The standard faucet supply line sizes are 3/8 inch compression fitting at the valve end and either 1/2 inch IPS or 7/8 inch ballcock thread at the fixture end. Length typically runs 12 to 20 inches for most sink installations, though toilet supply lines commonly use 12-inch braided stainless steel connections. Getting the right diameter and thread type before you shop saves a second trip to the hardware store — and prevents leaks that silently destroy cabinets.
This guide covers every measurement you need, explains how spigot thread standards differ from compression fittings, and helps you match supply lines to your specific fixture type.
Standard Faucet Supply Line Sizes at a Glance
Supply line sizing involves two separate measurements that often confuse homeowners: the connector diameter at each end and the overall length of the line. These are completely independent. A line rated "3/8 inch" refers only to the compression nut end that connects to the shutoff valve — the fixture end can be a completely different thread size.
Here is how the most common residential supply line configurations break down:
| Application |
Valve End (Inlet) |
Fixture End (Outlet) |
Common Lengths |
| Bathroom faucet |
3/8" compression |
1/2" IPS or M10 |
12", 16", 20" |
| Kitchen faucet |
3/8" compression |
1/2" IPS |
16", 20", 30" |
| Toilet fill valve |
3/8" compression |
7/8" ballcock |
12", 16", 20" |
| Ice maker / dishwasher |
1/4" compression or 3/8" |
1/4" compression |
60", 72", 120" |
| Outdoor spigot line |
1/2" or 3/4" NPT |
3/4" garden hose thread |
Varies |
Standard faucet supply line size configurations by fixture type
Notice that outdoor spigot connections follow a completely different standard. A spigot — the outdoor hose bib mounted to your house exterior — uses 3/4-inch garden hose thread (GHT) at the outlet and typically 1/2-inch or 3/4-inch NPT pipe thread at the wall connection. These are not interchangeable with indoor supply line fittings, and mixing them causes thread cross-contamination and leaks.
How to Measure Faucet Supply Line Size the Right Way
Measurements that seem simple often trip people up because pipe sizing in North America uses nominal dimensions — the number stamped on the fitting does not match what a ruler shows. A 3/8-inch compression nut actually measures closer to 5/8 inch across the outside. This discrepancy comes from the iron pipe sizing (IPS) system, which dates to the 1800s and has never been updated to reflect actual dimensions.
Measure the Shutoff Valve Outlet First
The shutoff valve under your sink or behind your toilet is your starting point. Most residential shutoff valves in the United States have a 3/8-inch compression outlet, which is by far the most common size you will encounter. However, older homes built before 1970 may still have 1/2-inch iron pipe stub-outs with no shutoff valve at all. In those cases, you either install a shutoff valve first or use a supply line with a 1/2-inch IPS inlet nut.
To confirm your valve size, look at the outlet end (the smaller end pointing up or toward the wall) and measure the outside diameter of the threads. A 3/8-inch compression valve outlet will measure approximately 0.625 inches (5/8 inch) across the threaded portion.
Measure the Faucet Shank or Toilet Fill Valve Inlet
At the fixture end, measurement depends on what you are connecting to:
- For faucets: the threaded tailpiece that hangs down through the sink deck is typically 1/2-inch IPS male thread. The supply line's female nut threads onto this. Some European and modern designer faucets use M10 x 1 metric thread instead — check the faucet documentation before purchasing a line.
- For toilets: the fill valve inlet is a 7/8-inch ballcock thread, also called 7/8-inch slip joint. The supply line nut slides over the threaded shank and tightens against a rubber washer — no thread engagement occurs at the fill valve end.
- For outdoor spigots: the spigot body itself connects to your home's water pipe via NPT threads, while the hose end uses GHT threads. Supply lines running to an outdoor spigot inside a wall are copper or PEX — not the flexible braided lines used under sinks.
Measure the Required Length
To get the correct length, measure the distance from the center of the shutoff valve outlet to the center of the faucet shank inlet. Then add 2 to 3 inches to allow a gentle curve without kinking. Supply lines should never be pulled straight or forced to bend at a sharp angle. A line under tension will eventually fail at the fitting junction — usually at the worst possible time.
