Water Supply Pipe Sizing Guide

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WSFU Measures Demand on the Incoming Pipe. DFU Measures Load on the Leaving Pipe.

IPC Table E103.3(2) assigns each fixture a water-supply fixture unit — a Hunter-curve number for how hard that fixture hits the cold, the hot, or both. A private lavatory is 0.7 total (0.5 cold / 0.5 hot). A private kitchen sink is 1.4. A private water closet with a flush tank is 2.2, cold only. A private bathroom group on a flush tank is 3.6 total. Those numbers live in Appendix E, not Chapter 7.

Drainage fixture units live in Chapter 7 and Chapter 9. A private lav is 1 DFU. The same lav is 0.7 WSFU. A private WC is 3 or 4 DFU depending on gallons per flush, and 2.2 WSFU if it is a tank. The vent guide uses DFU because a vent is sized from the drain it serves. The drain-slope guide does not use fixture units at all; it uses pipe diameter and fall. If you paste a DFU total into the supply calculator you will size a water pipe from a waste table. The comparison page exists so that mix-up has a URL: WSFU vs DFU.

Hot-only and cold-only branches do not get the total column. Table E103.3(2) note 1: the separate hot and cold loads are three-fourths of the total. Size the cold main on the cold column plus cold-only fixtures (toilets, hose bibbs). Size the hot main on the hot column. Size the meter and the building supply on the combined total. That is why a house with 16.5 WSFU on the meter can still have a 3/4 in hot branch if the hot-only total is under the 3/4 in cell.

How to Total WSFU for a House

Walk the fixtures, not the rooms. One bathroom group (WC tank + lav + tub/shower) is 3.6 WSFU in Table E103.3(2), not 2.2 + 0.7 + 1.4 = 4.3. The group number already includes diversity inside that room. A second bathroom group adds another 3.6. Kitchen sink 1.4. Clothes washer 1.4. Dishwasher 1.4 hot. Two hose bibbs are not in E103.3(2)’s residential rows the way they sit in some UPC tables; if your jurisdiction uses a 2.5 WSFU hose bibb, add them. A typical three-bedroom, two-bath house lands in the mid-teens of WSFU on the combined column. That is the 16.5 number this calculator defaults to, and it is the number the Williams / UPC 610.4 worked example uses for a 1 in building supply at ~54 psi and 115 ft.

Do not double-count a bathroom group and then add the WC again. Do not use public-occupancy rows for a house. Do not treat a manifold home-run as one giant branch — each home-run is sized for that fixture’s WSFU, and the manifold trunk is sized for the total. Parallel systems have their own listing rules (UPC 610.4 / 610.13). This calculator is the trunk/branch table method.

The WSFU Table Is the Same. The Inside Diameter Is Not.

IPC Table E104.1 and UPC Table 610.4 do not have a copper column and a PEX column. IAPMO ER-0253 and ICC-ES PMG 1006 say you may use those tables for Uponor / SDR 9 PEX at the same nominal size. That is the published permission. It is also why this calculator does not invent a fake PEX WSFU table.

IPC Table E105.1 is the table that actually splits materials. Ounces of water per foot of tube, which is a stand-in for internal area:

At 1/2 in: copper Type L 1.55 oz/ft, CPVC CTS SDR 11 1.25, PEX CTS SDR 9 1.18. At 3/4 in: copper L 3.22, CPVC 2.67, PEX 2.35. At 1 in: copper L 5.49, CPVC 4.43, PEX 3.91. PEX at 1 in holds about 71% of the water copper Type L holds. Same stamp on the coil, smaller hole, more friction per foot, higher velocity at the same gpm.

UPC 610.12 still caps copper tube at 8 ft/s cold and 5 ft/s hot, and caps copper fittings in non-copper tubing the same way. A 1/2 in PEX branch that Table 610.4 permits on WSFU can still be a noisy, erosive fitting if the peak gpm is high. This calculator bumps one nominal size when you pick PEX or CPVC and the Table E104.1 cell is 80% or more used. That bump is a friction check from E105.1, not a second code table. A 200-foot PEX main with a 20 psi elevation hit still wants Appendix E charts, not a bump.

Developed Length, Then 1.2, Then the Next Column

IPC E104.1 Step 2: actual length of pipe between the source and the most remote fixture, including the hot run through the water heater or the cold branches, multiplied by 1.2 to cover fittings. Then pick the Table E104.1 column equal to or greater than that number. A 100-foot developed run becomes 120 ft, which uses the 150 ft column. That is why the results panel prints both lengths.

UPC 610.8 does not print that 1.2 factor. You pick the length column equal to or longer than the required length, then walk down to a WSFU cell that covers the load, then read meter and building-supply sizes from the two left columns. No building supply less than 3/4 in. The calculator uses entered length for UPC and entered × 1.2 for IPC so the two books stay honest.

Remote fixture means the farthest outlet this segment serves, not the farthest outlet in the building, unless you are sizing the building supply. A 20-foot lav branch does not inherit the 120-foot house developed length. Work from the remote outlet back toward the meter, adding WSFU as branches join, the same way you add DFU on a drain. The supply calculator sizes one segment at a time. The drain calculator sizes fall. The vent calculator sizes air. Three worksheets, one house.

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