Plumbing fixture units explained

"Fixture units" is two different measurements wearing one name. A water supply fixture unit (WSFU) weights how hard a fixture pulls on the supply. A drainage fixture unit (DFU) weights how much it dumps into the drain. The same lavatory is 0.7 WSFU and 1 DFU, and the two numbers are not convertible.

Neither is a flow rate. A fixture unit is a weighting invented so that a building full of fixtures can be sized without pretending they all run simultaneously — which is exactly what would happen if you added up gallons per minute.

Sum the WSFU of every fixture and you have a demand weight, not gallons. Hunter's curve converts that weight into a probable peak flow, and the curve is deliberately non-linear: doubling the fixtures does not double the flow, because the probability that all of them run at once falls as the count rises.

The hot and cold columns do not add to the combined figure. A shower cannot draw both at full rate at once, so the code assigns a combined value below the sum. Use the branch figures on the branches and the combined figure on the trunk.

Drainage fixture units count discharge, and the ratios are different because the physics is different. A water closet is the heaviest fixture on the drain at 3 DFU and one of the lighter ones on supply relative to a hose bibb. The bibb, which is 2.5 WSFU, contributes 0 DFU — it discharges to grade.

Two full bathrooms, a kitchen, a laundry and two hose bibbs. On the supply side that is two water closets at 2.2, two lavatories at 0.7, two tub/showers at 1.4, a kitchen sink and a dishwasher at 1.4 each, a washer at 1.4 and two bibbs at 2.5 — about 20.3 WSFU.

On the drainage side the same house is two closets at 3, two lavatories at 1, two tubs at 2, sink and dishwasher at 2 each and a washer at 2 — 20 DFU. The totals happening to land close together is a coincidence of this house, not a rule.

Sizing and flow

Twenty WSFU reads as roughly 20 gpm on the flush-tank curve; twenty DFU sits comfortably inside a 3" building drain at 1/8" per foot. Both numbers fall out of the fixture unit calculator if you would rather count with buttons.

IPC and UPC publish different fixture unit schedules. Several fixtures carry different weights, and the UPC counts some fixtures the IPC does not. A total computed on one schedule is not submittable to a jurisdiction that adopted the other.

Establish which code your authority having jurisdiction (AHJ) adopted before you total anything — the IPC vs UPC comparison covers where the two diverge on a house.