IPC 709.1 · 710.1 · Drain sizing

How to size drain pipe with drainage fixture units

Assigning DFUs, reading the building-drain and branch/stack tables, slope and branch intervals, and how the UPC differs — with a worked example and a free link to the official tables.

Need the actual tables? Read IPC Tables 709.1 and 710.1 free at codes.iccsafe.org →

Which table sizes this pipe?

Pipe	What it is	Sized with
Fixture drain / trap arm	From one trap to the branch	Minimum trap and fixture-drain sizes (709 and trap tables)
Horizontal fixture branch	Horizontal drain collecting fixtures, feeding a stack or the building drain	Table 710.1(2), horizontal-branch column
Stack	Vertical soil or waste pipe running through one or more floors	Table 710.1(2), stack columns — by number of branch intervals
Building drain and building sewer	The lowest horizontal piping carrying everything out of the building	Table 710.1(1), by pipe size and slope
Continuous or semicontinuous flow (pumps, AC condensate, etc.)	Not a fixture with a DFU value	Converted to DFU by flow rate (709.3)

Our own summary, not the code text. Read Tables 709.1, 710.1(1), and 710.1(2) free at codes.iccsafe.org (2021/2024 IPC) and verify against the edition adopted by your AHJ.

How to read the tables, step by step

Step	What you do	Watch out for
1	Assign each fixture its DFU from Table 709.1	Use the bathroom-group line when a water closet, lavatory, and tub/shower share a room — it is less than the sum of the parts
2	Work downstream, totaling DFU at every section of pipe	Each section is sized for everything upstream of it, not the whole building
3	Horizontal branches: find the smallest size whose horizontal-branch capacity in Table 710.1(2) meets the total	Branches that serve water closets carry minimum-size rules no matter how low the DFU
4	Stacks: count branch intervals, then use the matching stack column; check the per-interval limit too	A stack can never be smaller than the largest branch connected to it
5	Building drain: choose the slope you will actually install, then read Table 710.1(1) in that slope column	Minimum slope depends on pipe size (Table 704.1); flatter slope = less capacity
6	Never reduce pipe size in the direction of flow	A 4-in. drain that drops to 3 in. downstream fails inspection

Our own summary, not the code text. Read Tables 709.1, 710.1(1), and 710.1(2) free at codes.iccsafe.org (2021/2024 IPC) and verify against the edition adopted by your AHJ.

Worked example — single-family house, building drain

Input / step	Value	Source
Two full bathrooms (1.6 gpf water closets)	2 bathroom groups × 5 DFU = 10 DFU	Table 709.1 — bathroom group
Kitchen sink, laundry, and other fixtures	Add each one’s DFU from Table 709.1	Table 709.1
Say the total comes to	18 DFU	Your fixture schedule
Building drain installed at 1/4 in. per ft	3-in. capacity at that slope: 42 DFU	Table 710.1(1)
Result	3-in. building drain is adequate (18 ≤ 42); many designers still run 4 in. to the sewer by local rule or preference	Check local amendments and the sewer connection size

Worked example values only. Read the full tables free at codes.iccsafe.org.

IPC vs. UPC — how the drainage tables differ

Topic	IPC (ICC)	UPC (IAPMO — common in western states)
Fixture-unit table	Table 709.1	Table 702.1
Public vs. private use	One value for most fixtures, plus bathroom-group lines	Separate private and public values for many fixtures
Pipe capacity table	Tables 710.1(1) (building drain by slope) and 710.1(2) (branches and stacks)	Table 703.2 — horizontal and vertical capacity plus maximum lengths
Result on the same fixtures	DFU totals and pipe capacities are not interchangeable	Size with one code from start to finish

Our own comparison, not the code text. Consult the UPC through IAPMO where your jurisdiction adopts it.

Common mistakes

Mistake	What to do instead
Summing every fixture in a bathroom separately	Use the bathroom-group value where the fixtures qualify
Reading the building-drain table in the wrong slope column	Use the slope you will actually install, and confirm the minimum slope for that size
Using the stack column for a horizontal branch	Branches and stacks have separate columns with different capacities
Ignoring branch-interval limits on a stack	Check both the total stack load and the load per branch interval
Leaving out continuous flows (sump or ejector discharge, condensate)	Convert them to DFU per 709.3 and add them in
Mixing IPC DFU values with UPC pipe tables	Stick to one code throughout

Our own summary, not the code text. Read Tables 709.1, 710.1(1), and 710.1(2) free at codes.iccsafe.org (2021/2024 IPC) and verify against the edition adopted by your AHJ.

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From fixture list to pipe size

Read Tables 709.1 and 710.1 free on codes.iccsafe.org. ICC offers free read-only access to the 2021 and 2024 International Plumbing Code. This page explains how to use the tables; it does not reproduce them.

Drain sizing is bookkeeping. Every fixture gets a drainage fixture unit value from Table 709.1. Walk the system downstream, keep a running total at every section, and size each section for what it carries. Horizontal branches and stacks come from Table 710.1(2); the building drain and sewer come from Table 710.1(1), where capacity depends on slope.

Worked example. A house with two full baths counts each as a bathroom group at 5 DFU — 10 DFU before the kitchen and laundry. If the total reaches 18 DFU and the building drain runs at 1/4 inch per foot, a 3-inch drain listed for 42 DFU at that slope handles it. Flatten the slope and the capacity drops, so read the column for the slope you will really install.

Western jurisdictions on the Uniform Plumbing Code (IAPMO) use UPC Table 702.1 for fixture units — often with separate private and public values — and Table 703.2 for pipe capacity and maximum lengths. The numbers are not interchangeable with the IPC.

Every drain needs air: see plumbing vent sizing. For the roof side, storm drain sizing and hanger spacing; for the count of fixtures that feed all of this, minimum plumbing fixtures. Field PM keeps rough-in inspections, test records, and photos with the job.

FAQ

What is a drainage fixture unit (DFU)?+

A DFU is a load factor that rates how much a fixture adds to the drainage system, based on how much water it discharges, how fast, and how often. You total the DFU on each pipe section and pick a pipe size whose listed capacity covers it. The values are in IPC Table 709.1, readable free at codes.iccsafe.org.

How do I size a building drain?+

Total the DFU on the drain, pick the slope you will install, and read IPC Table 710.1(1) in that slope column for the smallest pipe whose capacity meets the total. For example, a 3-inch building drain at 1/4 inch per foot is listed for 42 DFU. Then check local rules on minimum building-drain and sewer size.

Why is a bathroom group rated lower than its fixtures added up?+

People rarely flush, run the lavatory, and shower at the same moment in one room, so the code rates the group as a whole. Use the bathroom-group line only when the fixtures actually qualify as a group under the definition.

What is a branch interval?+

A branch interval is a vertical stretch of stack — at least about a story high — where horizontal branches connect. Stack capacity depends on how many intervals the stack has, and the code limits how much load can enter at any one interval.

How is UPC drain sizing different?+

The UPC assigns its own DFU values (Table 702.1), often with separate private and public ratings, and sizes pipe with Table 703.2, which also caps the length of some pipe sizes. The two codes give different answers for the same fixtures, so use the code your jurisdiction adopted.

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