Fixture Unit Calculator

A Water Supply Fixture Unit (WSFU) is a standardized measure of the probable hydraulic demand on a water supply system. Defined in IPC P-2903, it accounts for the flow rate and usage frequency of plumbing fixtures. For example, a lavatory is typically 1 FU, while a bathtub is 4 FU. Higher values indicate greater water demand.

The conversion uses Hunter's Curve methodology. For small systems (under 60 FU), each FU roughly equals 1 GPM. As FU increases, the GPM-per-FU ratio decreases due to diversity factor—not all fixtures operate simultaneously. Example: 100 FU converts to approximately 40 GPM, while 500 FU yields about 150 GPM.

Combined FU represents total demand regardless of temperature. Hot FU counts fixtures requiring hot water only (dishwasher, washing machine). Cold FU counts cold-only fixtures. Most fixtures use both (lavatory, shower), contributing to all three counts. Pipe sizing uses combined FU for mains, hot FU for hot water lines.

Building type determines the diversity factor—the likelihood of simultaneous fixture use. Residential buildings use a high diversity factor (0.7-0.85) since fewer fixtures run concurrently. Office buildings use lower factors (0.4-0.6) due to intermittent use patterns. Hospitals use higher factors (0.75) for 24-hour operation.

Flushometer (flush valve) toilets have higher FU ratings (6-10 FU) because they demand high instantaneous flow rates of 25-35 GPM for 4-8 seconds. Tank toilets (3-4 FU) fill gradually at 2-4 GPM. This affects main pipe sizing—buildings with many flushometers need larger supply mains.

Per IPC P-2903, pipe sizing depends on total FU, material, and velocity limits. For copper Type L: up to 9 FU uses 3/4" pipe, 14-22 FU needs 1", 39-75 FU requires 1-1/4", 151-360 FU needs 1-1/2", and 360+ FU requires 2" or larger. PEX and CPVC may require one size larger due to higher friction.