Free Tool — Plumbing
Pipe DFU Calculator
Add fixtures to total drainage fixture units (DFU) and review the 2021 IPC Table 710.1(1) building-drain or sewer size at ¼ inch per foot. Verify your jurisdiction's adopted code and local amendments before design or installation.
Pipe DFU Calculator
2021 IPC Table 710.1(1) — building drain/sewer at ¼ in/ft
Private Water Closet (1.6 gpf)
3 DFU each
Bathroom Sink
1 DFU each
Kitchen Sink
2 DFU each
Bathtub
2 DFU each
Shower
2 DFU each
Clothes Washer
2 DFU each
Dishwasher
2 DFU each
Floor Drain (2")
2 DFU each
Additional Floor Drain
2 DFU each
Laundry / Utility Sink
2 DFU each
Urinal (1.0 gpf or less)
2 DFU each
Drinking Fountain
0.5 DFU each
Total DFU Load
0
Add fixtures above to calculate drain size.
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Practice 5 plumber exam questions
Real IPC/UPC questions — each with an instant AI explanation.
Plumber / IPC-UPC · Question 1 of 5
According to the IPC, what is the drainage fixture unit (DFU) value for a residential water closet (1.6 gpf or less)?
Pick an answer to see the explanation + an instant AI breakdown.
What Are Drainage Fixture Units (DFU)?
A drainage fixture unit (DFU) represents the relative load a plumbing fixture places on a drainage system. The 2021 IPC Table 709.1 assigns different values by fixture and use: a private 1.6-gpf water closet is 3 DFU, a public 1.6-gpf water closet is 4 DFU, a private lavatory is 1 DFU, and a residential clothes washer is 2 DFU.
IPC Table 710.1(1) maps total DFU loads to building-drain and sewer capacity by slope. At ¼ inch per foot, a 2-inch pipe is listed for up to 21 DFU, a 3-inch pipe for 42 DFU, and a 4-inch pipe for 216 DFU. A building drain serving a water closet has a 3-inch minimum. Horizontal fixture branches and stacks use the different capacities in Table 710.1(2), so the calculator above must not be used for those pipe sections.
For exam study, first identify the code edition, fixture classification, pipe section, and slope named in the question. Then total the applicable Table 709.1 fixture values and use the matching Table 710.1 column. Actual exam content and adopted plumbing codes vary by jurisdiction.
DFU Values Reference Table (IPC Table 709.1)
Common drainage fixture unit values from IPC Table 709.1. Use this chart to verify the calculator above or for quick reference on a job.
| Fixture | DFU | Min Trap Size |
|---|---|---|
| Water Closet (private, 1.6 GPF) | 3 | Fixture outlet |
| Water Closet (public, 1.6 GPF) | 4 | Fixture outlet |
| Urinal (1.0 GPF or less) | 2 | Fixture outlet |
| Bathroom Sink (lavatory) | 1 | 1¼" |
| Bathtub (with/without shower) | 2 | 1½" |
| Shower (5.7 GPM or less) | 2 | 1½" |
| Kitchen Sink (residential) | 2 | 1½" |
| Dishwasher (residential) | 2 | 1½" |
| Clothes Washer (standpipe) | 2 | 2" |
| Laundry / Utility Sink | 2 | 1½" |
| Floor Drain (2" trap) | 2 | 2" |
| Drinking Fountain | ½ | 1¼" |
| Bidet | 1 | 1¼" |
| Bathroom Group (private 1.6 GPF water closet) | 5 | — |
Selected study values from 2021 IPC Table 709.1. Fixture classifications and footnotes matter; verify the complete table and the code adopted in your jurisdiction.
Pipe Size Capacity: Max DFU by Slope (IPC Table 710.1(1))
Maximum drainage fixture units a building sewer, building drain, or horizontal branch of the building drain can carry, by pipe size and slope. Steeper slope carries more flow. Any building drain serving a water closet must be at least 3 inches.
| Pipe Size | ⅛ in/ft | ¼ in/ft | ½ in/ft |
|---|---|---|---|
| 2" | — | 21 | 26 |
| 2½" | — | 24 | 31 |
| 3" | 36 | 42 | 50 |
| 4" | 180 | 216 | 250 |
| 5" | 390 | 480 | 575 |
| 6" | 700 | 840 | 1,000 |
| 8" | 1,600 | 1,920 | 2,300 |
Source: 2021 IPC Table 710.1(1) (building sewer, building drain, and horizontal branch of the building drain). Horizontal fixture branches and stacks use Table 710.1(2), which lists different values. 1¼" and 1½" sizes carry ≤3 DFU and are omitted. Verify against the code edition adopted in your jurisdiction.
