Drainage Pipe Size for Houses per SNI 8153: 3-Inch Toilet Drain, Slope and Vents
Under SNI 8153:2015, the drainage pipe size in a house is set by the number of drainage fixture units (UBAP in Indonesian), not by the plumber's habit. The basic rules: any pipe serving a toilet must be at least 3 inches (80 mm), 2-inch pipe is only for small loads such as basins, showers and floor drains, and the reference slope for horizontal pipes is 20.8 mm per metre (about 2 %). A house with more than three toilets on one drain needs a 4-inch pipe.
Drainage pipe size per fixture (Table 10)
| Fixture (private/house) | Minimum trap | Fixture units |
|---|---|---|
| Toilet, 6 litres per flush (LPF) | 3 in | 3.0 |
| Toilet above 6 LPF | 3 in | 4.0 |
| Single lavatory (basin) | 1¼ in | 1.0 |
| Shower, single trap | 2 in | 2.0 |
| Bathtub or combination bath/shower | 1½ in | 2.0 |
| Domestic kitchen sink | 1½ in | 2.0 |
| Domestic clothes washer | 2 in | 3.0 |
| Floor drain | 2 in | 2.0 |
The table is taken from UPC 2012 (IAPMO). A note under the table says toilets shall be counted as 6 fixture units, and the drain pipe for some fixtures must be at least 2 inches (63 mm). To stay on the safe side, use the higher figure in your calculation.
Horizontal pipe capacity and slope
Table 11 gives the maximum load on horizontal drainage pipes: 8 fixture units for 2 inches, 35 for 3 inches and 216 for 4 inches. A 3-inch pipe is limited to three toilets on a horizontal branch and four toilets on a vertical stack. These figures apply to a slope of ¼ inch per foot (20.8 mm/m). If the slope is only ⅛ inch per foot (10.4 mm/m), the horizontal load is multiplied by 0.8. Individual vent pipes must be at least 1¼ inches (32 mm).
For the building drain, Annex D (Table D.4) gives a 3-inch pipe a capacity of 20, 42 and 50 fixture units at slopes of 10.41, 20.83 and 41.65 mm/m, limited to two toilets (three toilets for single-family dwellings). A 4-inch pipe takes 180, 216 and 250 fixture units at the same slopes. A drain serving toilets must not be smaller than 3 inches.
homeX worked example: drainage pipe size
A house with two bathrooms (each with a toilet, basin, shower and floor drain), one kitchen sink and one washing machine: counting the toilet as 6 units, the total is 2 x (6 + 1 + 2 + 2) + 2 + 3 = 27 fixture units. That is below the 35 units for a 3-inch horizontal pipe at 20.8 mm/m, with two toilets, so 3 inches meets the table. At only 10.4 mm/m the capacity is 35 x 0.8 = 28 units, which leaves little margin. This is only an illustration; the final design belongs to a plumbing designer.
Installation details that are often missed
- A trap water seal of 5–10 cm, and no more than 60 cm from the fixture outlet to the trap.
- Floor drains must have a removable strainer with an open area of at least ⅔ of the pipe cross-section, a diameter of at least 80 mm and a trap height of 5.5–10 cm.
- Traps must have an accessible cleanout.
- Maximum vent distance from the trap (Table 14): 150 cm for 2-inch, 180 cm for 3-inch and 300 cm for 4-inch pipes.
- Wastewater must not be discharged into drains reserved for rainwater.
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Note: summary of SNI 8153:2015; local regulations and plumbing designers may require stricter rules. Data as of 24 October 2026.
FAQ
What size should a toilet drain pipe be under SNI? The toilet trap is 3 inches, and under SNI 8153:2015 a drain serving toilets must not be smaller than 3 inches (80 mm).
What slope is recommended for a wastewater pipe? The SNI 8153:2015 capacity tables use ¼ inch per foot, or 20.8 mm/m (about 2 %). At 10.4 mm/m, horizontal capacity is multiplied by 0.8.
Can wastewater go into a rainwater drain? No. SNI 8153:2015 prohibits discharging wastewater into drains reserved for rainwater.
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Sources (checked 24 October 2026): BSN — SNI 8153:2015 Plumbing systems in buildings (BSN display copy on the Internet Archive).