Vent pipe sizing based on fixture units and drain pipe sizes. Helps ensure proper drainage and prevent siphoning in your plumbing system.
| Fixture | Drainage Fixture Units (DFU) |
|---|---|
| Toilet (Tank Type) | 3 |
| Toilet (Flushometer) | 6 |
| Bathtub | 2 |
| Shower | 2 |
| Bathroom Sink | 1 |
| Kitchen Sink | 2 |
| Dishwasher | 2 |
| Washing Machine | 2 |
| Floor Drain | 1 |
| Vent Size | Max Fixture Units | Max Length (Developed) |
|---|---|---|
| 1-1/4" | 1 DFU | 45 ft |
| 1-1/2" | 2-8 DFU | 60 ft |
| 2" | 9-24 DFU | 120 ft |
| 2-1/2" | 25-42 DFU | 180 ft |
| 3" | 43-84 DFU | 212 ft |
| 4" | 85-256 DFU | 300 ft |
Developed length = total run including vertical and horizontal sections
| Drain Size | Max Fixture Units | Typical Use |
|---|---|---|
| 1-1/4" | 1 DFU | Single sink |
| 1-1/2" | 3 DFU | Sink, shower |
| 2" | 6 DFU | Kitchen sink, washing machine |
| 3" | 20 DFU (horiz) / 48 DFU (vert) | Toilet, branch drain |
| 4" | 160 DFU (horiz) / 256 DFU (vert) | Main building drain |
Individual Vent
One vent pipe serving one trap - most common for toilets and distant fixtures
Common Vent
One vent serving two fixtures on opposite sides of wall (back-to-back sinks)
Wet Vent
Drain pipe that also serves as vent for other fixtures (bathroom groups)
Stack Vent
Vertical main drain that extends through roof as vent
| Trap/Drain Size | Max Distance to Vent |
|---|---|
| 1-1/4" | 2 feet 6 inches |
| 1-1/2" | 3 feet 6 inches |
| 2" | 5 feet |
| 3" | 6 feet |
| 4" | 10 feet |
Distance measured from trap weir to vent connection
❌ S-Trap (illegal in most codes):
Trap that drains directly down through floor - creates siphoning
❌ Undersized vent pipe:
Insufficient airflow causes slow drainage and gurgling
❌ Trap too far from vent:
Exceeding maximum distance causes trap seal failure
❌ Vent sloping down toward drain:
Creates water trap in vent line
Typical Bathroom (toilet, sink, tub/shower):
Fixture Units: 3 (toilet) + 1 (sink) + 2 (tub) = 6 DFU total
Common Configuration:
Note: Exact configuration varies by code and layout. Consult local plumbing code for specific requirements.
When in doubt, consult licensed plumber or building inspector BEFORE installation
Proper plumbing vent sizing is essential for a functional drain-waste-vent (DWV) system. Vent pipes allow air into the drainage system, preventing siphoning of trap seals and ensuring smooth water flow. Without adequate venting, you'll experience slow drains, gurgling sounds, and potentially dangerous sewer gas entry into your home.
This guide covers fixture unit calculations, vent pipe sizing based on total DFU load, maximum developed lengths, and critical venting requirements per the International Plumbing Code (IPC) and Uniform Plumbing Code (UPC). Understanding these relationships helps you plan bathroom additions, kitchen remodels, and whole-house plumbing systems that will pass inspection and function properly for decades.
Whether you're a DIY homeowner tackling a bathroom renovation or a professional plumber verifying calculations, this reference provides the essential sizing tables and configuration rules you need for code-compliant plumbing work.
Plumbing vent sizing directly affects system performance and code compliance. Using this guide helps ensure your project meets requirements without costly rework or failed inspections.