Drainage Fixture Units (DFU) Calculator
Count your fixtures and this tool totals the drainage fixture units (DFU) that a drain and vent have to carry — the first number you need before sizing any waste pipe.
Calculator
Your fixtures add up to 10 DFU. Drainage fixture units are a labeled IPC/UPC planning convention that sizes the drain and vent — feed this into the drain pipe size and vent size tools. Your local code governs the exact values.
Every drainage system is sized from drainage fixture units (DFU). A DFU is not a measure of gallons; it is a dimensionless load number that captures how much waste flow a fixture typically contributes, and — just as importantly — how intermittently it does so. A toilet that dumps its bowl in a few seconds loads a drain far more than a lavatory that trickles, so a water closet is worth 3 DFU while a bathroom sink is worth 1. Add the fixtures on a branch, stack or building drain together and the total DFU tells you the minimum pipe size and vent size the code will accept.
This calculator uses the private-home values from the model plumbing codes (IPC and UPC agree on the common fixtures). Enter how many of each fixture drain into the pipe you are sizing, and read off the total. Then carry the number into the drain pipe size and vent size tools, which turn a DFU total into an inch dimension.
Formula
The total is a simple weighted sum of the fixture counts:
total DFU = Σ (fixture count × DFU per fixture)
with the labeled per-fixture values:
- Water closet (toilet) = 3 DFU
- Lavatory (bathroom sink) = 1 DFU
- Bathtub / tub-shower = 2 DFU
- Shower stall = 2 DFU
- Kitchen sink = 2 DFU
- Laundry tub / washing machine = 2 DFU
- Floor drain = 2 DFU
Worked example
Take a common single-bathroom home with a kitchen and a laundry: one toilet, one bathroom sink, one tub, one kitchen sink and one laundry hook-up.
(1 × 3) + (1 × 1) + (1 × 2) + (1 × 2) + (1 × 2)\n= 3 + 1 + 2 + 2 + 2 = 10 DFU
Ten DFU is the load this drain and vent must handle. Feed 10 DFU into the drain-size tool and you land on a 2 in branch (up to 6 DFU is a 2 in; 10 DFU steps up to a 3 in horizontal branch/building drain under the labeled band), and into the vent tool for a 1-1/2 in vent at a 40 ft developed length.
How to group and read fixture units
A few practical points. First, DFU tables distinguish between public and private occupancies; the values here are the private-home figures, which is what a house or apartment uses. Second, the way you group fixtures matters: size a single fixture’s trap arm from that fixture’s own DFU, a branch from the fixtures on that branch, and the building drain from every fixture in the house. Third, a few fixtures carry special rules — a 2 in or larger fixture drain, a continuous-duty pump, or a commercial fixture — that a residential table does not cover. When in doubt, size up and confirm with a licensed plumber.
The DFU method is deliberately conservative: it assumes fixtures never all discharge at once, which is true for a home but means the pipe is sized for a realistic peak, not a theoretical maximum. That is why a 3 in stack can serve a surprising number of fixtures. It also means DFU says nothing about slope, cleanouts or venting distance — those are separate checks handled by the other DWV tools.
Reference table
| Fixture | Drainage fixture units (DFU) |
|---|---|
| Water closet (toilet) | 3 DFU |
| Lavatory (bathroom sink) | 1 DFU |
| Bathtub (with or without shower) | 2 DFU |
| Shower stall | 2 DFU |
| Kitchen sink | 2 DFU |
| Laundry tub / washing machine | 2 DFU |
| Floor drain | 2 DFU |
Labeled IPC/UPC planning convention (private-home values). Your local plumbing code sets the exact figures — confirm with a licensed plumber.