Calculating Water Pressure Loss
Calculate Water Pressure Loss
Years ago, engineers developed tables to quickly calculate water pressure loss. They're 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 Do You Need to Know to Calculate Pressure Loss?
You need to know several things:
- size of pipe
- flow in gallons per minute (GPM)
- the distance which the water flows
Do Fittings Affect the Actual Distance?
Yes, fittings increase the actual distance. 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 - 1.25 GPM (H or C - times 2 if both handles are wide open)
- washing machine - 1.6 GPM (H or C - times 2 if H & C on at the same time)
- shower 2.5 GPM (both hot and cold on at the same time)
What are the Constants?
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. Type M is the standard grade of copper tubing used in residential construction.
What are the Pressure-Loss Constants for Small Pipe?
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. Okay, if you don't trust me and want to know more about the secret constant of 0.4335, click or tap here.
Can You Do A Sample Pressure-Loss Calculation?
Yes, 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
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?
I'd use no less than 2-inch.
I have a 400 ft run to a new house which is feeding off a 1 1/2 main. Should this house run be 3/4 or 1" PVC thanks
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!
I like to know what the answer is about running 400 ft from meter to house
There are lots of variables. It requires a phone consult.
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.
Bill, after reading my column and looking at the table about pressure loss, what do you think the answer is?
Off a 3/4 meter tap @60psi running 2000ft can we use 2" PVC to supply a 3 bath house
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
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.
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.
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.
The inside diameter of a pipe has nothing to do with pressure- it's volume of water.
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.
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
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?
Thanks!
H
Just look into the small flexible irrigation systems. Don't put in huge pipes to do what you need. Plants need just drip irrigation.
Look at this, or similar setups:
http://amzn.to/2mSMWPQ
Hey if I am running a water supply line about 250ft from the meter to a double wide mobile home what size PVC should I use?
2 inch
I have 1200 feet of 1-in PVC water line coming into my house but when running water from more than one fixture the drop in pressure is quite noticeable.
I have taken out all the galvanized and copper mess that was my plumbing and are replacing it with pex line. Are your calculations same for this expanding material as they are for copper? Also my water comes from the meter into my home in the basement as a 3/4” pvc and elbows up to 3/4” pvc of an aproximate length of two half or three feet where I have removed the mess. I have replaced that union with a new 3/4” pvc ball valve with a 3/4” shark bite pex fitting on the out flow end. Is it correct that I should use 3/4” runs with 1/2” rises to shut offs that then reduce to 3/8” hook ups to the toilet and sinks? I originally had crap pressure and I’d like to correct this due to my new instant gas hot water heater that I am also upgrading to. Thank you.
Good day,
I have a 1" line at the meter with 33 gpm out the meter. I have 1.25" flexible pipe running to the house approx 500' @ a 15' incline over the 500'. 60 psi.
I have a 1" pvc pipe coming out of the house. I am only getting 8 gpm. Thoughts?
i have a well supplying a 2 bath home. 1 inch line entering the house on one end. how bad will it affect pressure & volume if i divert that 1 inch line approx 100 feet to the other end of the house, & back to install a water filter in the garage?