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Drainage Fixture Unit (DFU) Calculator — IPC Table 709.1

Add up drainage fixture units (DFU) for residential or commercial buildings. The total DFU load feeds into horizontal drainage pipe sizing via IPC Table 710.1 / UPC Table 710.1.

By TradeCalc, Plumber Calculators — Code-Referenced — IPC Table 709.1, UPC Table 709.1, IPC Table 710.1

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How to Count Drainage Fixture Units (DFU)

What Is a Drainage Fixture Unit?

A Drainage Fixture Unit (DFU) measures the discharge load a plumbing fixture places on a sanitary drainage system. One DFU is defined as 7.5 gallons per minute of discharge (IPC Table 709.1 footnote). This is about 3× the 1 gpm basis of a Water Supply Fixture Unit (WSFU), because discharge flows are larger and more instantaneous than supply flows — a toilet flush dumps a full tank in seconds. DFU values are assigned per fixture type in IPC Table 709.1 / UPC Table 709.1 and account for both flow rate and discharge duration.

The Code Lookup Table

DFU values are tabulated in IPC Table 709.1 and UPC Table 709.1. Both codes list private (residential) and public (commercial) columns. Public values run higher for flush-valve fixtures because commercial toilets and urinals use flushometers that dump more water, faster.

Examples (private column): lavatory 1, bathtub 2, kitchen sink 2, shower 2, dishwasher 2, clothes washer 3, toilet (1.28 gpf) 2, floor drain 2.

Step-by-Step Worked Example

Scenario: A two-bathroom single-family home with the default fixture count loaded in the calculator above. Count the total DFU.

  1. 1 bathtub × 2 = 2 DFU
  2. 1 clothes washer × 3 = 3 DFU
  3. 1 dishwasher × 2 = 2 DFU
  4. 1 kitchen sink × 2 = 2 DFU
  5. 2 lavatories × 1 = 2 DFU
  6. 1 shower × 2 = 2 DFU
  7. 2 toilets (1.28 gpf) × 2 = 4 DFU
  8. 1 floor drain × 2 = 2 DFU
  9. Total = 19 DFU

At 19 DFU and 1/4 in/ft slope, IPC Table 710.1 shows a 2" pipe carries 21 DFU (barely fits) but good practice is a 3" main drain (24 DFU) for headroom. With diversity, the estimated peak discharge is ≈ 91.6 gpm — higher than the supply side because drains receive concentrated surge flows from toilets and appliances.

Practical Tips

  • 1 DFU = 7.5 gpm discharge. This is the raw instantaneous rate; actual simultaneous discharge is lower due to diversity (fixtures don't all flush at once). IPC Table 710.1 already bakes in diversity, so just feed total DFU into the pipe table directly.
  • Toilets drive the count. A 1.6 gpf tank toilet is 3 DFU vs 2 DFU for a 1.28 gpf HET. A commercial flush-valve toilet is 4–6 DFU. Switching a commercial building to 1.28 gpf flush-valve toilets can drop the load enough to downsize a 4" main to 3".
  • Don't forget floor drains and laundry trays. These are easy to miss in a takeoff. A floor drain is 2 DFU; a laundry tray is 2 DFU (private). A missed floor drain in a 4-fixture basement adds 2 DFU you didn't plan for.
  • Private vs public matters for flush fixtures. A 1.6 gpf tank toilet is 3 DFU private but 5 DFU public; a 1.6 gpf flush-valve toilet is 4 DFU private but 6 DFU public. Apartment buildings use the private column; offices and restaurants use public.

Code References

IPC Table 709.1, UPC Table 709.1, IPC Table 710.1

Frequently Asked Questions

What is a drainage fixture unit (DFU)?
A DFU is a discharge-load unit equal to 7.5 gallons per minute, defined in IPC Table 709.1 / UPC Table 709.1. Each fixture is assigned a DFU value based on its discharge rate and duration. A lavatory is 1 DFU, a 1.28 gpf toilet is 2 DFU, a clothes washer is 3 DFU. Summing DFU for a building gives the total sanitary load used to size horizontal drainage pipe via IPC Table 710.1.
How is DFU different from WSFU (water supply fixture unit)?
DFU measures drainage (discharge) load at 7.5 gpm per unit; WSFU measures supply (inflow) load at 1 gpm per unit. The same fixture has different values: a residential lavatory is 1 WSFU but also 1 DFU, while a 1.28 gpf toilet is 2.2 WSFU but 2 DFU. Drainage flows are larger and faster because a toilet dumps a full tank in seconds, so the discharge side is sized separately from the supply side.
How many DFU is a typical two-bathroom house?
A two-bathroom single-family home typically totals 15–20 DFU: two bathroom groups (each = toilet 2 + lavatory 1 + bathtub 2 + shower 2 = 7 DFU for 1.28 gpf fixtures), plus kitchen sink (2), clothes washer (3), dishwasher (2), and often a floor drain (2). At 20 DFU the main drain is usually 3" at 1/4 in/ft (IPC Table 710.1 capacity 24 DFU).
Do I apply a diversity factor to total DFU?
No — not separately. IPC Table 710.1 already incorporates diversity into its capacity numbers, so you feed the raw total DFU directly into the pipe-sizing table. The 7.5 gpm-per-DFU figure is the raw instantaneous rate, but the pipe table assumes not all fixtures discharge simultaneously. Do not multiply DFU by 7.5 to size pipe; use the table directly.