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How Do You Size Water Supply Pipe in Massachusetts? A 248 CMR Fixture-Value Guide

Updated 2026-10-03 · Checked against 248 CMR 10.14, 248 CMR 10.06, 248 CMR 10.10, 248 CMR 10.15, PSI bulletin 598

If you are asking, “What size water pipe do I need in Massachusetts?”, start with the Massachusetts method—not a generic fixture-unit-to-GPM chart. Under 248 CMR 10.14, the usual sequence is to identify fixture factor values, apply the applicable occupancy demand factor, determine the resulting capacity value, and select the nominal pipe size from the corresponding table.

That distinction matters on the Massachusetts Journeyman exam. A familiar plumbing book may teach a Hunter curve, a probable-flow conversion, or a developed-length pressure-loss calculation. Those methods can be useful engineering tools when a design specifically calls for them, but they are not a substitute for the 248 CMR 10.14 Tables 1, 2, and 3 procedure in an ordinary Massachusetts code question.

For broader exam preparation, keep this article alongside PressureTest’s Massachusetts Journeyman plumbing exam guide, the Massachusetts codebook reference, and plumbing math formulas.

The Massachusetts water-pipe sizing sequence

Think of the process as a table lookup with a few deliberate steps:

1. Identify the fixtures served by the pipe section.
2. Find each fixture’s water-supply factor value in the applicable Massachusetts table.
3. Add the factor values for the fixtures served by that section.
4. Identify the occupancy and use the applicable demand factor from the Massachusetts demand-factor table.
5. Apply that demand factor to the total factor value.
6. Use the resulting capacity value in the pipe-size table.
7. Select the pipe size whose listed capacity range accommodates the result.
8. Check any separate minimum fixture-flow-pressure or fixture-supply requirements stated by the code or by the question.

The important terms are “factor value,” “demand factor,” and “capacity value.” Do not silently replace them with “WSFU,” “GPM,” and “Hunter curve” unless the problem itself defines those terms or supplies the alternate chart.

A building main, a branch, and a riser generally go through the Table 1–3 process based on the fixtures served. A fixture supply, however, may have its own specific fixture factor value and minimum-size requirement. That means you should not assume that every segment is solved by one identical pressure-loss calculation.

A practical exam worksheet

Use a worksheet with these columns:

The table above is a solving format, not a replacement for the Massachusetts tables. On an exam question, read the fixture descriptions exactly. “Building main,” “branch,” “riser,” and “fixture supply” may lead you to different rows or notes.

Worked example: use the Massachusetts tables, not a generic flow chart

The following is a method example only. The numbers are intentionally supplied by the hypothetical question and are not being presented as Massachusetts code-table values.

Suppose an exam problem gives you a connected fixture-factor total, identifies the occupancy, supplies the applicable demand factor, and asks you to choose a pipe size. Do not invent a GPM conversion. Work only with the values the problem gives and the capacity ranges in the Massachusetts table.

The lesson is the order:

• The fixture information produces the total factor value.
• The occupancy table supplies the demand factor.
• The demand-factor step produces the capacity value.
• The pipe-sizing table maps that capacity value to a nominal size.

If the question gives a separate minimum flow pressure, elevation condition, or other hydraulic information, use it as directed. Do not add a Hunter-curve conversion merely because you have seen one elsewhere.

What if the problem asks about a fixture supply?

Treat that as a specific code question, not automatically as a building-main problem. First identify the fixture and locate its row in the Massachusetts water-supply table. Then check the listed factor value and minimum size for that fixture supply.

This is one of the easiest places to lose points: an examinee remembers the building-main sequence, then applies it mechanically to a fixture connection. The correct approach is to read the scope of the table row and any notes attached to it.

DFU is not the same thing as a water-supply factor value

Massachusetts plumbing questions often place water supply and drainage material close together. That makes terminology especially dangerous.

DFU means drainage fixture unit. DFU is used for sanitary drainage sizing under 248 CMR 10.15. Water-supply factor values in 248 CMR 10.14 are used for sizing water mains, branches, risers, and fixture supplies. They are related ways of representing fixture demand, but they are not interchangeable.

For example, the Massachusetts drainage table identifies these DFU values:

Those values come from 248 CMR 10.15 Table 1. They do not become water-supply factor values just because the same fixtures also consume water.

