Drainage Fixture Unit (DFU) Calculator
Size sanitary drainage pipe from drainage fixture units. Add fixtures to total the DFU load, then get the minimum building drain, branch, or stack size per IPC Table 710.1. Free, no sign-up.
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Explanation
A drainage fixture unit (DFU) is a dimensionless number that represents the probable discharge a plumbing fixture puts into the drainage system. Because fixtures rarely all drain at once, the code assigns each one a DFU weight and sizes pipe from the total — not from the sum of peak flows. This calculator totals the DFU load of your fixture schedule and returns the minimum building drain, horizontal branch, or stack size from the International Plumbing Code (IPC).
How DFU sizing works
Add up the DFU value of every fixture the pipe serves, then read the minimum size from the IPC sizing tables. Horizontal branches and stacks use one table; building drains and sewers use another that also depends on the slope, because a steeper pipe carries more.
| Application | Sizing table |
|---|---|
| Building drain / sewer | IPC Table 710.1(1) — varies with slope (1/16 to 1/2 in/ft) |
| Horizontal fixture branch | IPC Table 710.1(2) — branch column |
| Stack (3 branch intervals or less) | IPC Table 710.1(2) — stack column |
The pipe carrying fuel gas in the same building is sized a different way — by flow and run length rather than fixture units. For that, use the gas pipe size calculator.
Fixture DFU values
These weights are from IPC Table 709.1 for private use. Public fixtures and higher-flow models carry larger values; a flushometer-valve water closet, for example, counts more than a 1.6 gpf tank type.
| Fixture | DFU |
|---|---|
| Water closet, private (1.6 gpf) | 3 |
| Water closet, public (1.6 gpf) | 4 |
| Water closet, public (>1.6 gpf) | 6 |
| Bathroom group (1.6 gpf WC) | 5 |
| Bathroom group (>1.6 gpf WC) | 6 |
| Lavatory | 1 |
| Bathtub (with or without shower) | 2 |
| Shower (≤5.7 gpm) | 2 |
| Kitchen sink | 2 |
| Dishwasher | 2 |
| Clothes washer, residential | 2 |
| Clothes washer, commercial | 3 |
| Laundry tray | 2 |
| Floor drain | 2 |
| Service / mop sink | 2 |
| Urinal, flushometer valve | 4 |
| Urinal (≤1 gpf) | 2 |
| Bidet | 1 |
| Drinking fountain | 0.5 |
A bathroom group is a water closet, lavatory, and bathtub or shower counted together as a single weighted value rather than as separate fixtures.
Notes and limitations
Any drain that serves a water closet must be at least 3 inches, regardless of the DFU total, so this calculator floors those cases at 3 in. The tables here size sanitary drainage only — they do not size vents, and a stack of more than three branch intervals uses a separate column. Building drains and sewers below 2 in. and slopes other than those listed are not tabulated. Confirm fixture values, the slope, and the result against your adopted IPC edition and the authority having jurisdiction.