How to Calculate Pipe Size in Plumbing?

Hello everyone! If you want to know how to calculate the pipe size, don't worry, I'm here to help. To calculate the pipe size, you must use the formula of flow rate in GPM. It is the product of pipe area in square inches and velocity in ft/sec. I came to know about this as my father works in the plumbing company (J P Enterprises). I have shared the detailed pipe calculation process below.

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How Do I Calculate Pipe Size?

As I told you can calculate the pipe size using the below formula.

Flow Rate (GPM) = Pipe Area (square inches) x Velocity (ft/sec)

Here are the key factors for plumbing pipe size calculation.

1. Flow Rate: This tells you how much water is flowing through the pipe, measured in gallons per minute (GPM). 

2. Pipe Diameter: The size of the pipe affects how much water can flow through it. It’s measured in either inches or millimeters. 

3. Friction Loss: As water moves through the pipe, friction causes a pressure drop. Smaller pipes cause more friction loss. 

4. Pressure Drop: This is simply the decrease in water pressure as it travels through the pipe.

To calculate the pipe diameter, you'll need to know your desired flow rate and the velocity you've chosen. Once you have those, you can easily figure out the right pipe size. I hope now you know how to calculate size of pipe.

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Hey! You can learn how to calculate pipe size for water supply by using the following method. You will need to calculate the diameter of the pipe by determining the square root of 4 times the flow rate divided by pi multiplied by the velocity. For more information regarding this, you can continue to read my answer.

What are the Factors which Influence Pipe Size?

Before I discuss the calculation method, let me share the factors which influence the pipe size.

1. Pressure at the source point from where the water is supplied.

2. The needed pressure at the individual outlet fixture.

3. The loss of pressure of outlets which are kept above the water source.

Now, let me move ahead to the calculation part.

How to Calculate Water Supply Pipe Size?

Here is the explanation through which you can calculate the pipe size used for supplying water.

1. You can calculate the pipe size with the help of the flow rate of water. For this, we’ll find out the pipe diameter.

2. The formula for calculating the diameter of the pipe is as follows.

• D = Square root of 4 times the flow rate/pi times the velocity

• Here, you will have to be aware of the flow rate in inches per second.

• You can take pi as 3.14. 

This brings my answer to an end. I hope this helps you learn how to determine pipe size for water supply.

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Using the right plumbing pipe sizes in your house can be highly advantageous. It will be cost-effective while providing high pressure at the source of water supply. You even have the possibility of extra plumbing expansion in the future. I will share more details about plumbing copper pipe sizes and PVC pipes below. 

• You will need to draw suggested horizontal mains and branch lines. 

• You will need to collect the necessary information about the risers including the types and a number of fixtures. 

• You will need to calculate the demand weight. You can get the weight and standard plumbing pipe size from the table below. 

• You will need to figure out the water demand. The calculation should be made in gallons per millimetre and the total number of fixture units.

• Then, guess the length of the plumbing pipe for all stacks in the system.

• Calculate the maximum and minimum pressure needed for the highest fixture

• Determine the loss of pressure in the pipes

The mathematical calculation of the process to find plumbing vent pipe size is given below.

Using the continuity equation i.e.

Q = A x V

Where,

Q – Volumetric flow rate of water utility

A – Cross-sectional area of pipe (sq.m)

V – Velocity of water in the pipe (m/sec)

D – Inside diameter of the pipe (m)

Therefore,

Q = A x V

Q = (π x D x D x V) / 4

Therefore,

D = SQRT ( (Q x 4)/(π x V))

Here in this case,

Q = 1564 Lt/sec = 1.564 m3/sec

Recommended velocity = 2 m/s.

D = SQRT ( (1.564 x 4)/(3.14 x 2))

D = 0.997 mt = 997 mm

Hence selected pipe size is 1000 Nb (i.e 40 inch)

Knowing the outlet fixtures and the outside and inside diameter will help you choose the material for the pipe. The table below has a pvc plumbing pipe size chart. You can convert inches to mm to figure out plumbing pipe sizes in mm and inches.

I hope my explanation about plumbing pipe sizes has solved your query.

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