Faucet Supply Line Size Guide: Diameter, Length & Spigot Fittings

The Short Answer: What Size Is a Faucet Supply Line?

The standard faucet supply line size for most residential homes is 3/8 inch in diameter at the faucet connection end, with a 1/2 inch connection at the shut-off valve end. This combination covers the vast majority of bathroom and kitchen sink installations built in the last 30 years. If you are replacing a supply line under your bathroom sink right now, a 3/8-inch compression fitting on one end and a 1/2-inch female threaded connection on the other is almost certainly what you need.

That said, older homes, high-flow kitchen setups, and outdoor spigot connections introduce variations that are worth understanding before you walk into a hardware store. The wrong size can restrict water pressure, cause leaks at connection points, or simply refuse to thread onto your shut-off valve. This guide walks through every dimension you need — diameter, length, thread type, and material — so you buy the right part the first time.

Faucet Supply Line Diameter: The Two Sizes That Cover 95% of Homes

Supply line diameter controls how much water can flow through the line at any given moment. A line that is too narrow chokes flow and drops pressure at the faucet; a line that is oversized adds no benefit and creates unnecessary bulk under the sink cabinet.

3/8-Inch Diameter — The Residential Standard

A 3/8-inch supply line is the default for bathroom sink faucets and most single-handle kitchen faucets installed after roughly 1990. It handles a flow rate of approximately 15 to 20 gallons per minute (GPM), which is well above the 1.2–2.2 GPM that a standard residential faucet aerator actually delivers. In practice, the aerator is always the flow-limiting factor, so a 3/8-inch line never becomes a bottleneck for normal use.

When a plumber or a DIYer talks about a "standard faucet supply line," this is the size they mean. It fits standard compression-style shut-off valves and the threaded tailpieces found on virtually every faucet sold at home improvement stores today.

1/2-Inch Diameter — Kitchen Faucets and Older Plumbing

A 1/2-inch supply line supports flow rates between 20 and 30 GPM and is found in two main scenarios: kitchen faucets with pull-out sprayers or high-flow heads, and older homes where the original plumbing was roughed in with larger-diameter stub-outs. If your shut-off valve has a notably wide outlet port, or if the existing supply line looks thicker than a typical pencil, you are likely looking at 1/2-inch fittings.

Many modern kitchen supply lines are sold as "3/8-inch to 1/2-inch" — meaning the faucet end is 3/8-inch and the valve end is 1/2-inch. This hybrid configuration became popular because it bridges the older 1/2-inch valve infrastructure with modern 3/8-inch faucet tailpieces.

5/8-Inch and 3/4-Inch — Outdoor Spigots and Main Lines

These larger diameters are not used for indoor sink faucets. They appear on outdoor hose bibs, spigot connections, irrigation headers, and whole-house water mains. A garden spigot threaded connection is typically 3/4-inch male hose thread (MHT), which is a completely different standard from the compression or IPS fittings used under a sink. Confusing these with indoor supply line sizes is a common mistake when a homeowner tries to adapt outdoor plumbing for an indoor fixture — it never works without the correct adapter fittings.

Supply Line Length: How to Measure and Which Size to Buy

Length is the other critical dimension. A line that is too short puts constant tension on the connection nuts, which leads to micro-leaks at the threads over time. A line that is too long creates excess slack that can kink under the sink cabinet, restricting flow and creating a pressure drop at the faucet.

To measure correctly, hold a flexible tape measure from the outlet port on your shut-off valve up to the faucet's tailpiece inlet. Add 2 to 4 inches to that measurement to allow for a smooth, relaxed curve. The resulting number tells you the minimum supply line length to purchase — rounding up to the next available standard size is always the right move.

Standard Length Options Available at Hardware Stores

• 12 inches — Used when the shut-off valve is mounted very close to the faucet tailpiece, common in pedestal sinks where the valve is on the wall directly below the basin.
• 16 inches — The most common length for standard bathroom sink installations. This fits the typical scenario where the valve is roughly 10 to 12 inches below the faucet connection.
• 20 inches — Standard for kitchen sinks, where the cabinet is deeper and the shut-off valve often sits farther from the faucet body.
• 24 to 30 inches — Used for large farmhouse sinks, laundry room faucets, or any situation where the shut-off valve is offset from the fixture mounting point.

A reliable rule: if your measurement falls between two standard sizes, always buy the longer one. The extra few inches of line can be looped neatly against the back of the cabinet. A line that is even half an inch too short cannot be stretched without risking a fitting failure.

