What this page is, and what it refuses to be
This is an organizer. It counts fixtures, applies a weighting to each one, and gives you a total. That total — the water supply fixture unit load — is one of three inputs a pipe sizing table needs. The other two are the developed length of the run to the furthest fixture and the pressure available at the meter, and the calculator carries those alongside the total so all three sit in one place.
It stops there on purpose. It does not return a pipe size, because the pipe size comes out of the table in the code your jurisdiction has adopted, read against your fixture unit load at your developed length at your available pressure — and the tables are not the same between codes, between editions of the same code, or after a local amendment. A page that printed a size would be guessing which document you are being inspected against. Sizing a water service or a building supply is licensed work; it is not something to settle from a browser, and the plumber doing it will want the fixture count anyway, which is what this gives you.
Why fixture units exist at all
A house might have twenty fixtures. If every one ran at once, the supply would need to carry perhaps 40 gallons a minute. They never all run at once, and the probability that a large fraction of them run simultaneously falls as the building gets bigger — which is why a hotel with four hundred rooms does not need a pipe twenty times the size of the one serving twenty rooms.
The fixture unit is the weighting that captures this. It is a dimensionless number assigned to each fixture type reflecting how much it draws, how long it draws for, and how often it is used. Add the units up, take the total to a demand curve, and you get an estimated simultaneous flow that is far less than the arithmetic sum of the fixtures.
Two consequences catch people. First, fixture units are not gallons per minute and cannot be converted to them by multiplication. Second, they do not add linearly to demand: doubling the fixture units does not double the flow. The rough demand figure this calculator shows is there to convey that flattening, not to be used for anything.
The values, and why yours may differ
The weightings this page uses are working values in common circulation. Your adopted code has its own table and it governs. Where they most often differ:
| Fixture | Private use | Public use | Why the gap |
|---|---|---|---|
| Toilet, tank type | 2.2 | 2.7 | Same draw, used far more often |
| Toilet, flush valve | 6.0 | 10.0 | Draws hard and briefly rather than refilling slowly |
| Lavatory | 0.7 | 2.0 | Occupancy pattern, not fixture size |
| Shower | 1.4 | 4.0 | A gym runs its showers back to back |
| Kitchen sink | 1.4 | 4.0 | Commercial kitchens run continuously |
| Hose bib | 2.5 | 2.5 | Wide open for a long time either way |
Notice the combined figure for a fixture with both hot and cold is not the sum of the two sides. A lavatory is 0.5 on the cold branch and 0.5 on the hot, but 0.7 combined, because the fixture cannot draw its full share from both at once — you mix at the tap. This is a standard convention and it means the branch sizing and the service sizing use different totals from the same fixture list, which is why the calculator shows all three numbers.
Flush valve toilets are the item most likely to change an answer. A single flushometer draws in the region of 25 gallons a minute for a few seconds, which is why the weighting jumps so far above a tank toilet that refills quietly at 3 GPM for a minute. If a building has them, the supply is a different conversation entirely.
The two numbers this page holds but does not use
Developed length is the distance along the pipe, following the actual route with all its rises and detours, from the meter or well tank to the furthest fixture. It is not the straight-line distance and it is not the length of the house. Friction loss scales with it, so a sprawling single-storey house can need a larger service than a compact three-storey one with more fixtures.
Available pressure is what you have to spend. Out of it come the friction loss in the service and the distribution piping, the elevation lift to the highest fixture at 0.433 psi per vertical foot, the loss through the meter, and the loss through anything else in the line — a softener, a filter, a backflow device. Whatever is left has to be enough at the fixture. The pressure drop calculator works out that budget for a specific run once you know the size, which is the reverse of the sizing question but useful for checking whether an answer is plausible.
Plumbing work is regulated, and the requirements are not the same from one town to the next. There are several model codes, jurisdictions adopt different ones and then amend them, and the local authority having jurisdiction decides what applies to your building. Most permanent work needs a permit and an inspection. Anything that touches the water service, the gas line, or a backflow-prevention point is licensed work in essentially every jurisdiction. What is on this page is common working practice and arithmetic, not a requirement you can hold up to an inspector.
Questions people ask
How many fixture units can a 3/4 inch line carry?
That question does not have an answer without a length and a pressure, which is why this page does not try. The same 3/4 inch line serves a comfortable number of fixtures at 20 feet and 65 psi and almost none at 180 feet and 40 psi, because friction loss consumes the pressure budget along the way. Sizing tables are indexed on all three variables together for exactly this reason. Take your fixture unit total, your developed length and your available pressure to the table in the code your jurisdiction adopted, or to the plumber who will be pulling the permit.
Are fixture units the same as gallons per minute?
No. A fixture unit is a dimensionless weighting that accounts for the flow rate, the duration and the frequency of use of a fixture, and it converts to a demand flow only through a probability curve, not by multiplication. The curve is steep at low fixture counts and flattens out sharply as the building grows, because the chance of everything running at once collapses. Ten fixture units might correspond to roughly 9 GPM, a hundred to around 40 — ten times the units for four times the flow. The rough figure this page shows follows that shape and is there to make the point, not to be designed against.
Why is the hot plus cold total higher than the combined total?
Because a mixing fixture cannot take its full rated draw from both sides at the same time. When a shower runs at its rated flow, that flow is split between the hot and cold branches; each branch sees part of it. So the fixture is weighted at, say, 1.0 on each branch for branch sizing, but 1.4 rather than 2.0 when you are sizing the service that feeds both. Use the branch figures for branch pipe and the combined figure for the service, and do not add the branch figures to size the service.
Do I count a tub and shower combination as one fixture or two?
One, in the usual convention, when they share a single valve and only one can run at a time. Two separate valves in the same room that can both run are two fixtures. The general principle is that fixture units track simultaneous draw capability rather than fittings on the wall, so anything that physically cannot run at the same time as its neighbour is counted once. If your code table says otherwise for a particular arrangement, the table wins.
Can I use this total to size the drain as well?
No. Drainage fixture units are a separate system with separate values, and a fixture has both a supply fixture unit rating and a drainage fixture unit rating that are different numbers derived from different considerations. Drains are sized on flow, slope and venting, not on supply pressure, and a drain line runs partly full by design. The drain slope calculator covers the fall side of that question, and the venting side is not something to work out from a page.