Code Sources and Calculator Scope
This study tool uses selected fixture values from the 2021 IPC Table 709.1 and the ¼-inch-per-foot building-drain and sewer column in Table 710.1(1). It does not size horizontal fixture branches, stacks, vents, storm drainage, or water-supply piping, and it does not account for local amendments or the UPC.
Educational use only. Confirm the adopted code edition, pipe section, slope, fixture details, and authority having jurisdiction before design or installation.
Frequently Asked Questions
How many DFUs can a 2-inch pipe handle?
A 2-inch building drain or sewer is limited to 21 DFU at the standard ¼-inch-per-foot slope (2021 IPC Table 710.1(1)). That is the whole-pipe capacity, not a per-fixture figure, and it is below the 3-inch minimum a building drain serving a water closet must meet. Horizontal fixture branches and stacks are sized from Table 710.1(2) instead.
How many DFU for a 3-inch pipe?
A 3-inch building drain or sewer carries up to 42 DFU at ¼ inch per foot (2021 IPC Table 710.1(1)) — one size step below the 216 DFU a 4-inch drain carries. Horizontal fixture branches and stacks use Table 710.1(2), which allows less. Confirm the adopted code edition and any local amendments.
How many DFU is a toilet?
A private 1.6-gpf water closet counts as 3 DFU and a public 1.6-gpf water closet as 4 DFU under 2021 IPC Table 709.1. Counted as part of a bathroom group with a private 1.6-gpf water closet, the group is 5 DFU rather than the fixtures added individually. Other flush types and classifications carry different values.
What is the DFU value of a toilet?
Under 2021 IPC Table 709.1, a private 1.6-gpf water closet is 3 DFU and a public 1.6-gpf water closet is 4 DFU. A bathroom group with a private 1.6-gpf water closet is 5 DFU. Other flush types and classifications have different values.
What is the minimum drain pipe size for a toilet?
Under 2021 IPC Table 710.1(1), a building drain serving a water closet must be at least 3 inches. The final size also depends on total DFU and slope. Horizontal fixture branches and stacks use Table 710.1(2), not the building-drain capacities used by this calculator.
How many DFU can a 4-inch pipe handle?
A 4-inch building drain or sewer handles up to 216 DFU at the standard ¼-inch-per-foot slope (IPC Table 710.1(1)) — 180 DFU at ⅛-inch-per-foot, or 250 DFU at ½-inch-per-foot. A 3-inch drain handles 42 DFU and a 2-inch handles 21 DFU, both at ¼-inch-per-foot. Note that horizontal fixture branches and stacks use a separate table (710.1(2)) with lower values — a 4-inch horizontal fixture branch is capped at 160 DFU.
What is the difference between DFU and WSFU?
DFU (drainage fixture unit) measures relative load on the drainage system. WSFU (water supply fixture unit) measures demand on water-supply piping. They use different tables and are not interchangeable; classify the fixture and supply control exactly as the adopted code requires.
Are floor drains counted in the DFU total?
A floor drain is listed at 2 DFU in 2021 IPC Table 709.1, subject to its table footnote and Sections 709.4 and 709.4.1. A clear-water emergency floor drain is listed separately at 0 DFU. Verify the fixture classification and adopted code before adding it to a drainage load.
What is the difference between IPC and UPC?
The IPC and UPC are separate model plumbing codes published by ICC and IAPMO. Their fixture values, sizing tables, editions, and local amendments can differ. Check the authority having jurisdiction for the exact code and edition that applies to your exam or project.
Does this calculator work for vent sizing too?
No. This calculator is limited to the 2021 IPC Table 710.1(1) building-drain and sewer capacities at ¼ inch per foot. Vent sizing uses different requirements and must be evaluated separately.
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