Vent pipe size calculator

How the vent pipe size is read

This page reads a vent size from a table and does not derive one. The stack is a soil or waste stack with a stack vent or a vent stack, and its load is the sum of the drainage fixture units of every fixture on it, from Table 709.1 of the International Plumbing Code. Table 906.1 lists, for each stack diameter and a load in DFU, the longest developed length of every vent diameter that is allowed. The size is the smallest listed vent whose longest length is at or above the length of the vent. Stacks of 1-1/4 to 15 inch are covered, and individual, branch and circuit vents are not.

Load in DFU

load = sum of (fixtures x DFU per fixture), Table 709.1Each fixture carries a drainage fixture unit value, and the stack carries their sum. The opening example, a two-bath house with a kitchen sink, a dishwasher, a clothes washer and a laundry tray, adds up to 20 DFU, the same house as on the sanitary drain page.

Row of the table

row = first row of the stack diameter whose DFU is at or above the loadTable 906.1 has a few rows for each stack diameter, and a 3 inch stack has rows at 10, 21, 53, 102 DFU. A load between two rows is read in the higher one, and there is no interpolation. Reading 20 DFU on a 3 inch stack gives the 21 DFU row, while 22 DFU gives the 53 DFU row.

Vent size

size = smallest vent in the row whose longest developed length is at or above the lengthEach row lists the longest developed length of every vent diameter it allows. The 21 DFU row of a 3 inch stack allows 32 ft for 1-1/2 inch, 110 ft for 2 inch, 270 ft for 2-1/2 inch and 810 ft for 3 inch. A 60 ft vent takes 2 inch, which carries 110 ft, and a 30 ft vent takes 1-1/2 inch.

Minimum size

vent >= one-half the stack diameter, and >= 1-1/4 inchSection 906.1 sets a floor of one-half the drain served and 1-1/4 inch. The first column of every row in the table is at or above it, so a 4 inch stack never takes a vent under 2 inch, even for 10 ft.

Developed length

length = distance along the vent pipe to the open airNote a of Table 906.1 measures the developed length from the vent connection to the open air. It follows the pipe through every offset, so it is at least as long as the straight distance. The ICC example of a 3 inch stack with 102 DFU and 25 ft takes a 1-1/2 inch vent, and 26 ft takes 2 inch.

Worked examples from the table

The default case is a house with two baths, a kitchen sink, a dishwasher, a clothes washer and a laundry tray, 20 DFU on a 3 inch stack. Table 906.1 has no row for exactly 20 DFU, so the load is read in the next row, 21 DFU. A vent of 60 ft takes 2 inch, which carries 110 ft in that row, and a vent of 30 ft fits 1-1/2 inch. A vent of 150 ft takes 2-1/2 inch.

Table 906.1, the longest developed length in feet by stack diameter, load and vent diameter, as this page reads it. A hyphen is a vent size the table does not list.

Eight cases below run through the same functions the form uses. A 3 inch stack with 102 DFU and 25 ft takes 1-1/2 inch, which is the example in the ICC guide to the 2021 code, and one more foot, 26 ft, takes 2 inch.

Standards and data sources

• ICC, Methods of Venting Plumbing Fixtures and Traps in the 2021 International Plumbing Code - Table 906.1 for stacks of 1-1/4 to 8 inch, up to the 3,400 DFU row of 8 inch, note a on the developed length, and Example A, a 3 inch stack with 102 DFU and 25 ft that takes a 1-1/2 inch stack vent.
• UpCodes, Texas IPC 2021, Chapter 9 Vents - Table 906.1 in full, 38 rows from 1-1/4 to 15 inch stacks, and the text of Sections 906.1 to 906.3. The UpCodes print shifts the 1-1/4 inch column of the 2 and 2-1/2 inch stack rows down one row, and this page takes those cells from the ICC print and the Virginia code.
• UpCodes, Maryland Plumbing Code 2024 (the 2024 IPC), Chapter 9 Vents - A second print of the same table in the 2024 edition. Every cell agrees with the Texas print of the 2021 edition, so the table did not change between the two.
• Virginia Plumbing Code 2006, Chapter 9, Table 916.1, as posted by InspectApedia - An independent print of the whole table from an IPC-based code. It agrees in every cell but two that were edited later, 720 against 729 ft for the 8 inch stack at 3,400 DFU with an 8 inch vent, and 570 against 571 ft for the 10 inch stack at 15,000 DFU with a 10 inch vent. This page uses the 2021 values.

