Plumbing Pipe Sizes: The Complete Supply, Drain, and Vent Chart

Standard plumbing pipe sizes fall into a simple pattern: supply lines run small and drains run large. A home water service is 3/4 to 1 inch, interior supply branches are 1/2 inch, and fixture stops drop to 3/8 inch. Drains climb from a 1-1/4-inch bathroom sink to a 1-1/2-inch tub or kitchen sink, 2 inches for a shower or washer, 3 inches for a toilet, and 4 inches at the building drain.

The master plumbing pipe size chart

This chart maps every common fixture to four numbers at once: minimum trap size, drain size, vent size, and supply size. Most references show supply or drainage alone. Reading them together is how a plumber sizes a rough-in. Values below follow the International Plumbing Code (IPC); the Uniform Plumbing Code (UPC) differs on a few, so confirm your local adoption.

For material choices behind these sizes, see our guide to choosing the right pipe material, and for room-by-room layout, the bathroom rough-in dimensions.

What size are water supply pipes?

Water supply pipes are the small side of the system. The service line from the street or meter is 3/4 to 1 inch, the cold main inside the house is 3/4 inch, and branch runs to fixtures are 1/2 inch. Individual stops at a toilet or bathroom sink step down to 3/8 inch. The water heater feeds off a 3/4-inch line, hot and cold.

Two things push a supply line up a size: total demand and distance. A home 500 feet or more from the main may need a 1-1/4 or 1-1/2-inch service to hold pressure, and upper-floor branches sometimes go one size larger. Upstream of every branch sits a shutoff and angle-stop valve matched to the pipe.

What size are drain pipes for each fixture?

Drain pipes run larger than supply because they move waste by gravity, not pressure. A bathroom sink drains through 1-1/4 to 1-1/2 inches, a tub or kitchen sink through 1-1/2 inches, a shower or washing machine through 2 inches, and a toilet through 3 inches. Branches combine into a 3 or 4-inch building drain that leaves the house.

Slope carries as much weight as diameter. IPC sets 1/4 inch per foot for pipe 2-1/2 inches and smaller, and 1/8 inch per foot for 3-inch through 6-inch runs. Too little slope leaves solids stranded; too much lets water outrun the solids. Every drain connects with sized transition and reducing fittings, never a hard step-down that traps waste.

What size vent pipe do I need?

Vents keep trap seals from siphoning and let sewer gas escape above the roof. The working rule: a vent is at least half the diameter of the drain it serves, and never smaller than 1-1/4 inches. A 1-1/2 or 2-inch fixture drain usually vents with 1-1/2 inches, and a 3-inch toilet drain vents with 2 inches. The main stack through the roof is commonly 3 inches.

Cold-climate roofs often require a larger vent terminal, 3 inches or more, so frost cannot seal the opening shut. Air admittance valves (AAVs) are allowed in many areas where a full vent is impractical, but not all jurisdictions accept them, so check local code before you plan an island sink or a remote fixture around one.

What are fixture units, and how do they set pipe size?

A fixture unit is the code’s load rating for a fixture. Drainage fixture units (DFU) size drains and vents; water supply fixture units (WSFU) size supply pipe. You add the fixture units on a run, then read the minimum pipe size from the code table. A lavatory is about 1 DFU, a tub or shower about 2, a kitchen sink 2, and a toilet 3.

The table below shows how many DFU each drain size carries under IPC. A horizontal fixture branch caps lower than a building drain of the same diameter, and a 3-inch branch is limited to two toilets. UPC assigns slightly different values, so treat these as a planning baseline and verify locally.

On the supply side, WSFU works through the Hunter curve. A private lavatory is roughly 0.5 to 1 WSFU, a tank toilet about 2.2, a shower near 2, and a tub about 4. A full bathroom group lands near 6 to 8 WSFU, which a 3/4-inch main handles comfortably while a 1/2-inch branch serves one or two fixtures.

What size is the building drain and house sewer?

The building drain is the horizontal pipe that collects every branch and carries it to the house sewer at the foundation. In a typical single-family home it is 3 inches minimum and 4 inches in common practice, because a 3-inch drain is capped at two toilets under IPC while 4 inches carries a whole house with room to spare.