Thread Types and Fitting Compatibility: Where Most Mistakes Happen

Getting the diameter and length right still leaves one more variable to address: the type of connection at each end of the supply line. Thread mismatches are the most common reason a new supply line leaks even after it has been tightened correctly.

Compression Fittings

Compression fittings use a brass ring (the ferrule) that compresses against the valve outlet as the nut is tightened, creating a watertight seal without thread engagement on the pipe itself. Most modern angle stop shut-off valves have a 3/8-inch or 1/2-inch compression outlet. This is the most forgiving connection type because it seals on the ferrule rather than on threads, making slight alignment imperfections less likely to cause problems.

IPS (Iron Pipe Size) Threaded Fittings

IPS connections use tapered or straight threads and require Teflon (PTFE) tape wrapped around the male threads before assembly. A 1/2-inch IPS thread is the standard for the supply end of many kitchen faucet supply lines. IPS threads are not interchangeable with compression fittings of the same nominal size — the thread pitch and sealing mechanism are entirely different.

NPT (National Pipe Taper) Threads

NPT is the North American standard for tapered pipe threads. The taper is 1/16 inch per inch of thread length, meaning the threads tighten as they screw in, creating a mechanical seal that is then made watertight with Teflon tape or pipe thread sealant. NPT 1/2-inch threads appear on older supply stub-outs coming out of the wall or floor and on some shut-off valve inlets.

Push-Fit (Quick-Connect) Fittings

Push-fit supply lines eliminate threading entirely. You simply push the fitting onto a compatible valve outlet and it locks with an internal collet and O-ring. These are popular for DIY installations because no wrench is needed, but they require that the valve outlet is specifically designed for push-fit connections. Attempting to push-fit a line onto a standard compression valve will not work and will leak immediately.

Outdoor Spigot Connections: A Different World

An outdoor spigot (hose bib) uses 3/4-inch male hose thread on its outlet — the same thread standard as a garden hose. The supply side of the spigot where it connects to the home's copper or PEX pipe typically uses a 1/2-inch sweat (solder) or 1/2-inch NPT connection. If you are running a supply line to an outdoor spigot rather than connecting a garden hose to it, you are working with the indoor supply side, and the relevant size is that 1/2-inch pipe connection — not the hose thread outlet. Many homeowners confuse these two ends when planning spigot replacements.

Supply Line Materials: Which One Should You Actually Buy?

The physical size of the supply line matters, but so does the material it is made from. Material selection affects burst resistance, lifespan, flexibility during installation, and how the line holds up to the specific water chemistry in your area.

Braided Stainless Steel

Braided stainless steel is the overwhelming choice for any permanent installation. The outer braid encases a flexible inner tube — typically PVC or PEX — and provides mechanical protection against kinking, crushing, and rodent damage. Quality braided lines are rated to burst pressures exceeding 800 PSI, far above the 40–80 PSI typical in residential water systems. They are resistant to corrosion, handle both hot and cold water lines without degrading, and are available in every standard size. For any kitchen or bathroom faucet replacement, braided stainless steel is the correct choice.

Braided Nylon

Braided nylon supply lines are lighter and less expensive than stainless steel. They work adequately for low-pressure bathroom applications but are not recommended for kitchen sink lines, which see higher flow demands and more frequent temperature cycling. Nylon braiding also provides less protection against physical damage from items stored under the sink.

Corrugated Copper

Corrugated copper supply tubes are rigid enough to hold their shape but flexible enough to bend into position. They are durable and completely immune to the corrosion issues that can affect low-quality braided lines at the brass connection nuts. The downside is that they are harder to work with in tight cabinet spaces and cost more than braided alternatives. Corrugated copper is most commonly seen in professional plumbing installations rather than DIY replacements.

PEX Tubing

PEX (cross-linked polyethylene) is color-coded — red for hot, blue for cold — making it easy to identify line function at a glance. PEX handles temperature fluctuations better than PVC and is resistant to scale buildup. It is commonly used as the inner tube inside braided supply lines and increasingly appears as a standalone rigid supply line in new construction plumbing.

How Long Do Supply Lines Actually Last?

Braided stainless steel supply lines have a typical service life of 10 to 12 years under normal residential water pressure (40–60 PSI). Homes with hard water or water pressure above 80 PSI should plan to inspect and replace supply lines every 5 to 7 years, as high mineral content accelerates corrosion at the brass ferrules and connection nuts. A supply line failure under a sink can discharge several gallons per minute — in a closed cabinet, that translates to thousands of dollars in water damage before it is noticed. Scheduled replacement is far cheaper than emergency water remediation.

How to Measure Your Existing Supply Line in Five Minutes

If you are replacing an existing supply line rather than installing a new faucet from scratch, the fastest approach is to measure what you already have. Here is a straightforward process:

1. Turn off the shut-off valve under the sink and open the faucet to release pressure from the line.
2. Place a small bucket under the connection points and disconnect both ends of the old supply line.
3. Measure the overall length of the old line from end to end. This is your baseline — purchase the same length or one size up.
4. Look at the connector nuts at each end. The size is often stamped directly on the nut — for example, "3/8 COMP" or "1/2 IPS." If the stamp is worn, use a caliper to measure the outer diameter of the threads.
5. Note whether each end uses a compression ferrule (a small brass ring visible at the connection) or whether it is a straight-threaded IPS connection (no ferrule, just threads).
6. Take the old supply line to the hardware store or use the measurements to order online. Matching the old part eliminates guesswork entirely.

If the connector nut markings are illegible and you do not own a caliper, a 3/8-inch outer diameter thread will pass through a 7/16-inch open-end wrench with slight clearance. A 1/2-inch outer diameter thread will not fit through a 7/16-inch wrench. This quick wrench test can confirm the diameter when measurement tools are not available.

Faucet vs. Spigot Supply Line Sizing: Understanding the Difference

The term "supply line" applies to both indoor faucets and outdoor spigots, but the sizing conventions are not the same, and this creates real confusion when a homeowner is working on multiple plumbing projects at once.

An indoor faucet supply line — the flexible hose running from the angle stop valve under your sink to the faucet tailpiece — uses the 3/8-inch and 1/2-inch compression or IPS standards described throughout this guide. An outdoor spigot, also called a hose bib, sillcock, or exterior faucet, is a different type of fixture. The outlet of a spigot where a garden hose connects uses 3/4-inch male hose thread (MHT), which has 11.5 threads per inch. This is specifically designed for garden hose female connectors and is not compatible with the plumbing thread standards used for indoor supply lines.

The supply side of a spigot — the connection between the spigot body and the home's water distribution pipe — is typically 1/2-inch copper sweat, 1/2-inch CPVC solvent-weld, or 1/2-inch PEX crimp, depending on the home's pipe material. Replacing a spigot means matching that connection type, which has nothing to do with the 3/8-inch or 1/2-inch supply line standards used for sink faucets.

In short: when people ask about faucet supply line size, they are asking about the flexible connector under the sink, measured in 3/8-inch or 1/2-inch. When the same question comes up in the context of a spigot or hose bib, the answer involves hose thread standards and pipe connection types — a completely separate sizing system.

Supply Line Sizing for Specific Fixture Types

Different fixtures around the house have slightly different supply line requirements. Here is a fixture-by-fixture breakdown:

Bathroom Sink Faucet

A standard bathroom sink faucet uses two supply lines — one for hot and one for cold. Each line is typically 3/8-inch diameter with a 3/8-inch compression connection at the shut-off valve and a 3/8-inch or 7/16-inch female threaded connection at the faucet tailpiece. Length is usually 16 inches, though pedestal sinks with wall-mounted valves may need 12-inch lines and some vanity configurations require 20-inch lines.

Kitchen Sink Faucet

Kitchen faucets also use two supply lines (hot and cold), but the lines are often 20 inches long to accommodate the deeper base cabinet. Diameter is most commonly 3/8-inch at the faucet end and 1/2-inch at the valve end — the hybrid configuration described earlier. If the kitchen faucet includes a built-in sprayer with a separate supply connection, a third line may be needed.

Toilet

Toilet supply lines run cold water only. They connect from a 3/8-inch compression shut-off valve to the toilet's fill valve inlet, which typically uses a 7/8-inch ballcock nut. Most toilet supply lines are sold as 12-inch or 16-inch lengths. Standard diameter at the valve end is 3/8 inch; at the toilet end, the supply line terminates in a larger nut sized specifically for the fill valve — this is not interchangeable with faucet tailpiece fittings.

Refrigerator Ice Maker and Water Dispenser

Refrigerator water lines are a special category. They typically use 1/4-inch outer diameter tubing — either copper, nylon, or braided — that connects to a saddle valve or a dedicated 1/4-inch angle stop. This is considerably smaller than standard faucet supply line sizes and is not interchangeable with 3/8-inch or 1/2-inch lines.

