Work out trap seal depth instantly with clear inputs, formula shown and shareable results.
A trap seal is lost two ways. Evaporation removes a slow, steady depth each day a fixture sits unused, and a pressure swing in the drainage system displaces water by half the pressure difference in inches of water gauge, because the level drops on one leg while it rises on the other.
Remaining seal
Remaining = installed depth - evaporation rate x days - pressure swing / 2
Time to failure
Days = (depth - pressure loss) / evaporation rate
Remaining = installed depth - evaporation rate x days - pressure swing / 2. A trap seal is lost two ways.
Floor drains are rarely used, so evaporation runs unchecked. A trap primer or a sealing insert is the usual remedy.
This calculator takes 4 inputs: Installed trap seal depth, Evaporation rate, Days unused, System pressure fluctuation. The pre-filled defaults are a realistic worked example — replace them with your own site or project figures.