Q&A / 

Calculating Water Pressure Loss

Calculating Water Pressure Loss

Years ago, engineers developed tables to quickly calculate water pressure loss. They are often referred to as hydraulic friction loss tables. Studies produced "constants" (numbers which you use to help in the calculation) which make the math fairly easy. What you need to know are several things: size of pipe, flow (in gallons per minute GPM) and the distance which the water flows (fittings increase the actual distance, so if you have lots of bends and tee's, 100 feet of pipe might really be 130 feet or so). Flow is the thing that stumps most people. Flow rates can also be found on tables. Here are some standard flow rates for residential fixtures: toilet - 2.5 GPM, kitchen sink (H or C - times 2 if both handles wide open) - 1.25 GPM, washing machine (H or C - times 2 if H & C on at same time) 1.6 GPM, shower 2.5 GPM (both hot and cold on at same time).

Now for our constants. I have listed several to show the relationship as the gallons per minute of flow increases. Listed are 1/2 and 3/4 inch copper pipe Type M (standard used in residential construction):

The constants listed are pressure loss per hundred feet of pipe run.

1/2 inch pipe:

1 GPM = 2
2 GPM = 7.22
3 GPM = 15.3
4 GPM = 26.0
5 GPM = 39.2
6 GPM = 55
3/4 inch pipe:

1 GPM = 0.354
2 GPM = 1.28
3 GPM = 2.71
4 GPM = 4.60
5 GPM = 6.94
6 GPM = 9.75

Notice the difference in the constants for different sized pipe, but yet the same GPM flow! It's HUGE. See why 3/4 is better?

Now, here is how you do a pressure loss calculation. By the way, we need one other constant and it is a number that relates to pounds per square inch. It's really pretty complicated, so I would appreciate it if you would just trust me on this one. The number we need to remember is 0.4335.

Here's the problem. Let's calculate what will happen to the pressure if we flush a toilet while a shower is running in a bathroom 100 feet away from where the water enters the house. We are using 1/2 inch pipe.

Toilet flow = 2.5 GPM Shower Flow = 2.5 GPM Total flow = 5 GPM

Constant from table multiplied by 0.4335 = Pressure loss in PSI
39.2 X .4335 = 16.99 PSI

If we substitute 3/4 inch pipe, here is the calculation:
6.94 X .4335 = 3.00 PSI

Quite a difference!

Column B372

SPONSORS / 

4 Responses to Calculating Water Pressure Loss

  1. Hello I have a calculation to figure out and you seem to have the easiest to understand math that I can relate to.
    This is for my daughters house in the country where a 6 inch water line is available to hook into. Once water goes through water vault 1inch hook up and 60 psi delivery. The distance we have to go is 2000 feet and have been told to use 2 inch black plastic tubing/pipe off a roll.
    Could you please tell me if this is correct or could we use smaller water line?

    • jim, your question requires lots of typing, plus I have some questions for you so I can give you the correct answer(s). I only do pithy answers here in the comment section. If you want to protect the investment you have in your house and not waste time or money *hoping* you make the right decision, you should talk to me on the phone for just 15 minutes. It'll be the best investment you've ever made in your home!

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>