and note the connection type (compression fitting or push-fit) so you buy the correct replacement.
  • Disconnect the old line by turning the compression nut by hand or with an adjustable wrench, then hand-tighten the new line and test for leaks before turning the water back on.
  • If the shutoff valve leaks or will not turn, or if you cannot hand-loosen the compression nut, stop and call a plumber rather than forcing it.

Gather Your Tools and Materials

You will need an adjustable wrench, a bucket or towel to catch residual water, and a replacement supply line. Most hardware stores stock universal lines in lengths of 12, 16, 20, and 30 inches; buy one slightly longer than you need rather than one that is too short, because a tight line under tension will fail faster.

Check the connection type at both ends before you buy. Most modern toilets use a compression fitting at the tank inlet (a nut that screws onto a threaded post) and either a compression fitting or a push-fit connector at the shutoff valve. If you are unsure, take a photo of both connections and show it to the hardware store staff—they can match the line to your setup in seconds.

Turn Off Water and Empty the Tank

Locate the shutoff valve, which is usually a small knob or lever on the wall or floor behind the toilet. Turn it clockwise (or flip the lever perpendicular to the pipe) until it stops. You should feel resistance; do not force it if it is stuck.

Flush the toilet to drain the tank. If water continues to run, the shutoff valve is not sealing properly—stop here and call a plumber, because you cannot safely work on the line with water still flowing. If the tank empties normally, place a bucket or towel under the connection points to catch any remaining water in the line.

Disconnect the Old Supply Line

At the tank inlet (the bottom of the tank), you will see a compression nut holding the supply line. Try to turn it counterclockwise by hand first. If it moves, continue turning until the line comes free. If it does not budge after gentle pressure, use an adjustable wrench to grip the nut while you hold the fitting steady with your other hand to prevent the tank inlet from twisting.

At the shutoff valve end, repeat the same process. Some valves have a small nut; others have a push-fit collar that you squeeze or twist to release. If either connection is corroded or stuck, do not force it—call a plumber rather than risk cracking the valve or the tank inlet.

Install the New Supply Line

Before connecting, hold the new line in place to confirm the length is correct. The line should reach comfortably from shutoff to tank inlet without being stretched or bunched. If it is too long, you can coil the excess behind the toilet, but avoid sharp kinks.

Hand-tighten the compression nut at the tank inlet first, turning it clockwise until you feel resistance. Then hand-tighten the nut at the shutoff valve. Once both are snug by hand, use the adjustable wrench to turn each nut an additional one-quarter turn—no more. Over-tightening crushes the ferrule (the seal inside the nut) and causes leaks.

Test for Leaks Before Closing Up

Turn the shutoff valve back on by turning the knob counterclockwise or flipping the lever parallel to the pipe. Watch both connection points for drips. The tank should fill normally within two to three minutes. If you see water pooling at either connection, turn the shutoff valve off again, wait 30 seconds, and tighten that nut by one-quarter turn more with the wrench. Repeat until the leak stops.

Once the tank is full and both connections are dry, flush the toilet twice to confirm the fill and drain cycles work smoothly. Leave the bucket or towel in place for an hour and check again—sometimes a slow leak takes a few minutes to appear. If everything is dry, you are finished.

When to Call a Plumber Instead

Stop and call a professional if the shutoff valve will not turn, leaks from the valve stem itself, or is stuck in the open position. Do not force a stuck valve; you risk breaking it and flooding your bathroom. Similarly, if the compression nut at the tank inlet will not loosen by hand or with gentle wrench pressure, the fitting may be corroded to the tank—forcing it can crack the porcelain.

If you have replaced the line and tightened both connections but water still drips from one end, the problem is likely a damaged ferrule or valve seat inside the fitting. This requires a plumber to disassemble and repair the valve or tank inlet.

Frequently Asked Questions

Can I use the same supply line if I just tighten the nut?

If the line itself is cracked or has a visible hole, no—water will continue to leak. If the leak is only at the connection and the line looks intact, tightening the nut one-quarter turn may stop it. If tightening does not work after two attempts, replace the line.

What is the difference between compression and push-fit connections?

Compression fittings use a nut and ferrule to squeeze the line onto a threaded post; they are adjustable and reusable. Push-fit connectors snap onto a barbed post and are faster to install but harder to disconnect. Most supply lines work with both types—check the packaging to confirm.

How tight should the compression nut be?

Hand-tight plus one-quarter turn with a wrench. If you turn it more, you risk crushing the ferrule and creating a leak. If it leaks after that, tighten it another one-quarter turn, but stop after two additional turns total—further tightening will not help.

Can I replace the supply line while the water is still on?

No. Always turn off the shutoff valve first. If you disconnect the line with water pressure on, water will spray out and you will not be able to see what you are doing. A stuck shutoff valve is a reason to call a plumber, not to work around it.

Do I need to replace the shutoff valve too?

Only if it leaks from the handle or stem, will not turn, or does not shut off the water completely. A working valve can last 20 years or more. If the valve is fine, the new supply line alone will solve the problem.