BUILDING DRAIN SIZE
About this calculator
The building drain is the lowest horizontal pipe inside the building, where every branch eventually empties before the line transitions to the building sewer at 5 feet outside the foundation. Its size is set by IPC Table 710.1 using a different DFU column from horizontal branches — the building drain is allowed higher loads at the same diameter because it always runs at full slope. This calculator returns the minimum building drain size for the total DFU load at both ⅛"/ft and ¼"/ft slopes, since slope choice affects capacity. ESTIMATE ONLY — verify with a licensed plumber and local plumbing code before installation.
How to use this calculator
Enter the total DFU served by the building drain (sum every fixture in the building — every lav, toilet, shower, sink, washer, etc.). Pick the slope: ⅛" per foot is allowed for 3" pipe and larger and minimizes vertical drop in tight basements; ¼" per foot is required for ≤2.5" pipe and adds capacity at every size; ½" per foot is the maximum before scour issues kick in.
The calculator returns the minimum building drain diameter from IPC Table 710.1 building drain column — note this is a different column than the horizontal branch table because building drains always run full and accept higher DFU loads per size. Any building drain carrying a toilet must be ≥3".
Worked example
For a typical 3-bath, 1-kitchen, 1-laundry house: total DFU ≈ 30 (3 × 6 DFU/bath + 4 kitchen + 8 laundry).
At ¼" per ft slope: 3" pipe handles up to 50 DFU on building drain → final 3".
At ⅛" per ft slope: 3" pipe handles up to 42 DFU → still 3" works.
Drop per 10 ft of run: ¼"/ft = 2.5", ⅛"/ft = 1.25". For a 50-ft building drain run, that's 12.5" vs 6.25" of vertical drop — meaningful in tight basements.
For a larger 5-bath home (60+ DFU): 3" insufficient at any slope → need 4". 4" at ¼" handles 216 DFU; at ⅛" handles 180 DFU. Both work.
For a multi-family building (200+ DFU): 4" pipe at ¼"/ft handles 216 DFU comfortably; 5" at ⅛"/ft handles 390 DFU for larger projects.
Common mistakes & waste factors
Using the horizontal branch table instead of the building drain table. The building drain column allows higher DFU at the same size because it always runs full. Using the branch table oversizes the building drain.
Forgetting the toilet rule. Any building drain carrying a WC must be ≥3". Common in basement remodels where someone tries to use 2.5" pipe to a single toilet.
Going ⅛"/ft on small pipe. ⅛" per foot slope is only allowed on 3" pipe and larger per IPC. Smaller pipe needs ¼"/ft minimum.
Ignoring the building sewer transition. The building drain becomes the building sewer at 5 ft outside the foundation. The sewer is sized separately (usually one size larger to handle increased velocity from depth changes) per local code.
Rules of thumb
IPC Table 710.1 building drain column at ¼"/ft slope: 2" up to 26 DFU, 2½" up to 31, 3" up to 50, 4" up to 216, 5" up to 480, 6" up to 840.
At ⅛"/ft slope (3"+ only): 3" up to 42 DFU, 4" up to 180, 5" up to 390, 6" up to 700.
Residential typical: 3" or 4" building drain handles most homes.
Drop per 10 ft: ⅛" = 1.25", ¼" = 2.5", ½" = 5". Choose slope based on available vertical drop in the basement.
Building drain transitions to building sewer at 5 ft outside the foundation. Sewer sizing per local code, often one size up.
Always add a cleanout at every direction change >45° and at the building drain start.