Plumbing: Total Drainage Fixture Units

All versions

Drain and vent sizing starts with a single number: the total drainage fixture units on the line. Each fixture carries a DFU value from the code table; multiply by how many you have and add them up.

Quick formula: Count times DFU for each fixture, then SUM:

=SUM(D2:D5)

Two toilets, two lavatories, a shower, and a kitchen sink total 12 DFU — the figure you carry into the drain-size table.

Functions used (tap for the full reference guide):

SUM

The example

A small bathroom group plus a kitchen sink, each fixture with its DFU value from the code table.

	A	B	C	D
1	Fixture	Count	DFU each	Subtotal
2	Toilet	2	3	6
3	Lavatory	2	1	2
4	Shower	1	2	2
5	Kitchen sink	1	2	2
6	Total DFU			12

The formula

Each subtotal is a count times a DFU value; SUM adds the column into the line total:

=SUM(D2:D5) // add every fixture's DFU subtotal for the line total

How it works

Two layers of arithmetic:

=B2*C2 in each row multiplies the fixture count by its DFU value — two toilets at 3 DFU is 6.
SUM(D2:D5) totals those subtotals into the drainage fixture units for the whole line.
That total is what you carry into the code's drain-size table to pick the pipe diameter.
Keep each fixture on its own row so adding a fixture is a new row, not a formula edit.

The DFU values come from your plumbing code's fixture table and vary a little by code and fixture type. Put them in the DFU column so a code update is a column edit, not a rewrite.

Try it: interactive demo

Interactive

Enter counts for a simple bathroom group and kitchen sink.

Toilets

Lavatories

Showers

Kitchen sinks

Variations

Weighted total with SUMPRODUCT

Multiply counts by DFU values in one step across the whole list.

=SUMPRODUCT(B2:B5,C2:C5)

Compare to a branch limit

Flag when the line exceeds what its pipe size can carry.

=IF(SUM(D2:D5)>E2,"Upsize","OK")

Pitfalls & errors

DFU values are not the same as fixture count — a toilet is one fixture but three DFU. Multiply by the code value, never just count heads.

Codes differ. The IPC and the UPC assign slightly different DFU values to the same fixture, so use the table for the code your jurisdiction has adopted.

Practice workbook

📊

Download the free Plumbing: Total Drainage Fixture Units practice workbook

Edit the yellow count and DFU cells; the subtotals and total DFU recalculate.

Frequently asked questions

What is a drainage fixture unit?

A DFU is a code-assigned number that represents the drain load a fixture puts on the system, accounting for flow rate and how often it is used. Totaling them sizes the drain and vent piping.

Do I use DFU for water supply too?

No — supply sizing uses water supply fixture units (WSFU), a different table. This total is for drainage and venting. Keep separate columns if you size both on one sheet.

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Function references: SUM