Slope holds at 1/8 inch per foot for a 4-inch line and 1/4 inch per foot for smaller runs. The house sewer beyond the foundation stays at 4 inches in most areas. Larger homes, added bathrooms, or long runs can call for a bigger main, which a licensed plumber sizes from the total DFU load.

Does pipe size affect drainage and pressure?

Yes, and correct is better than large. An oversized drain runs shallow and lets solids settle out, while an undersized drain clogs and gurgles. On the supply side, a pipe too small starves fixtures during simultaneous use, and one too large wastes water sitting in the line and slows hot-water delivery. Size follows the fixture-unit math, not a guess.

Pressure loss also climbs with distance, height, and fitting count, so a long or multi-story run may need a size up even when the fixture count is modest. This is why the same fixture can carry different pipe sizes in two homes. The chart gives the minimums; the load calculation and local code set the final number.

How code and material change the right size

The sizes here follow IPC, the most widely adopted model code, but many western states and cities use UPC, which differs on trap sizes, DFU values, and vent rules. Some areas add local amendments on top. Always confirm the code your jurisdiction has adopted before buying pipe or pulling a permit for anything beyond a like-for-like repair.

Material affects the number too. Nominal pipe size names the run (a 1/2-inch line), but PEX, copper, PVC, and CPVC each have their own inside diameter and flow at that nominal size. For a full breakdown of how each material performs and where it fits, read our guide to choosing the right pipe material before you commit a system to one size.

Frequently asked questions

What size water pipe do I need for a house?

Most homes use a 3/4-inch or 1-inch water service from the meter to the house, a 3/4-inch cold main inside, and 1/2-inch branches to fixtures. Toilets and bathroom sinks usually drop to a 3/8-inch supply stop. The water heater feeds off a 3/4-inch line. Fixture count and distance from the main can push the service up a size.

What size drain pipe does a toilet need?

A toilet (water closet) needs a 3-inch drain in almost every area, and many builders run 4-inch to cut clog risk on long runs. Code will not allow a 2-inch toilet drain because the trapway and flush volume demand 3 inches. A single 3-inch branch typically carries no more than two water closets under IPC.

What size pipe is used for a shower drain?

A shower drain is 2 inches in most current codes, though older 1-1/2-inch drains still exist. Two inches moves water fast enough to prevent standing water in low-threshold and curbless showers. The trap matches the drain at 2 inches. A tub drain, by contrast, is usually 1-1/2 inches because its flow is lower.

What size is a kitchen sink drain pipe?

A kitchen sink drain is typically 1-1/2 inches with a 1-1/2-inch trap, and 2 inches when a disposal, dishwasher, and double bowl all tie in. The 2-inch size gives more room for food waste and grease. Local code and the number of fixture units on the branch decide the final diameter.

What size vent pipe do I need?

A vent is generally sized at half the drain diameter it serves and never smaller than 1-1/4 inches. A 1-1/2-inch or 2-inch fixture drain commonly vents with a 1-1/2-inch pipe, while a 3-inch toilet drain vents with 2 inches. The main vent stack through the roof is often 3 inches, larger in cold climates to resist frost closure.

What are plumbing fixture units?

A fixture unit is a code value that rates how much a fixture loads the system. Drainage fixture units (DFU) size drains and vents; water supply fixture units (WSFU) size supply pipes. A lavatory is about 1 DFU, a shower or tub about 2, and a toilet 3. Adding up the DFUs on a run tells you the minimum pipe size from the code table.

Can I use 2-inch pipe for a toilet drain?

No. A 2-inch pipe cannot serve a toilet under IPC or UPC, because a water closet is rated at 3 DFU and needs a 3-inch minimum drain to pass the flush volume and solids. Two inches is the size for showers, washing machine standpipes, floor drains, and combined sink branches, not toilets.

Does drain pipe size affect how well it drains?

Yes, but bigger is not always better. A drain that is oversized for its flow runs shallow and can leave solids behind, while an undersized drain clogs and drains slowly. Correct slope matters as much as diameter: 1/4 inch per foot for pipe up to 2-1/2 inches, and 1/8 inch per foot for 3-inch and larger.

Last reviewed: September 2026.