What is the difference between DFU and WSFU?

DFU (Drainage Fixture Unit) measures load on the drain and vent system. WSFU (Water Supply Fixture Unit) measures load on the water supply system. They use separate tables because supply and drainage have different flow characteristics. Toilets, for example, have high WSFU values because of their flush demand, while floor drains have high DFU but zero WSFU.

How do I size a drain stack using fixture units?

Total the DFU values for all fixtures connected to the stack, then use IPC Table 710.1 to find the minimum stack diameter. For example, up to 24 DFU requires a 3-inch stack; up to 500 DFU requires a 4-inch stack. Always verify with your local plumbing inspector since local amendments may apply.

Fixture Unit Calculator

Calculate total DFU & WSFU, size drain stacks and supply mains per IPC/UPC code — add fixtures and get instant results

Fixture List

Select fixtures and enter quantities. Results update instantly.

Fixture Type	Qty	DFU ea.	WSFU ea.	Trap

Sizing Results

Total Drainage Fixture Units 0 DFU

Total Water Supply Fixture Units 0 WSFU

Drain Stack Minimum

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Building Drain Minimum

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Supply Main Minimum

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Estimated Peak Drain Flow

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Estimated Peak Supply Flow

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Total Fixtures 0

Fixture Types 0

Cold WSFUs 0

Hot WSFUs 0

Drain Only 0 DFU

Supply Only 0 WSFU

Largest Required Trap

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IPC Table 710.1 — Drain Stack Sizing

Maximum fixture units allowed on horizontal branches and stacks (3-story buildings — most residential). For taller buildings, consult IPC Table 710.1 footnotes.

Pipe Diameter (in)	Max DFU — Any Horizontal Branch	Max DFU — Stack (3 stories or less)	Max DFU — Stack (Total)
1¼"	1	2	2
1½"	3	4	8
2"	6	10	24
2½"	12	20	42
3"	20	48	72
4"	160	240	500
5"	360	540	1,100
6"	620	960	1,900
8"	1,400	2,200	3,600
10"	2,500	3,800	5,600
12"	3,900	6,000	8,400

IPC Table 604.3 — Water Supply Pipe Sizing

Required minimum diameter for building supply mains based on total WSFU and system pressure. Values shown assume 40–80 PSI service pressure at the meter.

Pipe Diameter (in)	Max WSFU — Flush-Tank Systems	Max WSFU — Flush-Valve Systems	Approx. GPM Capacity
¾"	14	—	4–6 GPM
1"	25	—	8–12 GPM
1¼"	50	15	14–20 GPM
1½"	100	30	22–30 GPM
2"	270	60	40–60 GPM
2½"	420	120	65–90 GPM
3"	640	180	100–140 GPM
4"	1,200	400	180–250 GPM

Flow Rate Reference — GPM by Pipe Size

Typical flow rates at normal domestic water pressure (60 PSI). Actual flow depends on pipe material, length, fittings, and pressure.

Pipe Size	Copper (GPM)	PEX (GPM)	CPVC (GPM)	Recommended Use
½"	3–5	3–5	3–5	Single fixture branch
¾"	6–10	6–9	5–8	2–4 fixture branch / small main
1"	12–18	10–15	9–13	Residential main (small home)
1¼"	20–30	17–25	14–22	Residential main (med. home)
1½"	30–45	26–38	22–32	Residential main (large home)
2"	55–75	46–62	40–55	Light commercial / apartment

IPC/UPC Fixture Unit Values

Standard fixture unit values per IPC Table 709.1 and UPC Table 702.1. Cold-only = no hot water connection (toilets, hose bibbs, floor drains).

Fixture	DFU	WSFU (H+C)	WSFU (Cold Only)	Trap Size	Notes
Bathtub / Shower Combo	2	1.5	—	1½"	Hot + cold supply
Shower Only	2	1.5	—	2"	Hot + cold supply
Toilet (Tank Type)	3	—	3.0	3" (integral)	Cold only; IPC §709.2
Toilet (Flushometer)	6	—	6.0	3" (integral)	Cold only; commercial
Lavatory / Bathroom Sink	1	1.0	—	1¼"	Hot + cold supply
Kitchen Sink	2	1.5	—	1½"	Hot + cold supply
Laundry Tray	2	1.5	—	1½"	Hot + cold supply
Dishwasher	2	1.5	—	1½" (standpipe)	Hot + cold supply
Clothes Washer	3	2.0	—	2" (standpipe)	Hot + cold supply
Utility Sink	3	2.25	—	1½"	Hot + cold supply
Floor Drain	2	—	0	2"–3"	Drain only; no supply
Hose Bibb (Sillcock)	0	—	2.5	—	Cold only; exterior

