Sizing Guide

For the Estimating of Flow Rates and Water Usage

Engineering Reference of Flow

You will find a listing of various water using fixtures and how to estimate the probable demand in residential properties, office buildings, schools, motels and hotels, shopping centers, and many other customers. In order to properly select water conditioning equipment, it is essential to determine flow demands. The recording of actual installations by the use of special meters and recording charts enables us to use the information as an excellent reference tool.* The following graphs and data are the result of such research conducted by the AWWA.

  • Hotels and Motels Hotels and motels are subject to wide fluctuations in water use, with peak periods of short duration. The example of a 216 room hotel had a maximum demand of 150 GPM, or 0.7 GPM/unit, which occurred at one time during the 24 hour period. The graph below represents the result of a recorded survey on a Texas hotel.peak-gpm-flows-commercial
  • Schools Flush valves with high flow requirements are normally used for sanitary purposes, and schools usually operate with uniform recess periods, both of which produce extreme water-flow-rate demands. Test results from a South Texas modern high school with 1390 students demonstrated the need to properly size equipment for these types of applications. In this particular application flows of 150 GPM were common, with peak demands reaching 210 GPM on many occasions.
  • Apartments Apartments, like hotels have wide variations in flow rates as shown below. The survey conducted illustrates the flows throughout a one week period.

*Utility engineers have used a wide variety of methods to estimate a customers peak water demand, which range from a rule of thumb procedure to detailed criteria. The information here has been taken from field experiments, utility surveys, technical publications, and hydraulic design methods, all of which are assembled into a condensed explanation of customer demand and how to determine the maximum flows that can be expected. The majority of the contents within this publication have been extracted from the American Water Works Association (AWWA) Manual M-22. In determining demands of commercial applications, professional engineering information and on site experience are essential in final analysis of determining probable flows. The AWWA manual lends this design experience to efficiently calculate flow demands in commercial applications.

Most types of fixtures and uses are listed in this publication to permit the estimating of the probable gallon per minute demand in residential, public buildings, motels and hotels, office, schools, shopping centers, and other customers. The following information which the AWWA assembled in the estimating of flows, is also in part, data that has been published from the National Bureau of Standards, using plumbing manual report BMS-66. This method includes a list of fixtures and a table of values for each fixture, as well as a value for the fixture if it is in public use.

  SUN MON TUES WED THUR FRI SAT
MIDNIGHT 10 10 10 10 10 10 10
6:00 AM 25 50 30 30 45 35 30
NOON 35 90 45 80 90 85 35
6:00 PM 30 40 45 30 30 30 30
Plumbing Fixtures Values

The following represents each individual fixture value as if each fixture was operated independently at 35 PSI inlet pressure. A bathtub for example flows at a rate of 8 GPM without any interference from other fixtures. As more fixtures are present, the probability of flow decreases. When encountering devices or fixtures not listed, the demand in gallons per minute should be determined and added to the total fixture count.

Fixture Type Fixture Value Based on 35 PSI Inlet Fixture Type Pressure
Bathtub Arrangement 8
Bedpan Washers 10
Combination Sink and Tray 3
Dental Unit 1
Dental Lavatory 2
Drinking Fountain (cooler) 1
Drinking Fountain (public) 2
Kitchen Sink: 1/2″connection 3
3/4″ connection 7
Lavatory: 3/8″ connection 2
1/2″ connection 4
Lavatory Tray: 1/2″ connection 3
3/4″ connection 7
Shower Head (shower only)

4
Service Sink: 1/2″. connection 3
3/4″ connection 7
Urinal: Pedestal Flush Valve 35
Wall or Stall 12
Wash Sink: (each set of faucets) 4
Water Closet: Flush Valve 35
Tank Type 3
Dishwasher: 1/2″ connection 4
3/4″ connection 10
commercial (nominal) 15
Washing Machine: 1/2″ connection 5
3/4″ connection 12
1″ connection 25
1-1/4″ connection 35
1-1/2″ connection 50
Hose (50 ft. length wash down): 1/2″ 6
5/8″ 9
3/4″ 12
1″ 25

Water pressure available has a significant influence on the gallon per minute flow of the application. To illustrate this all important factor, the chart below provides evidence that the water pressure factor must be included in your sizing.

Variations in Flows with a 50 Foot Garden Hose
Water Pressure PSI Flow GPM
10 7
20 9
30 11
40 13
50 15
70 18
100 22

Due to the variation illustrated above, compensation must be applied when calculating the flow demand on any application. Multiplication factors must be applied upon completion of converting fixture value to probable GPM flow. The chart in Figure A should be used for this important adjustment. Example: A probable demand of 50 GPM was determined. The application has an inlet pressure of 60 PSI. Using the chart below, a multiple factor of 1.34 should be used. 50 GPM x 1.34 = 67 GPM compensated flow demand.

Figure A
Pressure PSI Factor
20 0.74
30 0.92
35 1.00
40 1.07
50 1.22
60 1.34
70 1.46
80 1.57
90 1.68
100 1.78

*Charts are based on inlet pressure of 35 PSI. For other pressures, adjust by use of Figure A.