Limitations of this calculator

• Stack vents and vent stacks only, sized from Table 906.1. Individual, branch, circuit and relief vents are sized at one-half the drain they serve, not under 1-1/4 inch, and grow one size past 40 ft under Section 906.2, which the page does not apply.
• Stacks of 1-1/4 to 15 inch, up to 50,000 DFU on a 15 inch stack. A load above the last row of a stack, 20 DFU for 2 inch and 102 DFU for 3 inch, is refused, and so is a length above the last cell of the row. The page never takes a vent size that the table does not list.
• The table was read in the ICC venting brochure for the 2021 code, in two UpCodes prints of the 2021 and 2024 code and in the 2006 Virginia code. ICC's own code page returned an error to the reader, so the model code text itself was not read, and the 2018 edition was not compared cell by cell. Check the table in the edition your jurisdiction has adopted.
• You type the load and the length. The page does not add the DFU of the fixtures, does not measure a developed length from a drawing, and does not check the limit of 1 inch of water column on the pressure at a trap seal that the venting rules are written to hold.
• Wet vents, common vents, island fixture vents, combination drain and vent, waste stack venting, circuit and loop vents and air admittance valves are separate methods with their own rules. A vent header that joins several stacks is sized under Section 906.1 on the sum of their DFU and the longest vent length, and the page does not combine stacks for you.
• A stack of five branch intervals or more needs a vent stack under Section 904.2, and one of more than ten branch intervals needs relief vents under Section 908. The page takes the stack as given and does not count branch intervals.
• The roof opening of a vent is a separate rule. Where the 97.5 percent winter design temperature is 0 degrees F or lower, Section 903.2 asks for vent extensions through a roof or wall to be at least 3 inch, enlarged inside the building at least 1 ft from the thermal envelope, so a 1-1/2 or 2 inch result still needs a larger terminal there. Sizes are nominal diameters in inches and lengths are in feet, as the table prints them. The Universal Plumbing Code sizes vents with its own tables, and local amendments change the IPC table in some jurisdictions.
• This is a planning estimate, not a design drawing. A licensed plumber or engineer and the authority having jurisdiction decide the vent sizes and the installation.

Common vent sizing mistakes

1. Reading the load in a lower row. Table 906.1 has fixed rows, and a load between two rows is read in the higher one. On a 3 inch stack 21 DFU at 100 ft takes 2 inch, but 22 DFU is read in the 53 DFU row and the same 100 ft takes 2-1/2 inch, since that row carries only 94 ft with a 2 inch vent.
2. Measuring the length in a straight line. The length is the developed length along the pipe to the open air, as note a of Table 906.1 words it. A vent of 60 ft takes 2 inch on a 3 inch stack with 20 DFU, and a run of 150 ft once the offsets are counted takes 2-1/2 inch, so a length taken from the height of the building understates the size.
3. Adding the 40 ft increase to a stack vent. The extra size past 40 ft is in Section 906.2, which covers individual, branch, circuit and relief vents. A 3 inch stack with 21 DFU takes 2 inch at 60 ft from Table 906.1 alone, with no further size added. Branch and individual vents are sized at one-half the drain they serve and are not read from this table.
4. Taking a vent smaller than the table lists. A vent is never under one-half the stack diameter and never under 1-1/4 inch. On a 4 inch stack the smallest listed vent is 2 inch even for a run of 10 ft, and no stack above 2 inch lists a 1-1/4 inch vent.
5. Taking the vent size from the drain size. Vent and drain diameters come from different tables. The sanitary drain page sizes the drain, and this page reads the vent for a stack of that size. A 3 inch stack stops at 102 DFU in Table 906.1, and a load above that is refused here.

Common questions about vent pipe size

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