Free study tools · 248 CMR 10.15
Massachusetts DFU Calculator: Size a Branch and a Stack
Add up drainage fixture units the way the exam expects, using the fixture values in 248 CMR 10.15 Table 1. The calculator then finds the smallest horizontal fixture branch (Table 2) and stack (Table 3) that can carry the load, and shows every step so you can check your own math.
1. Pick your fixtures
2. Your result
How drain sizing by fixture units works
- Look up each fixture's DFU in Table 1 and multiply by how many there are.
- Add them up to get the total load on the pipe you're sizing.
- Go down the sizing table from the smallest pipe and stop at the first size whose maximum is at least your total.
- Check the water closet limit. A 3-inch horizontal branch and a 3-inch stack each carry not more than four water closets, no matter how few DFU you have. Five water closets means 4-inch.
That last step is the one people miss on the exam. Five tank-type water closets are only 20 DFU, well under the 34 DFU a 3-inch branch can take, but the water closet count still pushes you to 4-inch.
| Fixture | DFU |
|---|---|
| Water closet, tank type | 4 |
| Water closet, valve operated | 6 |
| Lavatory | 1 |
| Bathtub or tub & shower unit | 2 |
| Shower stall, residential | 2 |
| Kitchen sink, residential (with or without disposer) | 2 |
| Laundry connection, residential | 3 |
| Laundry/utility sink | 2 |
| Drinking fountain / water station | 1 |
| Floor/trench drain, 2-inch | 4 |
| Floor/trench drain, 3-inch | 5 |
| Floor/trench drain, 4-inch | 6 |
| Urinal, wall lip | 4 |
| Urinal, pedestal, siphon jet blowout | 6 |
| Urinal, waterless | 1 |
| Pipe size | Horizontal fixture branch (Table 2) | Stack (Table 3) |
|---|---|---|
| 1-1/2 in. | 3 | 4 |
| 2 in. | 6 | 8 |
| 3 in. | 34, not more than 4 water closets | 48, no more than 4 water closets |
| 4 in. | 160 | 240 |
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Study aid — confirm with the current 248 CMR and your inspector. Values are from 248 CMR 10.15 as published by Cornell LII (amended effective 12/8/2023) and were checked in September 2026. This tool is not a design service and does not cover every rule that can affect a drain size. PressureTest is independent and not affiliated with the Commonwealth of Massachusetts, the Board of State Examiners of Plumbers & Gas Fitters or PSI.