Figure B *Fixture Value Conversion Charts

Chart I Country Clubs, Hospitals, Hotels, Nursing Homes, Office Buildings, Shopping Centers, Schools, Restaurants

Chart II Apartments, Condominiums, Dormitories, Trailer Parks, Homes, Motels
Fixture Value Probable GPM Flow Fixture Value Probable GPM Flow
10 – 10 10
20 – 20 18
25 – 25 20
40 – 40 21
50 35 50 22
75 43 75 23
100 50 100 24
125 55 125 26
150 57 150 28
200 62 200 30
250 67 250 33
300 72 300 37
350 77 350 39
400 82 400 42
450 86 450 44
500 90 500 46
550 94 550 50
600 98 600 52
650 102 650 54
700 106 700 56
750 110 750 58
800 112 800 59
900 117 900 61
1,000 122 1,000 62
1,100 127 1,100 64
1,200 131 1,200 66
1,300 133 1,300 68
1,400 136 1,400 69
1,500 138 1,500 70
2,000 140 2,000 72
3,000 156 3,000 76
4,000 162 4,000 82
5,000 168 5,000 88
6,000 174 6,000 94
7,000 180 7,000 100
8,000 186 8,000 108
9,000 192 9,000 116
10,000 198 10,000 122
11,000 204 11,000 128
12,000 210 12,000 134
13,000 216 13,000 140

The following is an example of estimating the probable GPM demand for an apartment complex. Customer: 160 unit apartment complex pressure at meter: 50 PSI

Fixture Fixture Value Extended Fixture Values
205 tank water closets x 3 = 615
259 lavatories: 3/8″ x 2 = 518
138 dishwashers: 1/2″ x 4 = 552
10 washing machines: 1/2″ x 5 = 50
165 kitchen sinks: 1/2″ x 3 = 495
162 bathtubs x 8 = 1296
Total Fixture Value 3526

Fixture value: 3526 Conversion from Figure B, Chart II: 80 GPM Adjustment to 50 PSI inlet water pressure: 80 GPM x 1.22 = 97.6 GPM or 98 GPM. The probable peak demand therefore, in this example of a 160 unit apartment, would be 98 GPM.

In determining water consumption of any application, it is more desirable to obtain the actual water meter history. Generally, a six month history will be representative of the applications requirements. This can easily be accomplished by contacting the water service supplying the application. Such requests are considered public information. Many of these services record usage in cubic feet. To convert volume given in cubic feet to gallons, multiple by 7.5. Example: 50 cubic feet x 7.5 = 375 gallons. Another procedure in determining consumption, and in particular when a meter reading is not available such as on a well system, is the use of a clock recording method. Upon determining the GPM rating of a well pump, connect an inexpensive clock to the pump circuit. Set at 12:00 o’clock and record daily the number of minutes the pump ran. Multiply these minutes recorded by the GPM rate and the average total daily consumption can then be estimated more realistically. A third method that can lend credibility to an estimated daily usage is through comparison. By obtaining an actual meter recording usage of a similar operation, the customer will have more confidence in your projections. When it is not practical to utilize any of the methods thus far described, the estimating of usage can be achieved by the chart below.

  • Apartments Based on 3 persons/apt. Hot and cold = 150 gal./unit/day Hot only = 60 gal./unit/day
  • Barber Shops 55 gal./day/chair
  • Beauty Salons 270 gal./day/station
  • Boilers To determine daily makeup in gallons: 1. Multiply boiler h.p. by 4.25 2. Then multiply by hours per day of operation. 3. Then multiply by the % operating rating. 4. Then subtract the % condensate returns.Note: When ratings are given in pounds of steam per hour, divide by 500 to obtain GPM requirement. When ratings are given in BTU’s, divide by 12,000. For every 12,000 BTU’s, there is an equivalent of 1 h.p.
  • Camps Day (no meals)= 15 gal./day/person Resorts = 50 gal./day/person Tourist = 35 gal./day/person
  • Cooling Tower To determine daily makeup in gallons: 1. Multiply the tonnage by four (this includes 2 gal./day/hr./ton bleed off). 2. Then multiply by the number of hours per day of operation.
  • Dentist 4,000 gal./month/chair
  • Dormitories Hot and cold = 40 gal./person/day Hot only = 20 gal./person/day
  • Hospitals Meter reading preferred Hot and cold = 250 gal./bed/day Hot only = 170 gal./bed/day
  • Lawns 25 gal./square ft./season
  • Laundry Hot and cold = 2.5 lb. capacity is equivalent to gallons per cycle.
  • Livestock and Poultry Cow, beef = 12 gal./animal/day Cow, dairy = 20 gal./animal/day Goat = 2 gal./animal/day Hog = 12 gal./animal/day Horse = 12 gal./animal/day Mule = 12 gal./animal/day Sheep = 2 gal./animal/day Chickens = 10 gal./each 100/day Turkeys = 18 gal./each 100/day
  • Motels Hot and cold = 130 gal./unit/day Hot only = 60 gal./unit/day
  • Nursing Homes Hot and cold = 100 gal./bed/day Hot only = 50 gal./bed/day
  • Office Buildings Hot and cold = 20 gal./person/day Hot only = 3 gal./person/day
  • Restaurants Hot and cold = 15 gal./meal/day Hot only = 7 gal./meal/day Add on for bar or cocktail lounge = 2 gal./patron/day
  • Schools Elementary:Hot and cold = 13 gal./stu./day Hot only = 5 gal./stu./day Jr. High: Hot and cold = 20 gal./stu./day Hot only = 10 gal./stu./day Sr. High: Hot and cold = 35 gal./stu./day Hot only = 15 gal./stu./day
  • Shopping Centers 300 gal./day/1000 sq. ft.
  • Trailer Parks 150 gal./trailer/day