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


20 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!

  2. I have a 3/4" feed from the main to feed my new house. The distance is 300'. Will running a 1" line from a 3/4" feed give me more pressure at the house? I was concerned about the up sizing from a smaller feed.

  3. Hi,
    My hose is 3/4" inside diameter, 10 metres in length with a mains water pressure of 500kPa. How would i find the water pressure

  4. The city is providing 110 psi from the meter in a 1 1/2 inch pipe that supplys 26 trailers about 1000 ft of pipe how much water pressure is lost? The water supply lines to the trailers range from 3/4 inch to 1 inch.

  5. I have a 3/4 at the meter. with 60 PSI I'm running it 2000 feet do I need to run a 2" line or smaller if so what would my pressure loss be.

  6. Tim, The question is not worded properly. The size of the pipe does not effect the pressure, it effects the capacity, friction loss, thus pressure loss. In your answer you added the key words, pressure loss. The question does not state that, the diameter will effect pressure loss.

  7. Sorry Tim, your question did not mention any flow. if there is no flow the pressure will be the same everywhere. No flow, no pressure drop.

  8. I am doing a 1000ft run from the meter to my house. The water pressure at the meter is 180lbs. My house is 130' above the meter as well.
    What size of meter 3/4, 1", 1 1/2 or 2" and what size of pipe should I run to my house?
    Thanks for the information

  9. Hello there!

    This is a very informative article, thank you for breaking everything down for us! But I still need some help :).

    I'm a backyard farmer and have been trying to devise a more efficient way to water. My garden did not do well last year because of the dry summer and apparently not getting enough water, although I watered twice a day for two hours.

    At any rate, I'm thinking of running some PVC pipe this year. I have several raised beds, about 8" off the ground. I plan to in the PVC along the top of the beds (not on the dirt in the beds). I'm going to need about 200 feet of PVC pipe along with 30 or so connections. I need to measure the flow out of my spigot, but the water is shut off right now due to the freezing temperatures lol.

    Can I apply your formula to this build? All I really need to figure out is what the flow is? I was planning on using 3/4" or 1/2", thinking the 1/2" would create more pressure behind the pinholes and force the still trapped water further down the pipes. Sounds like I actually want 3/4" though?


Leave a Reply

You have to agree to the comment policy.