Common length stock at hardware stores runs: 12 inch, 16 inch, 20 inch, and 30 inch. For kitchen faucets where the shutoff valve is far from center on deep base cabinets, 30-inch lines are frequently necessary. Always buy a line slightly longer than your measurement rather than shorter.
Thread Types and Fitting Compatibility: Compression vs. IPS vs. NPT
The single biggest source of leaks in DIY supply line installations is threading an incompatible fitting. There are three distinct thread systems in common use, and they do not interchange even when the nominal size number appears identical.
Compression Fittings
Compression fittings use a brass olive (ferrule) that deforms around the pipe when the nut is tightened, creating a seal without thread engagement in the water path. They connect to smooth copper tubing or the outlet nipple of a shutoff valve. The 3/8-inch compression nut on most supply lines connects this way. Compression connections are reusable only once — after tightening, the ferrule is permanently deformed and must be replaced if the fitting is ever removed.
IPS (Iron Pipe Size) Threads
IPS threads are straight (parallel) threads that rely on a rubber or nylon washer to create the seal. The 1/2-inch IPS female nut at the faucet end of a supply line threads onto the male IPS tailpiece of the faucet. The thread engagement itself does not seal — the washer against the faucet seat does. This is why you should never apply thread tape (PTFE) to IPS connections on supply lines; it can prevent the washer from seating correctly.
NPT Threads (Including Spigot Connections)
NPT (National Pipe Taper) threads are tapered — they get slightly smaller in diameter toward the end. The taper itself creates the seal as the threads wedge together, supplemented by PTFE thread tape or pipe dope. A spigot's wall connection uses NPT threads, typically 1/2 inch or 3/4 inch. Never connect a flexible braided supply line directly to an NPT fitting without an adapter, as NPT's wedging action will over-stress the line's end fitting and cause failure.
The practical rule: if you are connecting under a sink to a shutoff valve and a faucet, you are working with compression (valve end) and IPS (faucet end). If you are working with any outdoor spigot connection or in-wall pipe threading, you are in NPT territory and need different materials entirely.
Braided Stainless, PVC, and Corrugated Copper: Which Supply Line Material to Choose
Supply line material affects lifespan, pressure rating, and appropriate use location. The most common options available today each have specific strengths and limitations.
SS
Braided Stainless Steel over EPDM Core
This is the current industry standard for residential sink and toilet supply lines. The inner hose is EPDM rubber or nylon, wrapped in stainless steel braid for burst resistance. Rated pressure is typically 800 PSI burst and 150 PSI working pressure — far exceeding residential supply pressure, which rarely exceeds 80 PSI. The braided exterior resists puncture from cabinet items and does not UV-degrade indoors. Expected service life is 8 to 10 years. Replace proactively at the 10-year mark regardless of visible condition; EPDM degrades from the inside out.
PVC
PVC or Vinyl Braided Lines
Cheaper than stainless, PVC braided lines are found in budget fixtures and older installations. They are functional at typical residential pressures but become brittle after 5 to 7 years, particularly when exposed to UV light or temperatures above 120°F. They are not recommended for installation near hot water lines where the ambient temperature under the cabinet rises significantly. If you find a vinyl supply line on a property you've purchased, replace it with braided stainless immediately — the failure mode is sudden, not gradual.
Cu
Corrugated Copper or Chrome Supply Lines
Corrugated copper lines are flexible, durable, and compatible with hot and cold applications. They are commonly used for exposed plumbing where aesthetics matter — a pedestal sink with visible supply lines beneath, for example. Chrome-plated brass supply tubes are rigid straight tubes (not coils), used when a clean professional appearance is required. These require a properly measured cut and are not forgiving of measurement error. Chrome tubes are also used in spigot installations where the connection runs through finished cabinetry.
Step-by-Step: Replacing a Faucet Supply Line Without Leaks
A supply line replacement that leaks almost always traces back to one of three errors: wrong fitting size, over-tightening, or forgetting to seat the washer before threading. The steps below address all three.
- Shut off the water at the angle stop valve beneath the fixture. Turn the handle clockwise until it stops. Open the faucet to release pressure and drain the line. If the valve does not stop water flow fully, the valve itself needs replacement — do not proceed with a leaking shutoff valve.
