How to Size a Home Drainage System

Learning how to size a home drainage system, and how to calculate drainage fixture units (DFUs) in a residential setting, can be invaluable skill for up-and-coming plumbers and DIYers. A DFU, or drainage fixture unit, is a numerical value assigned to each fixture that determines how much load that fixture is putting on the drainage system. So the higher the number, the more load. A DFU is calculated based on frequency and amount of water discharge — so frequency of use and how much it dumping into the system. These instructions are based on the 2018 Uniform Plumbing Code, so check with your jurisdiction to see what code you’re on prior to proceeding. Right now it is the year 2022, and the most current code is 2018 UPC here in Montana. If you’re on international plumbing code, you may want to reference a different set of instructions.

Watch the video below

How to calculate drainage fixture units

So the assumptions we’re making for this drawing is it’s an isometric view. This is a private residence. So all these water closets or toilets are 1.6 gallon gravity flush. So your standard flush 1.6 gallon. This is all one level, and this is all crawlspace piping in the horizontal position. There’s some clean outs being shown. Not all of them are there, but we can do a future video on clean out locations and requirements.

First we’re gonna assign the DFU or drainage fixture units to each individual fixture trap, arm, and trap arm size. The first table we’re gonna use in the code book is Table 702.1.. This table features all the plumbing appliances, all the fixtures, their minimum trap and trap, arm size, their drainage fixture unit for private and, uh, for public. But today we’re gonna be focusing on private. So in table 7 0 2 0.1, we have all the drainage fixture units.

So we’ll go down the list starting over here. So our bathtub is two fixture units. Our toilet here is three. The lab here is one moving to this next bathroom group, the shower, two DFU toilet, three lab, one over here in this bathroom group lab, one lab, one toilet, three bathtub two. And then the shower here is two as well. Moving over to our kitchen. We have a kitchen sink, which is two DFU. We have a laundry sink, which is bigger than a normal lab. This one’s two as well. And then we have a clothes washer over here, which is three. And then our last bathroom, we have our lab as one and the toilet is three. So those are the fixture units of our system here.

How to calculate minimum trap and trap arm size

If we look in Table 702.1 in the column of minimum trap and trap arm size, that’s the next thing we have to do for each fixture. So this tub is an inch and a half. This toilet is three inches. This lav is inch and a quarter, this shower is two inches. This toilet is three inches. This lav is inch in a quarter. This lav is inch in a quarter. This lav is inch in a quarter. This toilet is three inch. This tub is inch and a half. This shower is two inch. This kitchen sink is inch and a half, but this is where you have to read the sub notes. You’ll notice by kitchen domestic. There’s a little ≈ next to the word that means to reference these notes below. And Subnote 2 says to provide a two-inch minimum drain. We won’t get too in depth in some of these subnotes cause we’re trying to keep it pretty basic. But for the DIYers, that section and reference is important. The laundry sink, same concept, minimum trap and trap arm is inch and a half. But as you hit the drain, this also is under ç, which says provide a minimum of two-inch drain. So that section will be two inch. The clothes washer and standpipe is two inch. Hopping down to our last bathroom group: trap and trap arm for this lav is inch and a quarter. And the final toilet is three inches.

How to assign drainage fixture units to each pipe section

Next we’re gonna assign the proper fixture unit to each pipe section, always start with the highest part of the system and work your way down. So we’re gonna start at this upper left hand corner. And I have conveniently labeled these pipe sections, A, B, C, D, et cetera. Okay, so we’ll start with this group. And this farthest upstream section pipe section A  is picking up this tub, which is two drainage fixture units. So pipe section A is carrying two. As we work our way down, pipe section B is now picking up pipe section A and this new toilet. So we have two coming from upstream and three more fixture units from the toilet. So pipe section B is picking up five total drainage fixture units, and continue to work our way down. Next pipe section C is picking up the five fixture units from above at pipe section B, but then we’re also picking up this LA, which is one fixture unit.

So pipe section C is six total DFU demand on that section, moving upstream to pick up this group as we’re working our way, this way, we’re gonna pick up pipe section D, which is carrying a shower, the shower, according to our Table 702.1 is two drainage fixture units. So that pipe section is two, uh, E is picking up this tub is the only thing on this tub. So pipe section E is two as well. Uh, pipe section F is picking up both upstream is E and D. So it’s picking up two and two totaling four. So we’re just adding the total load together, um, as we work our way down. So as we’re working downstream here, now, Jay is picking up all of I and all of C. So it’s nine plus six. So Jay has 15 fixture units on it. Now we are working downstream towards N so looking upstream N is picking up all of M and all of J. So it’s 15 plus six there’s 21 fixture unit load on N

Okay. Now moving down to R we’re picking up all of this queue, plus all of this N so 21 plus seven is 28 total drainage fixture units in this section, R and lab. Lastly, picking up this guest bathroom pipe section S just has a toilet on it for three picking up one lab here. So pipe section T has the lab and the toilet together one plus three equals four here. Okay. And then very lastly, you is picking up everything above it here at so 28, plus this bathroom group of four is now a total of 32 DFU for a total fixture unit on you.