Required Trap Sizes (IPC Table 709.1)

Fixture	Minimum Trap Size	Max Trap Arm Length	Code Section
Lavatory / Bathroom Sink	1¼"	5 ft	IPC §909.1
Kitchen Sink	1½"	6 ft	IPC §909.1
Bathtub / Shower	1½"	6 ft	IPC §909.1
Clothes Washer standpipe	2"	5 ft	IPC §802.4
Floor Drain	2"–3"	N/A (floor mount)	IPC §412
Utility Sink	1½"	6 ft	IPC §909.1
Shower (gang/commercial)	2"	5 ft	IPC §909.1

Common Sizing Rules of Thumb

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Minimum Drain Slope

1/4" per foot (2%) for pipes 2½" and smaller. 1/8" per foot (1%) is acceptable for pipes 3" and larger per IPC §704.1.

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3" Minimum for Toilets

Any drain serving a toilet must be at least 3" diameter. A toilet cannot connect to a 2" drain under any circumstances (IPC §710.1 footnote).

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Wet Vent Limits

A wet vent may serve up to 4 fixture units. The vent pipe must be at least half the diameter of the drain it serves, minimum 2" (IPC §908).

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Building Drain Sizing

Size the building drain to handle the total DFU of all connected branches. Add 10–20% safety factor for future fixture additions.

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Flushometer vs. Flush-Tank

Flush-valve (flushometer) toilets draw 25 GPM instantaneously vs. ~3 GPM for tank toilets. Supply mains serving flushometer fixtures must use the flush-valve WSFU column.

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Typical Residential Main

Most single-family homes need a 1" supply main. Homes with 3+ bathrooms often require 1¼". Commercial projects start at 1½" and go up from there.

How to Use the Fixture Unit Calculator

Add fixture rows — click "Add Fixture" and select each fixture type from the dropdown. Start with the default row provided.

Enter quantities — type the number of each fixture type in the Qty column. Results update instantly.

Read the pipe sizing — the result card shows minimum drain stack, building drain, and supply main diameters per IPC code.

Check the Pipe Sizing tab for the full IPC tables used to derive the recommendations.

Verify with your AHJ — local amendments to the IPC or UPC may require larger pipes. Always confirm with your Authority Having Jurisdiction before rough-in.

Frequently Asked Questions

What is a drainage fixture unit (DFU)?

A DFU is an arbitrary unit that represents the probable discharge load from a plumbing fixture. It was calibrated so that 1 DFU equals approximately 7.5 gallons per minute of peak drain flow. A lavatory is 1 DFU, a toilet is 3 DFU, and a shower is 2 DFU. These values come from IPC Table 709.1.

Can I use a 3" drain for everything?

A 3" drain can handle up to 20 DFU on a horizontal branch and up to 48 DFU on a stack (3-story building). However, it cannot be reduced below 3" once a toilet is connected. For most single-family homes with 2–3 bathrooms, 3" or 4" covers the main stack. Individual fixture branches are sized separately.

What's the difference between IPC and UPC fixture unit values?

For most residential fixtures the values are identical. IPC and UPC diverge mainly for specialty commercial fixtures and in some table footnotes. For standard residential projects — toilets, lavatories, sinks, showers, washers — you can use either code's table and get the same answer.

Do I count both hot and cold WSFU separately?

Yes. Hot and cold supply systems are sized independently. A fixture with 1.5 WSFU (hot+cold combined) contributes 1.0 WSFU to the cold branch and 1.0 WSFU to the hot branch (not 0.75 each). The combined figure is for sizing the main only. This calculator shows total combined WSFU and also breaks out cold-only and hot-only totals in the stat grid.

Why does a flushometer toilet have 6 DFU vs 3 DFU for a tank toilet?

A flushometer (valve-type) toilet discharges its entire tank equivalent in one powerful flush lasting 8–12 seconds, producing a much higher instantaneous flow rate than a gravity tank toilet that refills slowly. The 6 DFU value accounts for this peak demand on the drain system.

How do I account for future fixtures in my planning?

Best practice is to add a 20–25% safety factor to your total DFU and WSFU before sizing pipes, or to size up one pipe diameter from the calculated minimum. This allows for fixture additions without re-running pipe. The calculator shows the code-minimum sizes — always discuss future-proofing with your plumbing contractor.