Fixture units: sizing by probability
What you'll learn
Size drain pipe the way the code does — by the odds fixtures run at once, not by adding up gallons — plus the two hard rules.
How big does the drain need to be? Your instinct says: add up what the fixtures can pour and size the pipe for the total. The code says no — and the code's way is smarter.
Sizing by probability
A lavatory can discharge a couple gallons a minute. But across a whole house, how often is it actually running — and how often at the same instant as the shower, the dishwasher, and both toilets? Almost never. So instead of flow rates, the code weighs each fixture in drainage fixture units (dfu): a load number that already has the odds of simultaneous use baked in.
| Fixture | dfu |
|---|---|
| Lavatory | 1 |
| Kitchen sink, shower, tub, clothes washer | 2 each |
| Water closet (private, 1.6 gpf) | 3 |
| Full bathroom group (WC + lav + tub/shower) | 5 |
Notice the bathroom group: 3 + 1 + 2 = 6 as parts, but 5 as a group — the discount is the probability. One person can't run all three at once.
Capacity jumps, not creeps
Each diameter has a maximum dfu load that depends on the pipe's job — horizontal branch, vertical stack, or building drain (where slope matters too). The ratings make sense until you see the jump:
A 4″ branch doesn't carry twice what a 3″ does — it carries eight times as much. Bigger pipes serve more fixtures, and the more fixtures behind a pipe, the smaller the odds they peak together, so the table compounds geometry with probability. This is why "one size bigger, just in case" is usually enormous overkill — and why going one size smaller can be a cliff.
Two hard rules
- Any line serving a water closet is 3″ minimum, no matter what the dfu math says. A single private WC is only 3 dfu — a 1½″ branch could "carry" that — but the solids need the bore.
- Pipe never shrinks in the direction of flow. Loads only accumulate downstream; diameters only hold or grow.
Count the fixtures, pick the kind of run, and the drain pipe sizing tool reads the minimum diameter straight off the IPC tables — with both rules applied.
Why this matters
Undersized drains clog on schedule; oversized drains waste money and, on marginal loads, flow so shallow they carry solids poorly too. The dfu system is how you land on right — and it's what the plan reviewer is checking when your isometric hits their desk.
Check your understanding
Question 1 of 2
A drainage fixture unit (dfu) measures: