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Pipes for water distribution in a building are available in various sizes. Determination of suitable pipe sizes for various purposes in a building based on various factors such as cost, pressure etc. are discussed.

Following points regarding pipe sizes in buildings are discussed:

  • What are the factors that influence the selection of water distribution pipe size in buildings?
  • Procedures used for the determination pipe sizes for water distribution in buildings

Suitable Pipe Sizes for Water Distribution in Buildings

Fig.1: Different Pipe Sizes to Distribute Water in Buildings

Factors Influencing Selection of Water Distribution Pipe Size in Buildings

  • Cost effectiveness
  • Pressure at the source of water supply
  • Required pressure at each outlet fixture (examples of outlet fixture are bathtubs, channel drains, bidets, drinking fountains, kitchen sinks, bathroom sinks, showers and others)
  • The loss of pressure of outlets placed above water source. Loss of pressure is commonly caused by friction of water. The friction is occurred due to flow of water in pipes, water flow meter, and backflow preventer.
  • Restriction on water flow velocity to avoid noises and pipe erosion.
  • Extra capacity of possible expansion in the future. Generally, the additional capacity if around 10 percent.
  • Economic considerations govern the selection process to a great extent. But other factors explained above may enforce the use specific pipe size other than the most economical ones.

Procedures for Determination of Pipe Sizes for Water Distribution in Buildings

  • Draw all suggested horizontal mains, risers and branch lines with necessary information includes types and number of fixtures and flow that is needed.
  • Calculate fixtures demand weight employing Table 1 and Table 2.
  • Specify water demand in gallon per millimeter using Figure 2 and Figure 3 and total number of fixture units.
  • Estimate equivalent length of the pipe for each stack in the system. This step should start from the street main.
  • Determine average minimum pressure in the street main. This can be obtained from Water Company or by test.
  • Specify minimum pressure required for the highest fixture
  • Calculate the loss of pressure in the pipes using the computed equivalent length of the pipe.
  • Finally, select the size of the pipe using Figure 4.


  • Unit sizes provided in the table 1 and table 2 is in mm, but the size of pipes in the chart is in inch. 1 inch= 25.4mm, 1 ft. = 304.8mm, 1 gallon/m=3.785L/m.
  • Water velocity should not be greater than 2.438m/s at valve seat to prevent noise and erosion. So, Pipe area should be at least equal to water flow rate divided by eight.
  • Sizes for fixture supply pipes provided in table 1 and table 2 are minimums.

Table-1: Fixture Units, Trap and Connection Sizes for Plumbing Fixtures for Domestic Waters

Types of fixtureDomestic water
Fixture-unit value as load factorsMinimum size of connections, mm
PrivatePublicHot waterCold water
Bathtub (with or without overhead shower2412.712.7
Combination sink and tray312.712.7
Combination unit and tray with food disposal unit4
Dental unit19.525
Dental lavatory1212.712.7
Dish water, domestic2
Drinking fountain129.525
Floor drains1
Kitchen sink2412.712.7
Kitchen sink, domestic, with food waste grinder3
Lavatory, barber, beauty parlor2
Lavatory, surgeon’s2
Laundry tray (1 or 2 compartments)2412.712.7
Shower, per head2412.712.7
Sinks Surgeon’s312.712.7
Sinks Flushing rim (with valve)219.0519.05
Sinks Service (trap standard)312.712.7
Sinks Service (P trap)2412.712.7
Sinks Pot, scullery,4
Urinal, pedestal, siphon jet, blowout1025.4
Urinal, wall lip512.7
Urinal stall550.819.05
Urinal with flush tank3
Wash sink (circular or multiple) each set of faucets212.712.7
Water closet, tank-operated3519.05
Water closet, valve-operated61025.4

Table-2: Fixture Units, Trap and Connection Sizes for Plumbing Fixtures for Drainage

Types of fixtureDrainage
Fixture-unit value as load factorsMinimum size of trap, mm
Bathtub (with or without overhead shower238.1
Combination sink and tray2
Combination unit and tray with food disposal unit3
Dental unit131.75
Dental lavatory2
Dish water, domestic238.1
Drinking fountain131.75
Floor drains250.8
Kitchen sink2 or 338.1
Kitchen sink, domestic, with food waste grinder2
Lavatory, barber, beauty parlor2
Lavatory, surgeon’s2
Laundry tray (1 or 2 compartments)2
Shower, per head250.8
Sinks Surgeon’s338.1
Sinks Flushing rim (with valve)676.2
Sinks Service (trap standard)3
Sinks Service (P trap)350.8
Sinks Pot, scullery,338.1
Urinal, pedestal, siphon jet, blowout676.2
Urinal, wall lip238.1
Urinal stall250.8
Urinal with flush tank238.1
Wash sink (circular or multiple) each set of faucets3
Water closet, tank-operated476.2
Water closet, valve-operated6

Domestic Water Demand Curve

Fig.2: Domestic Water Demand Curve Specify Rate of Flow Using the Number of Fixture Units Served
Domestic Water Demand Curve

Fig.3: Domestic Water Curve, Enlarged Portion for Low Water Demand
Determination of flow in copper tubing and other pipes

Fig.4: Determination of flow in copper tubing and other pipes that will be smoothed after service of 15 to 20 years

Read More:

Types of Plumbing Pipes used in Building Construction

Types of Plumbing and Drainage Systems in Buildings

Types of Pipe Joints in Plumbing System

Types of Pipe Fittings in Plumbing System for Different Purposes


ASPE. Plumbing engineering design handbook: Engineer’s guide to system design and construction. Chicago: American Society of plumbing engineers , v. I, 2004.

FREDERICK S. MERRITT, JONATHAN T. RICKETTS. Building design and construction hand book. 6th edition. ed. New York: McGRAW-HILL, 2001.

GESAP, Water Supply Facilities  2017. Accessed on: 30 Sep. 2017.

About Madeh Izat HamakareemVerified

Madeh is a Structural Engineer who works as Assistant Lecturer in Koya University. He is the author, editor and partner at