A total is the least useful number in the schedule
Take the list the calculator starts with: an upstairs bath at 8 fixture units, a kitchen at 4, a laundry at 4, a powder room at 5. Twenty-one in total. That number describes exactly one length of pipe — the last one, below the final connection, on its way out of the building. Every other length of pipe on the job carries less than it, and some of them carry a quarter of it.
The segment above the kitchen tie-in carries 8. The segment between the kitchen and the laundry carries 12. Downstream of the powder room it carries all 21. Those are four different questions about four different pieces of pipe, and a single total answers only the last one. Where this matters is on a remodel, where the existing main is already sized and already carrying something, and the thing you actually want to know is which segment the new work pushes over whatever figure your table gives.
The fixture unit value is the part you do not get to choose
Everything else on this form is a description of your building. The fixture unit value is a number out of a table, and which table depends on which code your jurisdiction adopted, which edition of it, whether it has been amended locally, and whether the building counts as private or public use. A water closet is 4 in some tables and 6 in others. A bathroom group is often weighted as a group rather than as the sum of its fixtures, which is a discount you lose if you enter the fixtures individually.
That is why the value sits on the line as an input rather than in a lookup inside the page. A calculator that carried its own table would be quietly telling you which document you are being inspected against, and it does not know.
Why the discount is already in the number
Fixture units exist because fixtures do not discharge together. Twenty fixtures in a house are almost never running at once, and a drain sized for twenty simultaneous discharges would be enormous and would run so slowly for so much of its life that it would silt up. The weighting compresses that: a fixture that discharges rarely and briefly weighs less per gallon than one that runs steadily.
The consequence people miss is that the discount is already applied. Adding the fixture units and then multiplying by a diversity factor of your own takes it twice, and produces a number smaller than anything the table was ever meant to receive.
Order matters, and the order is physical
The list is read as a sequence down the run. Whichever branch is furthest from the point where the drain leaves the building goes first, because it is the one with the least pipe below it. If you have listed them the other way round there is a switch on the form rather than a requirement to retype.
Branches that are genuinely parallel — two branches meeting the main at the same fitting — can be entered in either order, because the accumulation below them is the same either way. Only the intermediate segments move.
Questions people ask
What is a drainage fixture unit?
A weighting, not a volume. It is a single number standing in for how much a fixture discharges and how often, so that a schedule of fixtures which are never all running at once can be reduced to one figure a sizing table can take. The value for any given fixture comes out of the table in the code your jurisdiction has adopted, and the tables are not identical between codes or between editions of the same code.
Are drainage fixture units the same as water supply fixture units?
No, and mixing them up is the common mistake. They weigh the same fixtures on different grounds. A tank type toilet sends a large volume down the drain very quickly and draws only a trickle up the supply while it refills, so it is heavy on the drainage side and light on the supply side. A shower is roughly the reverse. Use a drainage table for drains and a supply table for supplies.
Does this page tell me what size drain I need?
No, and it will not. A size comes out of a table read against a fixture unit load, a slope and a use class in the code your jurisdiction has actually adopted, as locally amended. This page adds fixture units and carries the total down the run. If you enter the figure your table gives, it will subtract it for you, but that subtraction is arithmetic between two of your own numbers, not a compliance determination.
Why does the accumulation matter more than the total?
Because the total describes only the last length of pipe. With the default list, the segment above the kitchen carries 8 fixture units and the segment below the powder room carries 21. On new work you size for the heaviest segment and move on. On a remodel, what you want to know is which existing segment your new branch pushes past the figure it was sized to, and that is a question about the middle of the run, not the end of it.
How do I handle a bathroom group?
Check your table first. Many tables weight a complete bathroom group as a single value rather than as the sum of the water closet, the lavatory and the tub, and that group value is usually lower than the sum. If yours does, enter the group as one line with the group value and a count of 1. If you enter the fixtures separately you will get a larger number than the table intends.