How to total drainage fixture units
The arithmetic is a plain sum — no curve here. The catch is that a branch and a stack of the same size are not the same pipe.
Drainage fixture units add up directly, which makes this simpler than water supply. The design case is different: supply worries about simultaneous demand, drainage worries about carrying solids and keeping its own air column stable.
Step-by-step
- Find each fixture's DFU value in your code's table — by the trap you are actually installing.
- Total them. Straight sum, no probability curve.
- Read the capacity twice for your candidate size: once from the horizontal branch column, once from the stack column.
- Check both separately. A load a stack takes happily can overload a branch of the same nominal size.
Why the branch is smaller than the stack
A vertical stack and a horizontal branch of the same diameter do not carry the same load, and the stack carries more. The reason is how the water travels.
In a stack, water clings to the wall and falls as an annular sheet with an air core down the middle. That air core is the point: it keeps system pressure stable and stops traps being siphoned or blown. A stack works well because gravity pulls along the whole length of the pipe.
A horizontal drain has only its slope. It is designed to run part-full — commonly around half — because the space above the flow is the vent path, and because a horizontal pipe running full loses the ability to carry solids at the velocity it needs. Same diameter, less usable capacity, which is why the two columns exist.
The trap size sets the value
Fixture tables are listed by fixture name, so it is natural to read down the name column and stop. But the same appliance on a larger trap contributes a larger DFU value, and on a remodel the trap you inherit may not be the one the table assumed.
Passing on fixture units is not the whole job
A DFU total tells you the pipe can carry the flow. It says nothing about whether the system holds its water seals — and that is the failure people actually notice, a trap that gurgles or empties and lets sewer gas into the room. That is a venting question: vent sizing, trap-to-vent distance, and how the stack is vented at the top. A drain sized correctly and vented badly is a system that smells.
What this tool will not do
It contains no code tables. It does not know which edition your jurisdiction has adopted, what local amendments apply, or anything about your job, and it makes no determination of compliance. A licensed professional and the authority having jurisdiction decide that, and where they or the published code differ from the page, they are right.