Fixture connection field guide

Plumbing Supply Line Sizes: Match Both Ends Before You Buy

The correct supply line is not “a faucet line” or “a toilet line.” It is a connector that matches both end connections, the required length, the route, and the exact fixture or appliance instructions.

Plumbing supply line sizes at a glance

Use this table to narrow the shelf—not to skip the model manual or a visual check. Connection names describe the mating surfaces and sealing method, not just a rough diameter.

Identify the line before you disconnect it

The most reliable shopping list is a four-part record. Photograph the installation first, then write down:

1. End A: the shutoff-valve outlet. Identify the connection type and nominal size. Do not estimate it from the wrench size used on the nut.
2. End B: the fixture or appliance inlet. Check the faucet shank, toilet fill valve, refrigerator valve, or dishwasher manual. Factory-attached faucet hoses can eliminate the need for a separate faucet-side connector.
3. The finished route. Measure along the route the line will actually take. Leave enough length for a relaxed bend or required appliance service loop, but do not create a tight coil that rubs, catches, or kinks.
4. The condition around both ends. Look for corrosion, damaged threads, a bent outlet, cracked plastic, missing sealing surfaces, water staining, or an adapter stack that should be corrected instead of copied.

Faucet supply lines: check whether the faucet already has hoses

BrassCraft lists both 3/8-inch compression × 1/2-inch FIP faucet connectors and 3/8-inch compression × 3/8-inch compression connectors for faucet pigtails. That difference matters. Some faucets use threaded shanks that accept separate connectors; others arrive with integrated hoses that connect directly to the stops.

Do not copy an improvised adapter chain simply because it has not leaked yet. If the stop outlet and faucet connection do not have a clean, listed connector path, identify the correct valve, faucet hardware, or manufacturer adapter before proceeding.

Toilet supply lines: the large nut has a specific job

A widely used toilet connector configuration is 3/8-inch compression at the shutoff and 7/8-inch ballcock at the toilet fill-valve shank. Fluidmaster and BrassCraft both document that pairing. The large plastic or metal swivel nut belongs at the fill valve; it is not a generic pipe-thread connection.

Before buying, check that the fill valve is stable, the shank is not cracked, the stop works, and the line can reach without pulling sideways. Hold the fill valve as directed by its manufacturer so tightening the connector does not rotate the valve inside the tank.

For a toilet that refills on its own or runs between flushes, the supply line may not be the failed part. Use the running-toilet diagnosis guide before replacing unrelated parts.

Ice-maker and refrigerator lines: plan the movement, not just the distance

GE’s current guidance calls for a 1/4-inch OD cold-water line with 1/4-inch compression connections on current ice makers, using approved tubing for the appliance. It also calls for extra tubing so the refrigerator can be pulled forward for service.

That service loop should be controlled—not pinched beneath the appliance, sharply bent at the valve, or left where the refrigerator can roll over it. Confirm the refrigerator model’s tubing material and fitting instructions. Do not connect an ice maker to hot water.

Dishwasher supply lines: the manual decides the appliance end

Dishwasher hookups are a good reason not to memorize one universal chart. BrassCraft lists a 3/8-inch compression × 3/8-inch compression dishwasher connector, while Whirlpool instructions for current models may call for a 3/8-inch supply connection and a 3/4-inch hose fitting with a rubber seal and 90-degree elbow at the appliance.

Find the model number and installation instructions before buying the kit. Verify whether the elbow is included, which side uses a gasket, and where the instructions permit thread sealant. Whirlpool’s current connection guide cautions against overtightening and directs readers to the owner’s manual.

If water is coming from the air-gap area above the sink instead, start with Dishwasher Air Gap Leaking? What to Check Before Buying Parts.

Five mistakes that turn a simple connector into a leak

1. Matching only one end. “It fits the valve” is only half the purchase.
2. Measuring the nut instead of identifying the connection. Wrench size and nominal fitting size are not interchangeable.
3. Using thread tape everywhere. Compression joints and gasket-sealed swivel connections often seal somewhere other than the threads. Follow the connector and appliance instructions.
4. Choosing length by straight-line distance. A taut hose, sharp bend, crushed loop, or side load can create a failure point.
5. Replacing the connector while ignoring the valve. Corrosion, damaged threads, an unreliable stop, or a leaking valve changes the job.

A safe replacement screen

This is a decision screen, not a substitute for the exact fixture, connector, and appliance instructions.

1. Confirm the complete shutoff plan. Make sure the fixture stop closes normally and know where the whole-house shutoff is before loosening anything.
2. Remove water and power hazards. Relieve water pressure as directed. For an appliance, follow its electrical shutdown instructions as well.
3. Protect the area. Use a shallow container and towels, keep electrical items dry, and maintain access to the valve.
4. Compare old and new parts off the connection. Confirm both ends, length, washers or ferrules, bend radius, ratings, and manufacturer compatibility.
5. Start threads by hand. If the nut does not turn smoothly, stop. Cross-threading cannot be fixed by a larger wrench.
6. Restore water slowly and inspect. Check both ends immediately, again after operating the fixture or appliance, and again after the area has been dry long enough to reveal a new drop.

Shopping checklist: what belongs on the package

• Fixture or appliance type and exact model where applicable
• End A connection type and nominal size
• End B connection type and nominal size
• Length that follows the real route without tension or kinks
• Approved temperature, pressure, potable-water, and appliance use
• Included washers, ferrules, elbows, or manufacturer-specific adapters
• Installation instructions that explain tightening and sealant requirements

Four useful product categories: faucet connectors, toilet connectors, approved ice-maker tubing kits, and model-compatible dishwasher supply kits. Treat a specific connector as an example only after its instructions and exact fit have been confirmed.

Continue the fixture decision path

Sources

• BrassCraft Water Connectors — manufacturer examples for faucet, toilet, washing-machine, ice-maker, and dishwasher connection patterns.
• Fluidmaster Click Seal Toilet Connector Sheet — 3/8-inch compression × 7/8-inch ballcock toilet connector configurations.
• Whirlpool Dishwasher Water Supply Connection — model-instruction priority, supply-line and elbow example, inspection, and leak-check guidance.
• GE Appliances Ice-Maker Water Supply Guidance — 1/4-inch OD cold-water line and service-loop guidance.

Research checked September 3, 2026. The sizes shown are common manufacturer examples; confirm the exact connector and appliance instructions before purchase or installation.