Knowing how to size your water lines (PEX or copper) is essential if you’re a DIYer or new to the plumbing trade. This blog post will cover the steps to our preferred method, using the 2018 Uniform Plumbing Code. If you’re making an addition or starting from scratch, this tutorial should be a big help. So let’s dive in.
Sizing the water meter
Our main task is sizing our water meter or verify that our existing water meter has capacity to expand on whatever project we’re working on. In our example in the video, we have our water meter connected to a kitchen sink, a bathroom group downstairs (which comprises a lav, toilet, and shower), and a bathroom group upstairs with the same fixtures. (The blue represents the cold water lines and the pink our hot water lines.)
To size the meter, we’re going to need four key bits of information:
- Total fixture units
- Developed length of the most remote outlet
- Elevation of the highest fixture
- Water pressure
1. Calculate the total fixture units
The first thing we need to do is calculate our total fixture units, which is essentially how much demand we have on the water system in the house. Each fixture is assigned a value from a table in the 2018 Uniform Plumbing Code on how much demand it puts on that system. (Follow along in Table 610.3 below.)
When you’re calculating the total fixture units, decide if you’re the “public” or “private” column first. In our example, we’re a private residence. Starting with the bathtub combination, we see that water supply fixture units (WSFU) is 4. Now let’s look for our kitchen sink: “Kitchen, domestic with or without dishwasher” is what we want, which is 1.5 WSFU.
Keep in mind is the minimum fixture branch pipe size is also listed on this table — these fixtures cannot be supplied with anything smaller than what’s in that column.
Continuing on: the lav is 1 WSFU and the toilet is 2.5. The bath/shower combination is 4. And since our upstairs example mirrors the downstairs, we can simply duplicate the numbers for that.
Now? We add it all up: 16.5 water supply fixture units in total.
2. Calculate the developed length
The next thing we need to do is calculate the developed length from the farthest remote outlet. In other words: the fixture that’s the farthest away from our meter. We do this by tracing back along the center lines of the pipes until we reach the meter. That total distance is called the developed length.
Again, the most remote outlet means the fixture in the house that’s farthest from the meter. In our example, that fixture would be the upstairs shower. Now, just trace back to the meter, adding up the length of the center line as you go. If you’re following our video, our resulting number is 115 feet.
3. Calculate the elevation of the highest fixture
Now we calculate the elevation of the highest fixture. Calculating this is simple: it’s finding the difference between the elevation of the meter and our highest remote outlet. In our example, our highest fixture is 12 feet higher than the meter.
This number is important because it affects the water pressure in your system.
4. Calculate the pressure
To calculate the pressure in your water lines, you will need the elevation of the highest fixture (which we figured out in the preceding step). If your highest fixture is above your meter, then you will experience a loss in pressure. To account for this, you can expect to lose 0.5 PSI for ever foot of elevation gain. In our example, we multiply 12 with 0.5, which results in 6 PSI pressure drop.
However, if your highest fixture is below your meter (i.e., you have a basement), you will add 0.5 PSI for every foot of elevation drop.
Next, we need to calculate the static pressure of our system at the source of supply. If there’s pressure fluctuations throughout the day, you should use your lowest measured pressure. In our example, we read 60 PSI. To get the real pressure, we need to take our reading and subtract the elevation of our highest outlet. The result: 54 PSI. That’s the last variable we need to start plugging these into our tables. (Follow along in Table 610.4 below.)
The top part of the table is the Maximum Allowable Length. From Step 2 in our example, our developed length of the remote outlet is 115. That woudl place us between 100 and 150 on the table, and you always want to round up. Going down from column 150, we’re going to look for the pressure range. Our pressure range is 54 PSI, so we’re going to work in the middle table labeled “46 to 60 PSI.” The next thing we need is the total number of fixture units. We have 16.5. In the table below, you’ll see that 16.5 is between 14 and 28. Once again, we always round up. Looking at the leftmost columns, we’re going to find our meter size and building supply size: 3/4 inches and 1 inch, respectively.
We recommend you create a cheat sheet for your respective pipe sizes, so you don’t have to search for them later. For our example, our 1-inch pipe can have up to 30 fixture units, our 3/4-inch can have up to 14, and our 1/2-inch can have up to 4.
The 75% Method
Now that we’ve got the size of the meter verified, we need to size each branch of our system. We’re going to be using what’s called the 75% Method. The 75% Method means that if there’s both a hot water and cold water supplied to the fixture, we take that total fixture unit and multiply it by .75. The resulting number is the size of the each branch.
Sizing hot and cold water lines
Instead of writing it all out here, we recommend you watch the video above to see our demonstration of calculating each branch and adding it all up. However, here are a few tips to keep in mind:
- Work with hot and cold lines individually before adding up the total at the meter
- Start with your most remote outlet on each floor, as you will be working downstream
- If your branch has both hot and cold lines, use the 75% Method on the total fixture units to get the branch size
- Refer to Table 610.4 in the Uniform Plumbing Code handbook to calculate the correct pipe size
- As you move closer to the meter, don’t forget to include all the additional branches upstream from the branch your now calculating (i.e., if your downstream branch is 3 fixture units, add the total fixture units from everything upstream to it)
Resizing for PEX-B
Since the above steps are meant to size a universal water line, this can be used for PEX, copper, galvanized — whatever your system is. However, we get a lot of questions about PEX-B water lines that have the insert-style fittings: how do you account for sizing if the insert-style fittings cause restrictions in flow?
Our real answer: there’s not a great way to account for it, but we have some tips. If your water line is running into a lot of fixtures with that PEX-B insert-style fitting, err on the size of the larger pipe size. For example, if you’re on the border between 1/2″ and 3/4″ pipe size, go with the larger one. There’s no exact science to it. But if you’re on those borders between pipe sizes, it’s worth upsizing to the next pipe size to be safe.