Drainage fixture unit calculator

Adds up drainage fixture units and finds the minimum size of every horizontal branch, stack, building drain and building sewer in a layout, from the fixture unit tables of the 2024 International Plumbing Code (IPC) or the 2024 Uniform Plumbing Code (UPC). Every rule that sets a size is named. It runs in your browser. Nothing you enter is sent anywhere.

This calculator needs JavaScript. It runs fully in your browser and sends nothing to a server. The method and sources are on the methodology page.

1. Code

Code and occupancy 2024 International Plumbing Code (IPC). Sections 704, 709 and 710 with Tables 704.1, 709.1, 709.2, 710.1(1) and 710.1(2). 2024 Uniform Plumbing Code (UPC). Sections 702, 703, 708, 717 and 1003.3 with Tables 702.1, 702.1(1), 702.2 and 703.2.

2. Pipe sections

Add one section for each run of pipe that you want a size for: a horizontal branch, a stack, the building drain or the building sewer. Put the fixtures on the section their fixture drains connect to, and say which section each one discharges into. A section carries its own fixtures plus everything that discharges into it. A branch that repeats on every floor can be entered once with the number of copies.

Result

Minimum pipe size for each section
Section	Load (dfu)	Table	Minimum size	Cell (dfu)

Load is the drainage fixture units carried by the section, summed exactly (half units shown as .5). Cell is the value of the table cell that the selected size is checked against. A size can be larger than the cell alone requires: the rule under each size says why.

Layout as entered
Section	Discharges to	Kind	Slope or length	Fixtures and discharges

Show the math

For each section: every fixture is rated from the fixture unit table of the chosen code (IPC Table 709.1 or 709.2; UPC Table 702.1 or 702.1(1)) and the units are added, together with the load of every section that discharges into it. The sizes of the capacity table are then tried from the smallest up (IPC Table 710.1(1) for the building drain, its branches and the sewer, IPC Table 710.1(2) for branches to a stack and for stacks; UPC Table 703.2 for all of them), and the first size whose cell is not less than the load and that no table note excludes is kept. The minimum rules follow: the water closet minimum, the trap size of the fixtures, the clothes washer connection, and no reduction in the direction of flow. The largest of all of them is the size shown. Only the cells that decide this layout are shown. More on the methodology page.

This result reads the fixture unit tables of the code you selected, the 2024 International Plumbing Code (Sections 709 and 710) or the 2024 Uniform Plumbing Code (Sections 702, 703 and 717), for gravity sanitary drains inside one building. It is a calculation aid, not a design approval. Your local code official decides what is accepted. Under the IPC and the UPC, plumbing work needs a permit (IPC Section 105.1, UPC Section 104.1) and is inspected before it is covered (IPC Section 111.2, UPC Section 105.1). State or local law sets what applies where you work and who may do the work. Use the code edition and amendments your jurisdiction has adopted, and check vents, trap arms, cleanouts and fittings separately.

Printed from PlumbMath. Drainage sources last checked on October 4, 2026.

What this tool does not do

Vents and vent stacks, trap arm lengths, cleanouts, fittings and changes of direction.
Combination waste and vent systems, wet vents, circuit vents and single stack systems, which have their own sizing rules.
Pipe larger than 6 in. Building sewers 8 in. and larger have maximum and minimum loadings per slope under the UPC (Table 717.1).
Stack offsets in buildings of five stories or more (IPC Section 711), and stack reduction toward the top.
Indirect waste receptors, grease and oil interceptors, sumps, ejectors and pumps beyond converting their flow to fixture units, septic tank sizing, and storm or combined drainage.
UPC Appendix C and any appendix, the International Residential Code, and editions other than 2024. State and local amendments.
Under the UPC, note 7 of Table 703.2 (up to 8 public lavatories on a 1-1/2 in. vertical branch or horizontal sanitary branch at 1/4 in. per ft) is applied only to the Lavatory row and only to a horizontal branch that drains to a stack, not to lavatories in sets, to a stack, or to the building drain.

The tables read, the rules applied and the sources are on the methodology page. Drainage sources were last checked on October 4, 2026.