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Ducts, pipes & wiring

Trap Arm Length Checker

Check how far your sink, tub, shower or toilet sits from its vent against the plumbing code your area uses, before you glue the pipe.

Your fixture drain
Fixture

Toilets are under a separate rule in both codes. The UPC puts "similar fixtures" with them, the IPC / IRC other self-siphoning fixtures, and neither text lists which: if yours may be one, ask your building department before relying on the first choice.

Plumbing code your area uses

States and cities adopt one of the two model codes, usually with their own amendments (California's plumbing code is built on the UPC, Colorado's on the IPC). If you do not know which applies to you, ask your building department.

Trap size

The nominal size marked on the trap, not a measurement across it and not the size of the pipe it feeds.

Size of the pipe leaving the trap (the trap arm)

The nominal size printed on the pipe. Under the IPC / IRC it may be larger than the trap, never smaller (a 1-1/4" trap on a 1-1/2" pipe, for example); under the UPC the two have to be the same size.

ft
Measure along the middle of the pipe, around any bends.From the trap weir (where the outlet of the trap turns level) to the vent fitting under the IPC / IRC, or under the UPC to the inner edge of the vent, the near inside wall of the vent pipe.Feet, or feet and inches like 3' 6". A number with no mark is read as feet; for inches add the mark: 42".
Slope the pipe is laid at

Result
    Side view; the distance is the length along the pipe, squeezed to fit. For a trap, the dashed level line runs from the trap weir, the fall is drawn to the same scale as the pipe thickness, and the shaded strip by the trap is the two pipe diameters a vent must stay out of. For a toilet the distance starts at the closet flange and includes the drop below it.

    Rule: 2021 IRC Table P3105.1 or 2024 UPC Table 1002.2 · Tested against 39 worked examples, 12 printed in a published source · How this is worked out →

    Common cases

    Questions people ask

    How far can a P-trap be from the vent?
    It depends on the trap size and on which model code your area has adopted, because the two codes print different tables. Under the 2021 IPC / IRC table the limits are: 1-1/4 in trap 5 ft, 1-1/2 in trap 6 ft, 2 in trap 8 ft, 3 in trap 12 ft, 4 in trap 16 ft. Under the 2024 UPC table they are: 1-1/4 in trap 2 ft 6 in, 1-1/2 in trap 3 ft 6 in, 2 in trap 5 ft, 3 in trap 6 ft, 4 in trap 10 ft. These are for sinks, tubs, showers and similar fixtures, not toilets, and local amendments can change either.
    How far can a toilet be from its vent?
    A toilet (water closet) is not on the trap arm tables. Under the 2024 UPC, the table leaves out water closets and similar fixtures, and a note limits the developed length between such a fixture and its vent to 6 ft, measured from the face of the closet flange to the inner edge of the vent; the 2021 and 2018 editions say the top of the closet flange. Under the 2021 IPC / IRC, an exception says the distance for self-siphoning fixtures such as water closets is not limited by the table. This tool reports that as "not limited", not as a pass.
    Where is the trap-to-vent distance measured, and what is the trap arm?
    The IRC defines the trap arm as the part of a fixture drain between the trap weir and the vent fitting, and the UPC as the part between a trap and the vent. The IPC and IRC limit its developed length from the trap weir to the vent fitting; the UPC from the trap weir to the inner edge of the vent. Developed length is measured down the centre of the pipe and its fittings, around any bends, not in a straight line. The UPC describes the trap weir as the low point of the waterway where it leaves the trap and turns horizontal.
    How much can a trap arm slope or drop?
    The IPC / IRC table lists a slope of 1/4 inch per foot for 1-1/4, 1-1/2 and 2 inch traps and 1/8 inch per foot for 3 and 4 inch traps, which are also those codes’ minimum slopes for drain pipe of those sizes, and the total fall of the fixture drain from its slope may not be more than one pipe diameter. The UPC table carries a note to maintain 1/4 inch per foot. This tool fails a slope flatter than those figures and a fall over one diameter. A steeper slope is reported as not covered by the table, because the text read does not say it is acceptable. If you do not enter a slope, the table slope is assumed and the result stays amber, marked as holding only if the pipe is laid at it.
    Can the vent connect right at the trap?
    No. The IPC and IRC say a vent may not be installed within two pipe diameters of the trap weir, under the heading of crown vents. The UPC says the trap arm may in no case be shorter than two times its diameter, and lists crown-vented traps as prohibited. This tool checks that minimum as well as the maximum.