Vent distance calculator
Two limits govern a trap arm and most people only know one. The table caps the developed length from the trap weir to the vent fitting. The second, quieter rule caps the fall over that arm at one pipe diameter — break it and the trap siphons itself dry no matter how short the run is. This checks both.
Pre-filled with a lavatory on a 1-1/2" trap. Both limits are checked.
Table limit for this size
- Required slope
- 1/4"per ft
- Fall over your arm
- 1.25in
- One pipe diameter
- 1.50in
- Headroom left
- 1.0ft
Inside both limits: under the table length and under one pipe diameter of fall.
| Trap size | Slope | Max distance |
|---|---|---|
| 1-1/4" | 1/4" / ft | 5 ft |
| 1-1/2" | 1/4" / ft | 6 ft |
| 2" | 1/4" / ft | 8 ft |
| 3" | 1/8" / ft | 12 ft |
| 4" | 1/8" / ft | 16 ft |
Why two limits
A trap holds a water seal. The vent's job is to break the vacuum behind a discharging fixture so that seal is not pulled out. Distance is the first constraint: too far and the vent cannot relieve the pressure fast enough. Fall is the second: once the arm has dropped by more than the pipe's own diameter, the discharge fills the pipe cross-section and acts as a piston, dragging the seal with it.
A 1-1/2" arm at the required 1/4" per foot uses its whole diameter of fall in six feet — which is exactly why the table stops at six. The two rules are the same rule seen from two directions.
Developed length, not straight line
What to do when you are over
Three moves, in order of cost. Move the vent closer — often it is only a stud bay away. Wet-vent the fixture off an adjacent one, if your adopted code permits it for that group. Or add a vent, which on a finished wall is the expensive answer and the reason inspectors catch this at rough-in rather than at final.
An air admittance valve is a fourth option in some jurisdictions and is banned in others. Do not design around one without confirming with your authority having jurisdiction (AHJ) first.
Code notes
IPC 909.1 IRC P3105.1 Same table, same numbers, two documents. The exception for self-siphoning fixtures — water closets — is written into both: their developed length is not limited, because the trap refills from the bowl after every flush.