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What size drain pipe do you need? Sizing by fixture units in MA

Updated 2026-09-27 · Checked against 248 CMR 10.15, 248 CMR 10.03, 248 CMR 10.05, 248 CMR 10.08, 248 CMR 10.10

In Massachusetts you size a drain by adding up drainage fixture units (DFU) from 248 CMR 10.15 Table 1, then reading the pipe size off Table 2 (horizontal branches and building drains) or Table 3 (stacks). The short version: a 1-1/2-inch horizontal branch carries 3 DFU, 2-inch carries 6, 3-inch carries 34 (with not more than four water closets) and 4-inch carries 160.

That sentence answers "how many fixture units on a 3 inch drain" for a branch. But a 3-inch stack, a 3-inch branch and a 3-inch building drain each have different limits, and Massachusetts adds rules on top of the tables that change the answer. This page walks through all of it, then sizes a whole house from fixtures to foundation wall.

Step 1: fixture unit values (248 CMR 10.15 Table 1)

Every fixture gets a DFU value that stands for its probable discharge. These are the Massachusetts values for the fixtures you will see most, straight from 248 CMR 10.15 on Cornell LII:

Two Massachusetts details trip people up here:

• There is no bathroom group value. Table 1 has no line for a bathroom group. A full bath is added fixture by fixture: 4 + 1 + 2 = 7 DFU for a tank toilet, lavatory and tub.
• The kitchen sink is 2 DFU whether or not it has a disposer. Table 1 gives one value for both, so do not add anything for the disposer.

For pumps and other continuous flows, 10.15(2)(b) counts 2 fixture units for each gallon per minute of flow, except ejector pumps installed per 10.15(9) Table 5 with a maximum of 20 GPM, which the same paragraph carves out.

Step 2: horizontal branches and building drains (Table 2)

Table 2 is titled Maximum Loads in Fixture Units for Horizontal Drains. The first column is the horizontal branch. The next three are the building drain at three slopes.

Larger sizes continue in the regulation up to 15-inch pipe.

Notice three things. A 3-inch building drain has no rating at 1/8 in. per ft. The four-water-closet footnote rides on every 3-inch entry. And the building drain columns are nearly useless below 4 inches in real buildings, for a reason covered in Step 4.

Slope comes from 10.05(2): horizontal drainage piping 3 inches or smaller has a minimum uniform pitch of 1/4 inch per foot, and piping larger than 3 inches a minimum uniform pitch of 1/8 inch per foot. 10.05(2)(e) sends you back to Table 2 for building-drain pitch, and 10.05(2)(d) allows flatter pitch only on systems designed by a Massachusetts professional engineer.

Step 3: stacks (Tables 3 and 4)

For a soil or waste stack with one or two branch intervals, use Table 3:

For taller buildings, Table 4 limits the load per branch interval and the total on the stack, based on the number of branch intervals. Watch the footnote: on a 3-inch stack in Table 4 it reads no more than three water closets, not four.

Two sizing rules sit outside the tables. A stack is never smaller than the largest horizontal branch connected to it (10.15(4)). And drains are not reduced in the direction of flow: 10.05(4)(a)2 prohibits any fitting or connection that incorporates a reduction in pipe area. That is a prohibited-fitting rule; "never smaller downstream" is the practical result of it, not its wording.

Step 4: the building drain and the foundation wall

Here is the rule that overrides Table 2 for most real jobs, and it is one the national model codes do not have in this form.

248 CMR 10.03 defines the Building Drain as the lowest piping that conveys waste to the building sewer, "measured ten feet in developed length from the inside face of the foundation wall," and says: "The ten-foot section defined as the Building Drain shall be minimum four inch." On top of that, 10.15(6)(b)1 requires sanitary piping that passes through an exterior foundation wall to be no less than 4 inches in diameter, with narrow exceptions such as a residential laundry drain to a separate Board of Health-authorized dry well, or grinder pump discharge.

So even if a small house totals 27 DFU, which fits a 3-inch pipe on paper, the building drain leaving the house is 4-inch.

Two more floor-level rules: no portion of the drainage system installed underground may be less than 2 inches in diameter (10.15(6)(a)), and a kitchen sink shall not waste into any horizontal drain smaller than 3 inches that receives discharge from a flat bottom fixture (10.15(8)(c)2).