How to calculate pipe size for each section

The next table we need to reference is Table 703.2. And this is what pipe size we need for each section. Okay. To explain 703.2, it’s pretty easy to read on the top. Here is the size of pipes between inch and a quarter. And the table goes all the way up to 12. And my little cliff notes here, I’m referencing inch a quarter to four, cuz that’s the most common in residential. So we have the pipe size here in this first row. And then below that is how many DFU or drainage fixture units that pipe can support both in the vertical and horizontal position.

The difference in maximum fixture unit values between vertical and horizontal drainage piping is because of the two differing flow characteristics of vertical versus horizontal. What that means is that vertical pipe can hold more flow in this model. Most of our stuff is on the horizontal plane. So we’re gonna focus in this horizontal column. So let’s go back to pipe, section a okay pipe section a has a total of two drainage fixture units and it’s in the horizontal. So we cannot run inch in a quarter. We cannot run inch and a half. We have to run two inch pipe there, cuz this is the maximum fixture value that it can hold. So two is obviously more than one, so we gotta size it to two inch. So that’s gonna be a two inch pipe moving up. We’re picking up pipe section B. This has five fixture units, which could be carried by a two inch pipe.

But wait, the toilet minimum pipe size is three inch as we saw before and we can never run a larger trap into a smaller branch. So pipe section B, even though it’s five fixture units still has to be three inch. Moving down to this lav: we’re not really focusing on the vertical section, but we will quickly. This vertical lab is one fixture unit. This pipe section’s in the vertical. It can hold inch and a quarter can hold one. So that pipe is okay to be inch and a quarter in real life. Most supply houses are only gonna carry inch and a half. So realistically in real life, this is usually inch and a half. But if we’re sizing to the minimum size, cuz it saves money that can be inch and a quarter moving down. We have C, here, which is six fixture units.

According to our table, it can be two-inch but wait: remember we have the three-inch toilet upstream, so that still has to remain three inches. So C is three inch. Working towards the shower, we have two fixture units here. This minimum trap and trap arm is two inch for the shower. So this pipe is two inch. D and the horizontal here is two fixture units. If we go back to our chart, two fixture units and the horizontal has to be two inch. So D is two inches. E in the horizontal is picking up two fixture units, which is once again two inch And we’re working our way down: pipe section J is a total of 15 DFU. So if you look according to our table, that has to be three inches. Moving down, we’re picking up our clothes washer. That’s a two inch trap and trap arm size. It is three fixture units, so it has to be two inch anyway — so section O is two inches. Moving down to P picking up a laundry sink with its minimum trap and trap arm size of inch and a half. But when we hit the drain, that’s a minimum two inch pipe. As I explained earlier, which then flows into P, which is a total fixture unit of five. So that’s two inches. Lastly, we’re picking up this guest bathroom. Uh, this water closet is three inch. As we’ve said before, pipe section S is three inch picking up this LA with four fixture units is still three inch because it has a toilet upstream.

Now we’re connecting into U. So U has a total drainage fixture unit load of 32 — 32 is less than 35, so that’s a three-inch pipe. But wait — there’s a subsection note. That’ll get you confused here. Notes from Table 703.2, note number four: only four water closets or six unit traps allowed on a vertical pipe or stack and not to exceed three water closets or six unit traps on a horizontal branch or drain. So remember this system is mainly in the horizontal. And so if you count these toilets, section U has four toilets. And as we remember from that note, the horizontal is only allowed to pick up three. So this actually has to upsize to a four-inch pipe.

As I mentioned earlier, another key component to a drainage system is where to locate cleanouts are. So if there’s ever a blockage, you can snake the system and get it cleared. This article does not cover those, but we might cover that in the future — so more to come on that.

Subscribe to see more tutorials!

We hope everyone learned something today on sizing drainage for a private residence, we covered most of the basics. There are some advanced sizing with batteries of multiple fixtures and commercial settings that we could get into more in depth in a future article. To learn more plumbing tips and tutorials, subscribe to our YouTube channel.

Text Us