For a drainage refresher, see drain-pipe sizing by fixture units. For water-pipe questions, go back to 248 CMR 10.14 and look for the water-supply fixture factor value—not the DFU row from 10.15.

Pressure is a separate check from the table-sizing sequence

A common mistake is to turn every Massachusetts water-supply question into a complete hydraulic design calculation. The 248 CMR 10.14 fixture-value procedure selects pipe size through its factor-value, demand-factor, and capacity-value tables. Minimum flow pressure at fixtures is addressed separately in Table 4.

That means you should separate two ideas:

• Table sizing: use the Massachusetts fixture factors, occupancy demand factor, capacity value, and nominal pipe-size ranges.
• Pressure performance: check the separate minimum-flow-pressure requirement or any hydraulic information expressly included in the problem.

A pressure question may give facts such as elevation, available pressure, a developed length, fittings, or a required outlet pressure. If it does, follow the question’s stated method and units. If it does not, do not assume that you must manufacture a pressure-loss calculation before selecting the table answer.

This is also why “the smallest pipe that carries the calculated GPM” is not a safe universal rule for the Massachusetts exam. The code table may be asking for the pipe size associated with a capacity-value range, while a separate question may test pressure at a fixture. Those are not the same question.

Common Massachusetts exam traps

Trap one: using a national plumbing code table as if it were Massachusetts law

Massachusetts has its own plumbing regulations and amendments. An IPC or UPC table may use different terminology, factors, footnotes, or sizing logic. Use the Massachusetts source provided for the exam and verify the section in 248 CMR 10.14.

Trap two: converting fixture units to GPM automatically

A search result may call a chart a “fixture units to GPM chart.” That does not make it the Massachusetts answer. Unless the question supplies a GPM chart or asks for a flow conversion, stay with the 10.14 factor-value and capacity-value method.

Trap three: confusing DFU with water-supply factors

If the question says water closet, lavatory, or kitchen sink, do not grab the first fixture-unit value you remember. Confirm whether the problem is asking about drainage or water supply.

Trap four: overlooking occupancy

The same connected fixture total can be treated differently depending on the occupancy demand factor specified by the Massachusetts table. Read the occupancy description before applying the factor. Do not select a demand factor from memory.

Trap five: treating every pipe section identically

A building main, branch, riser, and fixture supply may be described differently in the table. A fixture supply can have a specific minimum size or factor value. Identify the exact section being sized before beginning the worksheet.

Trap six: stopping after the capacity value

The capacity value is not usually the final answer by itself. You still need to match it to the pipe-size range in the applicable Massachusetts table and observe any notes or limitations.

Trap seven: adding unrelated code sections

Do not answer a water-supply sizing question with a drainage table, trap-arm rule, vent-size rule, or gas-pipe table. The related topics are worth studying, but they answer different questions. PressureTest also has focused practice on vent-pipe sizing, trap-arm length, and plumbing isometric drawings.

A fast checklist before choosing an answer

Ask yourself:

• Am I sizing water supply or sanitary drainage?
• Did I identify the exact fixtures served by this pipe section?
• Am I using the Massachusetts water-supply table rather than the DFU table?
• Did I identify the occupancy demand factor?
• Did I calculate the resulting capacity value using the problem’s supplied values?
• Did I match that capacity value to the Massachusetts pipe-size table?
• Is this a building main, branch, riser, or fixture supply?
• Does the question separately ask about minimum fixture pressure?
• Did I read the table notes and footnotes?
• Am I importing a national-code rule that the question never supplied?

For code review, start with 248 CMR 10.14, then compare related water and fixture provisions in 248 CMR 10.06 and 248 CMR 10.10. Use the PSI bulletin for exam-administration information, not as a replacement for the Massachusetts code tables.

The one-sentence memory aid

For Massachusetts water-pipe sizing, remember: fixtures create factor values, occupancy supplies the demand factor, the result becomes a capacity value, and the capacity value points you to the pipe-size range.

Do not begin with GPM unless the problem tells you to. Do not use DFU unless the problem is about drainage. Do not turn a Table 1–3 sizing question into an IPC or UPC exercise.

Want to practice the exact sequence under time pressure? Open the PressureTest practice app, choose water-supply sizing, and explain each table lookup before selecting the pipe size. That explanation step is where you catch most Massachusetts exam mistakes.

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