Laundry Room and Utility Sink

Utility sinks and laundry faucets often use 1/2-inch supply lines throughout, since these fixtures are connected to 1/2-inch rough-in supply pipes with standard 1/2-inch threaded valves. Washing machine hoses are a different category entirely: they are 3/4-inch hose thread on both ends, connecting to a washing machine fill valve on one side and a dedicated hot/cold washing machine shut-off valve on the other.

Installation Tips to Prevent Leaks from Day One

Even a perfectly sized supply line will leak if it is installed incorrectly. The following practices come from standard plumbing trade practice and eliminate the most common sources of post-installation drips:

• Always start threading by hand. If the nut does not thread smoothly by hand for the first two full turns, stop — the threads are cross-threaded. Back out, realign, and start again. Forcing a cross-threaded connection creates a leak that no amount of tightening will fix.
• Use Teflon tape on IPS and NPT threads. Wrap three to four layers in the direction of the thread (clockwise when viewed from the thread end). Do not use Teflon tape on compression fittings — the ferrule creates the seal, and tape can actually prevent it from seating properly.
• Hand-tighten first, then use a wrench for exactly one quarter-turn past hand-tight. Over-tightening cracks the ferrule on compression fittings and strips the threads on IPS connections. More is not better when tightening supply line connections.
• Check for leaks slowly. After turning the shut-off valve back on, open it only 25% for the first 30 seconds. This prevents a water hammer surge that can knock a newly connected ferrule slightly out of position. Then open fully and inspect all connection points with dry hands or a dry paper towel.
• Replace both lines at the same time. If one supply line is old enough to need replacement, the other one is the same age and will fail soon. Replacing both during one work session is far more efficient than returning under the sink six months later.
• Avoid right-angle bends. The supply line should have a gentle, smooth curve from the valve to the faucet tailpiece. A sharp 90-degree bend near a connection point puts mechanical stress on the fitting and eventually causes a fatigue leak at the connection nut.

When You Need an Adapter — and When You Should Replumb Instead

Adapters exist to bridge size mismatches between supply lines and connection points. A 3/8-inch to 1/2-inch adapter, for example, allows a 3/8-inch supply line to connect to a 1/2-inch IPS port. These adapters are inexpensive, widely available, and work reliably when installed correctly.

That said, adapters are not always the right solution. If your shut-off valve is old, corroded, or difficult to fully close, the better long-term move is to replace the shut-off valve with a modern quarter-turn ball valve in the correct size, then use a supply line that fits directly without adaptation. Adding an adapter to a questionable valve just introduces another potential failure point at a connection that may already be weakened by age.

For outdoor spigot installations where someone is trying to connect an indoor-style flexible supply line to an exterior hose bib body, adapters can bridge hose thread to pipe thread — but this is a scenario where the correct solution is almost always to use the appropriate fixture type rather than adapting the wrong one. A spigot is designed to have a garden hose or irrigation component connected to its outlet, not a flexible sink supply line. Forcing an indoor supply line onto an outdoor spigot outlet creates an assembly that is neither code-compliant nor mechanically sound for long-term service.

Buying the Right Supply Line: What to Look for at the Store

Supply lines are sold by diameter, length, and end configuration. The packaging label on a standard supply line will typically read something like: "3/8-inch compression × 1/2-inch female IPS, 20 inches, braided stainless steel." Breaking that down:

• 3/8-inch compression — the valve end, fits a 3/8-inch compression shut-off valve outlet.
• 1/2-inch female IPS — the faucet end, threads onto a 1/2-inch male IPS tailpiece on the faucet body.
• 20 inches — overall installed length of the line.
• Braided stainless steel — the outer construction material; confirms it is the recommended type for residential use.

When buying in person, check that the connection nut threads feel clean and well-formed. Avoid lines where the brass connection nuts show discoloration or casting defects, as these are more prone to cracking when tightened. For online purchases, stick with supply lines that list a rated burst pressure (minimum 500 PSI is acceptable; 800 PSI or higher is preferred) and that carry a warranty of at least five years. These specifications indicate a product built with adequate quality control rather than a cut-rate alternative.

If you are uncertain which end configuration matches your setup, bring the old supply line with you. Hardware store staff can match it in under two minutes, and having the physical part eliminates any ambiguity about thread type or fitting size.