A toilet supply line is a small part, but the wrong size can turn a simple replacement into a frustrating trip back to the hardware store—or a leak under the tank. For most modern toilets, the connector you need is a 3/8-inch compression connection at the shutoff valve and a 7/8-inch ballcock connection at the toilet tank. The length varies, so you’ll also need to measure the route between the two.
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The Standard Toilet Supply Line Size
Most modern toilet supply lines are sold as 3/8-inch compression by 7/8-inch ballcock. Those measurements describe the two ends of the connector, not the inside diameter of the hose.
- 3/8-inch compression: Connects to the shutoff valve near the wall or floor.
- 7/8-inch ballcock: Connects to the threaded inlet on the bottom of the toilet tank’s fill valve.
The two ends are different because the shutoff valve and toilet fill valve use different kinds of fittings. A typical flexible toilet connector is designed to bridge that difference without adapters.
When shopping, look for packaging that says something like “3/8 in. compression x 7/8 in. ballcock.” The line may also be labeled as a toilet connector or toilet supply connector. Don’t choose a line based only on a general description such as “3/8-inch hose”—that may refer to one end, and the other end still has to match your toilet.
What the Size Numbers Mean
The 3/8-inch compression end
The shutoff valve, also called an angle stop or supply stop, is usually mounted on the wall or floor behind the toilet. On many newer installations, its outlet accepts a 3/8-inch compression fitting. The nut on the supply line tightens against that outlet, and a small tapered metal ring called a ferrule helps form the seal.
Compression sizing can be confusing because the measurement doesn’t necessarily match the outside diameter of the part you’re looking at. It identifies a standard fitting size. If your existing connector attaches to the shutoff with a small nut and you can see a compression-style outlet, a 3/8-inch compression end is likely—but check the valve before buying.
The 7/8-inch ballcock end
The toilet end attaches to the fill valve, the part inside the tank that lets water in after a flush. Most standard toilet fill valves have a threaded inlet sized for a 7/8-inch ballcock connector. The connector usually seals with a washer inside its nut.
That washer is what makes the seal. In most cases, you should not wrap the tank connection threads with plumber’s tape. Tape is intended for certain threaded joints that seal on the threads; this connection generally seals where the washer presses against the fill valve. Follow the connector or fill-valve maker’s instructions if they say otherwise.
Check Your Shutoff Valve Before You Buy
The 3/8-inch compression connection is common, but it isn’t universal. Older plumbing or a previous repair may leave you with a different valve outlet. The wall-side connection—the part where the valve attaches to the household plumbing—can also vary. That connection isn’t the same thing as the valve’s outlet where the toilet supply line attaches.
Before removing anything, take a clear photo of the valve and the existing line connections. If you’re replacing the line, compare both ends of the old connector with the new one. A familiar-looking thread isn’t enough: compression, pipe-thread, and other fitting styles are not interchangeable just because their sizes sound similar.
If the valve outlet doesn’t match a standard 3/8-inch compression connector, don’t force the parts together. A mismatched fitting may seem to tighten but fail to seal, which can cause a slow leak or sudden spray. Ask a plumbing-supply specialist to identify the valve, or have a plumber replace it with a compatible shutoff.
Choose the Right Supply Line Length
Toilet connectors come in several lengths, commonly around 9, 12, 16, and 20 inches, though available options vary by brand. The right length is the shortest one that reaches comfortably while leaving a gentle curve.
To measure, trace the route from the shutoff outlet to the tank inlet. Measure along the path the line will take, rather than measuring straight through the air. Give yourself enough length for the line to curve without being pulled tight, but don’t buy an unnecessarily long connector just to avoid measuring.
