Plumbing fixture unit calculator: dfu and wsfu totals

How a fixture unit count is actually taken off

You count fixtures by type off the plumbing fixture schedule, then run two separate sums: drainage fixture units (dfu) for the waste side and water supply fixture units (wsfu) for the supply side. There is no single fixture unit number for a job. A public water closet on a flushometer valve is 4 dfu and 10 wsfu, and those two figures go to two different code tables to size two different piping systems.

On a real set the counts come off the fixture schedule sheet, cross checked against the floor plans, because schedules and plans disagree more often than anyone likes. Fixtures drawn on the architectural plans that never made the plumbing schedule are the usual miss. So are fixtures on the sheet that sit outside the scope you are bidding, such as an owner furnished coffee station or a tenant fit-out area, which still have to be identified and excluded rather than quietly counted.

Once the counts are set, each type is multiplied by the value the adopted code assigns it and the two totals are what actually size pipe. The dfu total sizes the building drain and the building sewer. The wsfu total sizes the water service and the distribution mains.

The formulas

Total dfu = sum over fixture types of (count x dfu per fixture) Total wsfu = sum over fixture types of (count x wsfu per fixture)

Sizing the drain from that total is a table lookup, not a formula. Take the total dfu to IPC Table 710.1(1), pick your slope column (1/16, 1/8, 1/4, or 1/2 inch per foot), and read down to the smallest diameter whose listed capacity is at least your total. Footnote a to that table sets a floor: a building drain serving a water closet is not less than 3 inches, no matter how low the dfu total runs.

The supply side has the same shape. The wsfu total goes to IPC Table E103.3(3) to get an estimated demand in gallons per minute, and that demand, with available pressure and developed length, is what sizes the pipe.

IPC drainage fixture unit values used here

These are published values from Table 709.1 of the 2021 International Plumbing Code, with the minimum trap size the table lists alongside each one:

• Water closet, private (1.6 gpf): 3 dfu
• Water closet, public (1.6 gpf): 4 dfu
• Water closet, flushometer tank, public or private: 4 dfu
• Water closet, private, flushing greater than 1.6 gpf: 4 dfu
• Water closet, public, flushing greater than 1.6 gpf: 6 dfu
• Lavatory: 1 dfu, 1-1/4 in trap
• Urinal: 4 dfu
• Urinal, 1 gallon per flush or less: 2 dfu
• Urinal, nonwater supplied: 1/2 dfu
• Shower, total flow 5.7 gpm or less: 2 dfu, 1-1/2 in trap
• Kitchen sink, domestic: 2 dfu, 1-1/2 in trap
• Service sink: 2 dfu, 1-1/2 in trap
• Drinking fountain: 1/2 dfu, 1-1/4 in trap
• Floor drains: 2 dfu, 2 in trap
• Emergency floor drain: 0 dfu

The shower row is tiered by total flow through showerheads and body sprays, not by fixture count. 5.7 gpm or less is 2 dfu, greater than 5.7 to 12.3 gpm is 3 dfu, greater than 12.3 to 25.8 gpm is 5 dfu, and greater than 25.8 to 55.6 gpm is 6 dfu. A multi-head shower is not one shower for dfu purposes, and the trap grows with it, from 1-1/2 in to 4 in across those tiers.

IPC water supply fixture unit values used here

From Table E103.3(2) of the 2021 IPC, given as cold, hot, total:

• Water closet, private, flush tank: 2.2, 0, 2.2 wsfu
• Water closet, private, flushometer valve: 6.0, 0, 6.0 wsfu
• Water closet, public, flush tank: 5.0, 0, 5.0 wsfu
• Water closet, public, flushometer valve: 10.0, 0, 10.0 wsfu
• Water closet, public or private, flushometer tank: 2.0, 0, 2.0 wsfu
• Lavatory, private, faucet: 0.5, 0.5, 0.7 wsfu
• Lavatory, public, faucet: 1.5, 1.5, 2.0 wsfu
• Urinal, public, 3/4 in flushometer valve: 5.0, 0, 5.0 wsfu
• Urinal, public, 1 in flushometer valve: 10.0, 0, 10.0 wsfu
• Urinal, public, flush tank: 3.0, 0, 3.0 wsfu
• Shower head, private, mixing valve: 1.0, 1.0, 1.4 wsfu
• Shower head, public, mixing valve: 3.0, 3.0, 4.0 wsfu
• Kitchen sink, private, faucet: 1.0, 1.0, 1.4 wsfu
• Kitchen sink, hotel or restaurant, faucet: 3.0, 3.0, 4.0 wsfu
• Service sink, faucet: 2.25, 2.25, 3.0 wsfu
• Drinking fountain, 3/8 in valve: 0.25, 0, 0.25 wsfu

Floor drains take no water supply, so they carry no wsfu at all. They still carry 2 dfu each. Table E103.3(2) lists urinals under public occupancy only, and lists service sinks and drinking fountains under one occupancy heading rather than splitting them, which is why those three rows do not move when the calculator switches between private and public use.

Read the footnote to that table before you sum anything: for a fixture with both hot and cold supplies, the separate hot and cold loads are each three quarters of the total. That is why the cold column and the hot column do not add up to the total column, and why adding the two and calling it a wsfu total overstates the load.

