Trap Size Calculator
Determine the correct P-trap or S-trap size for any plumbing fixture per code.
Results
Visualization
How It Works
Every plumbing fixture must have a trap that maintains a water seal to prevent sewer gases from entering the building. Trap size is determined by fixture type and must match the minimum drain pipe size.
The Formula
Trap size = Code-specified minimum for fixture type. Trap seal must be 2" to 4" water depth.
Variables
- Trap size — Inside diameter of the trap in inches
- Seal depth — Water depth in the trap (2-4 inches per code)
Worked Example
A bathroom lavatory requires a 1-1/4" trap with a 1-1/2" drain connection. Kitchen sinks need a 1-1/2" trap.
Practical Tips
- Never reduce trap size below code minimum
- S-traps are prohibited in most codes — use P-traps
- Trap seal can be lost through evaporation — pour water periodically in floor drains
- Maximum distance from trap to vent: typically 6 feet for 1-1/2" trap