A line that’s too short puts strain on its ends and can stress the valve or fill-valve connection. One that’s much too long can bunch up, kink, or rub against nearby surfaces. A smooth curve is what you want—not a sharp bend, tight coil, or stretched line.
| What to check | Typical choice | Why it matters |
|---|---|---|
| Shutoff valve end | 3/8-inch compression | Must match the outlet on the valve |
| Toilet tank end | 7/8-inch ballcock | Fits most standard toilet fill valves |
| Connector length | Measure the actual route | Prevents strain, kinks, and excess slack |
How to Replace a Toilet Supply Line
Replacing a connector is usually manageable if the shutoff valve works properly and the fittings are standard. Take your time, and don’t treat the small size of the job as a reason to skip leak checks.
- Turn off the water. Turn the shutoff valve clockwise until it stops. If it won’t close, leaks around its handle, or feels like it may break, stop and get help rather than forcing it.
- Flush the toilet. Hold the handle down to empty most of the tank. A little water may remain, so keep a small container and towel nearby.
- Loosen the old connector. Use an adjustable wrench if needed, but avoid twisting or putting heavy force on the shutoff valve or the plastic fill-valve shank.
- Check the fittings. Look for damaged threads, cracked plastic, corrosion, or a worn washer. If the valve is badly corroded or won’t operate smoothly, replacing the connector alone may not solve the problem.
- Install the new line. Seat the connector squarely on both ends. Hand-tighten the nuts first so they don’t cross-thread. Tighten them according to the manufacturer’s directions; excessive force can damage plastic parts or the sealing washer.
- Turn the water back on slowly. Watch both connections as the tank fills. Wipe them dry, wait a few minutes, and check again. A dry paper towel can help reveal a slow drip.
Don’t add plumber’s tape to a compression fitting or a washer-sealed ballcock connection unless the product instructions specifically call for it. Tape won’t fix a damaged washer, a crooked connection, or a fitting that doesn’t match.
Common Mistakes to Avoid
- Buying by hose length alone: The right length won’t help if one end has the wrong fitting.
- Confusing the valve inlet with its outlet: The connection from the wall to the shutoff valve can be a different size or type from the connection between the valve and toilet.
- Forcing a fitting: If a nut won’t thread on smoothly by hand, stop and check the size and alignment.
- Using a line that’s too short: A taut connector puts unnecessary stress on the joints.
- Ignoring a worn shutoff valve: A connector replacement won’t repair a valve that leaks or fails to shut off the water.
When to Call a Plumber
Call a plumber if the shutoff valve is stuck, leaking, heavily corroded, or attached to plumbing you’re not comfortable working with. The same goes if the existing fittings don’t match a standard toilet connector or if the valve begins leaking while you’re loosening the old line.
A working shutoff valve is useful beyond this one repair: it lets you stop water to the toilet without turning off water to the whole home. If yours doesn’t operate reliably, fixing that problem is often more important than simply installing a new connector.
Read more related posts:
- 10 signs of subfloor rot around the toilet base You Shouldn’t Ignore
- 10 reasons toilet handle sticks or feels loose plus simple fixes
- 10 causes of weak toilet flushing power easy fixes
Frequently Asked Questions
Is every toilet supply line 3/8-inch by 7/8-inch?
No, but it’s the common size for modern toilets with a standard shutoff valve. Check the valve outlet and the toilet fill-valve inlet before buying, especially in an older home or after previous plumbing repairs.
How do I know what length toilet supply line I need?
Measure the route from the shutoff valve to the tank inlet, following the path the connector will take. Choose a line that reaches with a gentle curve and no tension.
Can I use plumber’s tape on the toilet supply line?
Usually, no. The shutoff-side compression fitting and the tank-side ballcock connection typically seal with their own fitting surfaces or washers, not tape on the threads. Check the product instructions for the specific connector you bought.
Can I reuse my old toilet supply line?
It’s generally better to install a new connector when you’ve removed the old one. A used line may have a compressed washer, corrosion, or damage that isn’t obvious until it starts leaking.
Conclusion
For most modern toilets, the standard connector is 3/8-inch compression at the shutoff valve and 7/8-inch ballcock at the tank. Measure the route to pick the right length, confirm both fittings match, and install the line without forcing or overtightening it. When the valve looks damaged or the connections don’t match, pause and get the parts identified before turning the water back on.
