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Drain & Vent Size Calculator

Size drain, waste and vent piping from drainage fixture unit loads and slope.

Fixtures draining to this segment

Count every fixture upstream of the pipe you are sizing. Fixture unit values are private-use figures from the drainage fixture unit table; a fixture group such as a full bath can also be taken as a single group value under some code sections.

3 DFU · 3" trap minimum

1 DFU · 1-1/4" trap minimum

2 DFU · 1-1/2" trap minimum

2 DFU · 2" trap minimum

2 DFU · 1-1/2" trap minimum

2 DFU · 1-1/2" trap minimum

2 DFU · 2" trap minimum

2 DFU · 1-1/2" trap minimum

1 DFU · 1-1/2" trap minimum

2 DFU · 2" trap minimum

1 DFU · 1-1/4" trap minimum

Horizontal run

More fall is not better. Too much slope lets water outrun the solids.

ft

Used to calculate total fall, which sets your invert depth.

Branch intervals on the stack
ft

From the connection at the drain to the terminal above the roof.

Minimum building drain

3"at 1/4 in. per ft

18 drainage fixture units over 60 ft, dropping 15.0 in.
Total drainage fixture units9 fixtures counted
18 DFU
Fixture branchServing the water closet, 1.6 gpf
3"
Horizontal branchCarrying the full fixture load
3"
StackThree or fewer branch intervals
3"
Vent stackGood for 110 ft developed length
2"
Total fall over the run0.250 in/ft × 60 ft
15.0 in

Slope meets the code minimum for this size

A 3" drain requires at least 0.125 in. per ft, and you have selected 0.250 in. per ft. Over 60 ft that is 15.0 in. of fall, so plan the invert depth at the far end accordingly.

Three-inch minimum applied for water closets

No water closet may discharge into a drain smaller than 3 in., so the horizontal branch, stack and building drain are all held to 3 in. minimum regardless of what the fixture unit count alone would allow.
Building drain and sewer capacity, drainage fixture units by slope
Size1/8 in. per ft1/4 in. per ft1/2 in. per ftStatus
2"Not permitted2126Below minimum size
2-1/2"Not permitted2431Below minimum size
3" — minimum364250Carries this load
4"180216250Carries this load
5"390480575Carries this load
6"7008401,000Carries this load
8"1,6001,9202,300Carries this load
Horizontal branch and stack capacity, drainage fixture units
SizeAny horizontal branchStack, ≤ 3 intervalsStack, > 3 intervalsPer interval
1-1/2"3482
2"610246
2-1/2"1220429
3"20487220
4"16024050090
5"3605401,100200
6"6209601,900350
8"1,4002,2003,600600

What this assumes

  • Drainage fixture unit values and capacity tables follow standard model plumbing code tables.
  • Horizontal drains 3 inches and smaller assume 1/4 inch per foot of fall; 4 inches and larger assume 1/8 inch per foot.
  • Vent sizing assumes the vent does not exceed the developed length limits for its diameter.

Important

DWV design is one of the most heavily inspected parts of a plumbing permit. Fixture unit values differ between the IPC and UPC; verify the code adopted in your jurisdiction.

Understanding the result

Slope is a range, not a minimum

Drains need enough fall to carry solids, but too much fall lets water outrun the solids and leaves them behind. A quarter inch per foot is the standard target for pipe up to 3 inches; steeper is not better.

What vents actually do

Vents equalize pressure so draining water does not siphon the water out of a trap. A trap that loses its seal lets sewer gas into the building, which is why vent sizing and distance limits are enforced as strictly as drain sizing.

Wet venting has its own rules

A wet vent carries both drainage and venting in the same pipe under specific conditions — bathroom group wet venting is the most common residential application. Wet vent sizing uses different fixture unit limits than dry vents, and the horizontal wet vent segment has maximum length restrictions that vary by code.

Cleanouts are part of the design

Every change of direction greater than 45°, every horizontal run, and the base of every stack needs a cleanout accessible for rodding. Missing cleanouts do not show up in a fixture unit calculation but fail inspection and make future blockages far more expensive to clear.

Building drain versus branch

Fixture units accumulate toward the building drain, which is the horizontal pipe between the base of the stack and the building sewer. A 3-inch building drain handles up to 42 DFU in most code tables; a 4-inch building drain handles up to 140 DFU. Undersizing the building drain while correctly sizing branches is a common design error.

Common questions

What size drain pipe for a bathroom?
A full bathroom group (toilet, lavatory, tub/shower) totals roughly 6 to 8 DFU. The toilet requires a 3-inch drain; the lavatory and tub can share a 2-inch branch. The bathroom group branch to the stack is typically 3 inches. A half bath with only a toilet and lavatory can use a 2-inch branch in some jurisdictions.
What is the minimum drain pipe size for a toilet?
A toilet requires a 3-inch drain minimum in virtually every U.S. jurisdiction. Some codes allow a 3-inch closet flange connected to a 3-inch branch; no toilet may connect to a 2-inch drain regardless of fixture unit count.
What slope for a 3 inch drain pipe?
A 3-inch horizontal drain requires 1/4 inch per foot of fall — roughly 2% grade. Over 10 feet, that is 2-1/2 inches of drop. A 4-inch drain may use 1/8 inch per foot. Too little slope causes standing water and blockages; too much lets water outrun solids.
How many fixture units can a 2 inch drain handle?
A 2-inch horizontal drain typically handles up to 21 DFU under standard plumbing code tables. That covers a lavatory (1 DFU), shower (2 DFU), and washing machine standpipe (3 DFU) on one branch — but not a toilet, which requires 3-inch regardless of DFU count.
What size vent pipe for a bathroom?
A single bathroom group vent is typically 2 inches. The vent must rise undiminished to at least 6 inches above the flood level rim of the highest fixture served before reducing. A 1-1/2 inch vent is allowed only for individual fixture vents serving one fixture with 1.5 DFU or less.
How far can a toilet be from the vent?
A toilet on a 3-inch drain may be up to 10 feet from the vent under typical IPC distance tables, measured along the pipe from the trap weir to the vent connection. Beyond that distance, the fixture is unvented and will siphon. Each pipe diameter has its own limit — 2-inch drains allow only 6 to 8 feet.
What size drain for a kitchen sink?
A kitchen sink is 2 DFU and requires a 1-1/2 inch drain minimum; 2 inches is standard practice. A dishwasher connected to the sink drain adds 2 DFU. The combined kitchen branch is typically 2 inches to the stack.
Can a shower and toilet share a drain?
Yes, on a common branch sized for the combined DFU. A shower (2 DFU) and toilet (4 DFU) total 6 DFU, which fits a 3-inch branch. The toilet sets the minimum pipe size at 3 inches even though the shower alone could use 2 inches.
What size pipe for a washing machine drain?
A washing machine standpipe is 3 DFU and requires a 2-inch drain minimum. The standpipe itself must be 2 inches and extend 18 to 30 inches above the trap weir. A 1-1/2 inch standpipe will not pass inspection.
How many DFU is a toilet?
A toilet is 4 DFU under the IPC and 3 DFU under the UPC. This difference matters when you are near a pipe size limit. Always confirm which code your jurisdiction adopted before sizing a borderline branch.

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