Drainage Fixture Unit (DFU) Calculator

This example uses private gravity water closets of 1.6 gallons per flush or less at 3 DFU each, lavatories at 1, showers/tubs at 2 and domestic dishwashers at 2. Enter counts only for those fixture types. Public-use, higher-flow and other fixtures require different applicable table entries.

The output sums the stated loads; it does not select a pipe diameter.

A total alone cannot determine drain diameter. Horizontal branches, stacks and building drains have separate limits, and fixture outlet, slope, vent and local requirements can govern. This page does not infer that a two-inch line can carry toilets.

Supply fixture units are a different measure and cannot be substituted for DFU.

Total drainage fixture units from private gravity toilets, 1.6 gpf or less

Private gravity toilets, 1.6 gpf or less	2
Lavatory sinks	2
Showers or tubs	1
Dishwashers	1
Total drainage fixture units	12
Number of fixtures included	6

Common error: Sizing a drain from the total alone and ignoring the requirements for the toilet outlet. Some fixtures set a minimum pipe size on their own, whatever the total.

Drainage Fixture Unit (DFU): formula

Total example DFU = private ≤1.6-gpf toilets×3 + lavatories×1 + showers/tubs×2 + domestic dishwashers×2

IAPMO: Uniform Plumbing Code · City of Fairfax: Drainage fixture-unit table

Drainage Fixture Unit (DFU): changing Private gravity toilets, 1.6 gpf or less

Total drainage fixture units: assumed Private gravity toilets, 1.6 gpf or less scenarios.
Private gravity toilets, 1.6 gpf or less	Total drainage fixture units	Number of fixtures included
0	6	4
1	9	5
2 (example)	12	6
3	15	7
4	18	8

Drainage Fixture Unit (DFU): compare input ranges

Drainage Fixture Unit (DFU) Calculator: Total drainage fixture units with each input moved on its own, every other input held at the worked-example value.
Input	Tested from	To	Total drainage fixture units: low input	Total drainage fixture units: high input	Swing
Private gravity toilets, 1.6 gpf or less	1	3	9	15	6 (50%)
Showers or tubs	0	2	10	14	4 (33%)
Dishwashers	0	2	10	14	4 (33%)
Lavatory sinks	1	3	11	13	2 (17%)

Drainage Fixture Unit (DFU): individual input comparisons

Drainage Fixture Unit (DFU) Calculator: change one input at a time; other values remain at the stated example.
Input	Value	Total drainage fixture units	Number of fixtures included
Private gravity toilets, 1.6 gpf or less	0	6	4
	1	9	5
	2	12	6
	3	15	7
	4	18	8
Lavatory sinks	0	10	4
	1	11	5
	2	12	6
	3	13	7
	4	14	8
Showers or tubs	0	10	5
	0	10	5
	1	12	6
	2	14	7
	3	16	8
Dishwashers	0	10	5
	0	10	5
	1	12	6
	2	14	7
	3	16	8

Total drainage fixture units: Private gravity toilets, 1.6 gpf or less against Lavatory sinks

Drainage Fixture Unit (DFU): Total drainage fixture units at combined Private gravity toilets, 1.6 gpf or less and Lavatory sinks settings.
Private gravity toilets, 1.6 gpf or less / Lavatory sinks	0	2	4
0	4	6	8
1	7	9	11
2	10	12	14
3	13	15	17
4	16	18	20

Drainage Fixture Unit (DFU): values and definitions

Worked-example inputs and the results they produce for the Drainage Fixture Unit (DFU) Calculator.
Quantity and definition	Value used
Private gravity toilets, 1.6 gpf or less — Number of gravity-flush toilets on this drain.	2
Lavatory sinks — Number of lavatory (bathroom) sinks.	2
Showers or tubs — Number of shower or tub/shower combinations.	1
Dishwashers — Number of dishwashers on this drain.	1
Total drainage fixture units	12
Number of fixtures included	6

Private gravity toilets, 1.6 gpf or less: half and double

Drainage Fixture Unit (DFU) scale comparison; assumed inputs.
Private gravity toilets, 1.6 gpf or less	Total drainage fixture units
1	9
2	12
4	18

Drainage fixture units rate how much each fixture loads a drain, so a pipe can be sized from the sum instead of from each fixture. Two toilets, two lavatories, one shower and one dishwasher come to 14 units in this model, and the drain table gives a minimum diameter of about 2 inches for that load. The unit values and table come from the plumbing code your jurisdiction has adopted, and they differ between codes, so the result is a first check.

Use the fixture unit values and pipe-size tables from your local code edition, and let the plumber or inspector confirm the size. Branch and stack limits differ from the horizontal drain limit.

Set slope on the drain slope calculator, water heater capacity on the water heater sizing page and supply flow on the flow rate calculator. The pipe volume and friction loss pages cover supply piping, and the rectangular and cylinder calculators handle geometry.

Drainage Fixture Unit (DFU) Calculator