- Disconnect the old supply line. Use a basin wrench or adjustable wrench on the upper nut (faucet end) and standard adjustable wrench on the lower nut (valve end). Turn counterclockwise. Have a small towel or bucket ready — residual water will drip from the line and valve outlet.
- Inspect the shutoff valve outlet and faucet tailpiece threads. Look for corrosion, stripped threads, or debris. Clean with a wire brush if needed. Confirm whether the valve outlet is 3/8-inch compression or 1/2-inch IPS by measuring across the threads with a caliper or thread gauge.
- Select the correct replacement supply line. Match both end sizes and buy a length 2 to 3 inches longer than your measured distance. For a toilet, ensure the fixture-end nut is the 7/8-inch ballcock size, not the 1/2-inch IPS used for faucets. These are sold separately and labeled differently in stores.
- Hand-thread the faucet end first. Place the rubber washer inside the nut (check that it is present and undamaged), then thread the nut onto the faucet tailpiece by hand until snug. Tighten with a wrench: one-quarter turn past hand tight for IPS connections. Do not use thread tape on this connection.
- Connect the compression end to the shutoff valve. Slide the nut onto the valve outlet, then thread clockwise. Tighten to hand tight plus one quarter to one half turn with a wrench. Compression fittings do not require tape or compound. Over-tightening collapses the olive and splits the nut — stop at hand tight plus one-half turn maximum.
- Turn on the water slowly and check for leaks. Open the shutoff valve one quarter turn, then look at both fittings. Wait 60 seconds. If no drips, open fully. Check again after 10 minutes with water pressure stabilized. A single drop indicates improper seating — close the valve, disconnect, inspect the washer, and reconnect.
Outdoor Spigot Supply Line Sizes and Connection Standards
The term spigot applies specifically to an outdoor hose bib — the faucet mounted through your home's exterior wall that provides outdoor water access. Spigot connections follow a different sizing standard than indoor supply lines, and the two systems are not directly compatible.
Most residential outdoor spigots in North America use 1/2-inch or 3/4-inch NPT male threads at the pipe connection (the end inside the wall) and a 3/4-inch GHT female outlet for garden hose attachment. The in-wall pipe supplying the spigot is typically 1/2-inch copper (type L or M) or 1/2-inch CPVC, although 3/4-inch supply lines are used when the spigot services irrigation systems or high-volume applications like filling pools.
Frost-Free Sillcock Length and Sizing
Modern exterior spigots are almost universally frost-free sillcocks. Instead of shutting off water at the exterior wall, a frost-free spigot places the shutoff seat 8 to 12 inches inside the insulated wall cavity, where temperatures stay above freezing. The body length of a frost-free sillcock must match your wall thickness. Common sizes are 8 inch, 10 inch, 12 inch, and 14 inch. If you replace a spigot and install a body too short for your wall, the seat will be in the cold zone and the spigot will freeze and burst in winter.
To determine the correct frost-free sillcock size: measure the wall thickness from the interior pipe stub-out to the exterior siding surface, then add 2 inches. That is your minimum body length. The NPT thread size at the pipe end must match your existing supply pipe — 1/2-inch pipe uses a 1/2-inch NPT fitting, 3/4-inch pipe uses 3/4-inch NPT.
Spigot Thread Standards vs. Indoor Supply Line Threads
The garden hose thread (GHT) at a spigot outlet is 3/4 inch with 11.5 threads per inch — this is sometimes labeled as 3/4-11.5 NH (National Hose). This is completely different from:
- 3/4-inch NPT: 14 threads per inch, tapered — used for pipe connections
- 3/4-inch IPS: parallel threads — used for indoor plumbing fittings
- 3/8-inch compression: used exclusively for shutoff valve to supply line connections indoors
Attempting to thread a GHT fitting onto an NPT spigot port will result in cross-threading because the thread pitch is different (11.5 TPI vs. 14 TPI). The threads may appear to engage for the first two turns, then lock and strip. If you need to connect different thread standards at a spigot location, use a purpose-made adapter with the correct thread specifications on each end — these are sold at plumbing supply houses and labeled with both thread types.
Common Faucet Supply Line Problems and What Causes Them
Most supply line failures fall into predictable categories. Understanding the failure pattern tells you whether the line itself, the fitting, or the connected valve is the source of the problem.