The IPC and the UPC do not agree, and private is not public

There is no universal fixture unit table. The values above are IPC. If your jurisdiction has adopted the Uniform Plumbing Code, you use UPC Table 702.1 instead and the numbers move. A service sink or mop basin is 2 dfu under IPC Table 709.1 and 3.0 dfu under UPC Table 702.1. A urinal is listed at 4 dfu under IPC, while UPC lists a urinal with an integral trap at 1.0 gpf at 2.0 dfu. UPC also splits fixtures across private, public, and assembly occupancy columns, where IPC Table 709.1 mostly carries one value per fixture and reserves the private and public split for water closets and a handful of others.

The practical rule: a dfu total built from IPC values gets sized against IPC tables, and a UPC total against UPC tables. Mixing a UPC count into an IPC sizing table, or the reverse, produces a number no plan reviewer will accept and can leave a drain undersized. Find out which code and which edition the authority having jurisdiction has adopted, and write it on the takeoff, because local amendments are common and they change values.

The private versus public split is the other place counts go wrong. IPC Chapter 2 defines public, for the classification of plumbing fixtures, as applying to fixtures with unrestricted exposure to walk-in traffic, and private as applying to fixtures that are not public. That is a use test, not a building type test. A restroom inside a leased suite that only staff reach is not automatically private, and a fixture in a residential building's shared amenity space is not automatically private either. Getting that call wrong on a job with 6 water closets on flushometer valves moves the dfu total by 6 and the wsfu total by 24, which is enough to change a pipe size.

What the fixture count misses

The count on the plan is the easy part. Three things routinely get missed after it.

First, fixture units do not scale with flow. A fixture unit is a probability weighting, not a flow rate. IPC Chapter 2 defines a drainage fixture unit as a measure of probable discharge that depends on the volume rate of discharge, the duration of a single operation, and the average time between operations. The supply side behaves the same way. On the flushometer valve column of IPC Table E103.3(3), 10 wsfu reads 27.0 gpm and 80 wsfu reads 61.2 gpm. Eight times the load, a bit over twice the flow. Anyone who totals fixture units and multiplies by a gpm per unit figure will size the service badly.

Second, the low flow urinal discount has a condition attached. Table 709.1 lists a urinal at 1 gallon per flush or less at 2 dfu instead of 4, which is tempting on a current spec. Footnote e to that table says that for the purpose of computing loads on building drains and sewers, water closets and urinals shall not be rated at a lower drainage fixture unit unless the lower values are confirmed by testing. Take the discount without that confirmation and the building drain load is understated. This calculator uses the 4 dfu base value for that reason.

Third, and this one costs money rather than a redline: where your scope ends is not a plumbing calculation. IPC Chapter 2 puts the end of the building drain 30 inches in developed length past the exterior wall of the building, and the building sewer runs from that point to the point of disposal. That code line almost never matches the contract line. Whether the plumbing sub carries the sewer to 5 feet out, to the property line, or to the main, and who carries the excavation, bedding, backfill, compaction, and the tap fee, is settled in the division of work and the scope sheet. No fixture unit total answers it. Read the front end before you price the underground, and state your assumed boundary in writing on the proposal.

Worked example

Take the calculator defaults: public use, water closets on flushometer valves, 6 water closets, 4 lavatories, 2 urinals, 0 showers, 1 kitchen sink, 1 service sink, 2 drinking fountains, and 2 floor drains, with the building drain at 1/4 inch per foot.

Drainage side, from Table 709.1: water closets 6 x 4 = 24, lavatories 4 x 1 = 4, urinals 2 x 4 = 8, showers 0 x 2 = 0, kitchen sink 1 x 2 = 2, service sink 1 x 2 = 2, drinking fountains 2 x 0.5 = 1, floor drains 2 x 2 = 4. Total is 24 + 4 + 8 + 0 + 2 + 2 + 1 + 4 = 45 dfu.

Supply side, from the total column of Table E103.3(2) at public occupancy: water closets 6 x 10.0 = 60.0, lavatories 4 x 2.0 = 8.0, urinals 2 x 5.0 = 10.0, showers 0, kitchen sink 1 x 4.0 = 4.0, service sink 1 x 3.0 = 3.0, drinking fountains 2 x 0.25 = 0.5, floor drains 0. Total is 60.0 + 8.0 + 10.0 + 4.0 + 3.0 + 0.5 = 85.5 wsfu. Run the cold column separately and it sums to 81.75 wsfu, the hot column to 11.25 wsfu. Those are the two numbers you take to the cold main and the water heater branch, and note that they do not add to 85.5.

Now the drain. 45 dfu at 1/4 inch per foot in Table 710.1(1): a 2 in drain carries 21, a 2-1/2 in carries 24, a 3 in carries 42, and a 4 in carries 216. 45 is past the 3 in limit of 42, so the building drain is 4 in. That result is worth sitting with, because the same 45 dfu at 1/2 inch per foot fits a 3 in drain, where the table allows 50. Three dfu of load, or a quarter inch of fall, is the difference between a 3 in and a 4 in run and everything downstream of that choice. Fix the slope on the drawing before anyone prices pipe.

From a fixture count to a priced line

The two totals are the quantity half of the estimate. The priced line comes from the fixtures and trim, the carriers, the pipe and fittings by size and material, hangers, sleeves, insulation, and the labor unit on each, all against your own price book. That is the part Ruh automates: it reads the drawing set, measures the scope, and prices the measured quantities against the contractor's own price book, with every line traceable back to the sheet it came from. For the underlying term see the fixture unit glossary page, for the piping system see DWV, drain waste vent, and for the install sequence see plumbing rough-in. If you are pricing the trade end to end, the guide to bidding plumbing as a subcontractor walks the whole bid, and the plumbing trade page covers the wider scope.