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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.

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