The compression olive is either not seated properly or has been over-tightened and split. If the leak starts immediately after installation, the nut was hand-tightened only — add one quarter turn with a wrench. If it leaks after years of service, the olive has corroded or deformed; replace the entire supply line rather than just retightening.
The rubber washer inside the IPS nut is missing, degraded, or rotated out of position. Disconnect, inspect the washer, replace if cracked or hardened, and reconnect. This is the most common cause of leaks on newly installed supply lines — the washer sometimes shifts during hand-threading.
A bulge on a braided stainless supply line indicates the inner EPDM hose has already failed and water is pressing against the braid. This line is about to burst. Shut off the valve immediately and replace the line. Do not attempt to add clamps or tape — the failure will occur at the point of maximum weakness regardless.
A supply line pulled taut between the valve and fixture applies constant tensile stress on both fitting connections. This is a slow failure — the connections gradually loosen under vibration and thermal cycling. Remove the line, measure properly, and install a longer replacement. A 16-inch line installed where a 20-inch line belongs will fail within 2 to 5 years.
Water appearing at the exterior wall around a spigot body indicates the NPT thread connection inside the wall has failed or was never sealed with PTFE tape. This often happens when a frost-free sillcock is installed without threading tape or pipe dope on the NPT connection. Addressing this requires opening the interior wall to access the connection — it cannot be fixed from the exterior.
A supply line that whistles under flow has a partial obstruction, most commonly a washer that has swollen and partially blocked the line interior, or a debris particle lodged in the fitting. The sound is harmless temporarily but indicates reduced flow and impending blockage. Replacing the line eliminates the problem in 90% of cases.
Frequently Asked Questions About Supply Line Sizing
Are faucet supply lines universal?
No. Supply lines come in multiple end configurations and lengths. The most common combination — 3/8-inch compression by 1/2-inch IPS, 12 to 20 inches — fits the majority of bathroom and kitchen faucet installations in North America, but toilet applications require a 7/8-inch ballcock end, and some faucets use metric thread tailpieces. Always verify your specific fitting sizes before purchasing.
Can I use a longer supply line than I need?
Yes, as long as the excess length does not create kinks or sharp bends. A braided supply line with excess length should be looped gently — not kinked — inside the cabinet. Avoid letting the loop rest against sharp cabinet edges. A gentle single loop with a minimum 4-inch radius causes no issues and is common in kitchen installations where the shutoff valve is offset significantly from the faucet.
What size supply line do I need for a touchless kitchen faucet?
Most touchless and pull-down kitchen faucets use the same 3/8-inch compression by 1/2-inch IPS supply line as conventional faucets. The electronic components in touchless faucets connect to power, not to the water supply lines. Check the faucet's installation manual to confirm tailpiece thread size — some brands use proprietary adapters that ship with the faucet and attach to a standard 1/2-inch IPS supply line.
Does the inner diameter of the supply line affect flow rate?
At typical residential pressures (40 to 80 PSI), the inner diameter of flexible supply lines has minimal impact on faucet flow rate. The faucet aerator is the flow-limiting component in virtually every residential installation. Supply line inner diameter becomes a meaningful factor only in commercial applications with high-flow fixtures or when running long runs of over 10 feet. For standard under-sink connections under 30 inches, any properly rated supply line will deliver adequate flow.
How often should faucet supply lines be replaced?
Braided stainless steel supply lines should be replaced every 8 to 10 years regardless of visible condition. EPDM inner hoses degrade from the inside and show no external warning before failure. Vinyl or PVC braided lines should be replaced at 5 to 7 years. If you are buying a home built before 2015 and cannot verify when the supply lines were last replaced, replacing all lines at the time of purchase costs under $50 per fixture and eliminates a significant potential water damage risk.
What is the difference between a spigot and a faucet supply line connection?
A spigot (outdoor hose bib) uses NPT pipe threads at the wall connection and GHT garden hose threads at the outlet — both are entirely different from indoor faucet supply line threads. Indoor faucet supply lines use compression fittings at the valve end and IPS threads at the faucet end. The two systems serve different pressure classes, temperature ranges, and connection methods. Using indoor supply line fittings on an outdoor spigot connection is not acceptable plumbing practice and will result in leaks.
English
русский
日本語
عربى
Español