Quick answers: toilet, shower, washer, kitchen sink

What size drain pipe for a toilet? Plan on 3-inch. 248 CMR's drain tables attach every water-closet limit to 3-inch and larger pipe (the four-toilet footnotes in Tables 2 and 3, the three-toilet footnote in Table 4), and 10.05(4)(b)2 speaks of a three-inch closet bend. There is no sentence in 248 CMR 10.00 that says "water closet drain minimum 3 inch," so when you cite it, cite the tables. (Above the toilet connection, 10.16(6)(b) lets the stack continue at 2 inches as the toilet's vent, so not every pipe near a toilet is 3-inch.) For a floor-outlet toilet, keep the vertical drop from finish floor to the centerline of the horizontal drain at 20 inches or less, or the toilet must be individually vented (10.16(10)(c)).

What size drain pipe for a shower? 10.08 Table 1 lists a 2-inch minimum trap for a shower stall, and 10.08(2)(b)2 says the trap is the same size as the drain it discharges into. 10.10(4)(b)2 separately allows a 1-1/2-inch drain for a shower with one 2.0 GPM head, and 10.10(4)(b)3 steps the drain up with total flow when more than one head may run at once. Those two sections pull against each other on a single-head shower, so do not quote either one alone as the whole rule: on the exam, answer from the section the question cites, and on the job, rough in 2-inch and you satisfy both. A tub or tub and shower unit drain is 1-1/2 inches minimum.

What size drain for a washing machine? 2-inch trap (10.08 Table 1). The standpipe shall be not less than 2 inches in diameter and extend not more than 30 inches nor less than 18 inches above the base of the machine (10.10(8)(c)). The laundry connection is 3 DFU, which fits a 2-inch branch (6 DFU) with room for a laundry sink (2 DFU).

What size drain for a kitchen sink? The trap is 1-1/2 in. (10.08 Table 1) and 10.15(8)(c)1 requires not less than a 1-1/2-inch branch waste that connects independently to the drainage system. If that kitchen sink is an island, see how Massachusetts vents it with a bow vent.

Worked example: sizing a two-story house

Every trap arm in this house still has to meet the trap-to-vent distances: 5 ft for 1-1/2-inch and 6 ft for 2-inch. Those are in our trap arm length post.

Where Massachusetts differs from the national codes

If your study material came from an IPC, IRC or UPC book, these are the places the numbers will not match 248 CMR:

• No bathroom group. The IPC table has a bathroom group line. Massachusetts adds the fixtures one at a time.
• One toilet value per type. Massachusetts rates a tank toilet at 4 DFU and a valve-operated toilet at 6, without changing the value for flush volume.
• Four toilets on a 3-inch branch. The 3-inch horizontal branch and the 3-inch stack (one or two branch intervals) both allow up to four water closets. (Table 4, for stacks with three or more branch intervals, drops that to three.) If you remember two toilets on a 3-inch branch, that is not 248 CMR.
• A 3-inch horizontal branch carries 34 DFU. If you memorized a smaller number for a 3-inch horizontal branch, it is not the Massachusetts Table 2 value. (Stacks are a different table: Table 4 sets its own per-interval loads on taller 3-inch stacks.)
• The building drain is defined by length. The last ten feet inside the foundation wall is the Building Drain, and it is 4-inch minimum.

Common exam traps

1. Using the branch column for a stack, or the reverse. 3-inch is 34 DFU as a branch and 48 as a stack with one or two branch intervals.
2. Forgetting the toilet count. A 3-inch branch with 30 DFU and five toilets fails, even though 30 is under 34.
3. Sizing a building drain at 1/8 in. per ft in 3-inch. Table 2 has no rating there.
4. Reducing in the direction of flow. A 3-inch branch cannot drop to 2-inch after the last toilet; the reducing fitting itself is what 10.05(4)(a)2 prohibits.
5. Rounding a bathroom to a group value. Add 4 + 1 + 2.
6. Forgetting the foundation-wall rule. A building drain or foundation-wall question with a small DFU total still has a 4-inch answer (10.03, 10.15(6)(b)1). Keep the terms straight: the building sewer begins where the building drain ends.

Part II of the exam puts these rules on isometric drawings. The Massachusetts journeyman exam guide explains the blueprint, the code book has all four 10.15 tables in study form, and the practice app generates new risers to size every time.

Want the full set of sizing questions? Start 7 days free and work them under exam timing.

Independent study aid — always confirm requirements in the current 248 CMR and the PSI Candidate Information Bulletin.