What Size PEX For The Bathroom? [A Complete Guide]

We may get commissions for purchases made through links in this post.

Excited about using your new PEX pipes but you’re confused about what size to get? It’s a common question regarding bathroom installations.

What size PEX for the bathroom to use?

Usually, ½ and ¾ of inch PEX pipes are used in your bathrooms. Every size has its purpose. You have to figure out which size pipes to use. Factors such as fixture units, elevation, length of pipes, and pressure have to be taken into account.

Want to know more in detail? We’ve put together a guide on choosing PEX sizes for bathrooms.

So let’s get to it!

Why Does Size Matter?

When it comes to your plumbing, size matters. Having the wrong size would result in added/decreased water pressure for your supply.

PEX size also determines the flow rate of your water. A typical 1-inch line has a flow rate of 9 GPM (Gallons per minute). 

On the other hand, a 1 ¼  inch pipe provides 16 GPM. You can see how even a quarter of an inch affects your GPM.

This is why your appliance’s size depends on the size of your pipe. Choosing a smaller size for your pipes may lead to complications.

Since the GPM won’t match your designated faucet, you have to enlarge its hole.

Figuring Out Bathroom’s PEX Size [6 Step Guide]

As the above section explains, you can’t randomly place PEX of any size. There’s a blueprint for figuring out what size you have to use.

This calculation requires 6 steps of analysis, which are mentioned below.

Total Fixture Units

Fixture units are one of the most important values in plumbing. This is a representation of how much demand you have per your water system. 

To determine PEX size, we’ve to calculate the total number of fixtures.

Each fixture has an assigned value per the plumbing code manual. For example, a showerhead has a value of 2. While a bathtub/shower combo has a value of 4.

Sinks have a value depending on their type (kitchen, washup, etc.). Bidets have a value of 1. Each of the fixture values has a minimum fixture pipe size assigned to it.

Let’s assume our total fixture unit to be 16.5.

Length Of The Farthest Outlet

This is the distance of the pipes from your water meter which is the centerline. You have to calculate the distance of the pipes.

You could use measuring tape and markers to figure it out. This includes the horizontal and vertical pipelines needed. 

If you’re unsure about which measuring tape to use, pick from our recommendations below:

Measuring Tape-1 
Measuring Tape-2

Remember to calculate the length in feet. For simplicity, let’s assume the length is 115 feet.

Highest Elevation

This is the maximum elevation one of your fixtures has. Suppose you have an upstairs lavatory. You would need to take into account the vertical height of the bathroom. 

The height is measured from the centerline of your water meter. This is needed to figure out the pressure drop or rise.

The general rule of thumb is that 0.5 PSI of pressure rises per foot of elevation. And 0.5 PSI increases per foot drop from the centerline.

Pressure Calculation

Next, we need to calculate our minimum static pressure from the supply source. If your pressure varies, it’s ideal to pick the lowest setting.  Your well pump’s inefficiency to provide pressure may cause the variance.

Let’s assume your minimum pressure is 60 psi.

Now suppose, the maximum elevation is 12 feet. This would decrease the pressure by 6 PSI. So the residual pressure is now 54 PSI.

Picking Your Limits

As per our example, the maximum allowable length we had was 115. The next part of the calculation requires your PSI range.

The PSI range we have to choose from is between 46-60 PSI. This is because our residual pressure is 54 PSI.

Next, the total fixture unit we had was 16.5 units. Now you have to choose corresponding sizes according to your fixture units. Each of these fixture units has a corresponding PEX size allotted for it.

The maximum pipe size for the branches is 1 inch. It’s the corresponding size for the values of 16.5 units, 115 feet within the 46-60 PSI range. The lowest is ½ an inch.

For the meter and street service, you have a maximum size of ¾ of an inch.

Applying The 75% Method

This is for when you have both hot and cold water supplies. You take a branch’s fixture unit and multiply it by 75% to get the fixture unit for that pressure range.

If one of your branches connects to a combination of bathtub/showers, it has a value of 4. Now, multiply it by 0.75 which gives a value of 3.

The maximum allowable size for 4 is ½ an inch. If your value was greater than 4 and less than 14, it would have been ¾ inches. Having a value greater than 14 and less than 28 would result in 1 inch.

The values for your fixture unit, distance, elevation, and pressure may be different from ours. If so, use the plumbing code to figure out the fixture unit’s values.

In conclusion, this is how you figure out which sized PEX you have to use. If you use the wrong size PEX for your ductworks, there may be leaks. This is why it’s crucial for you to insulate your pipework too.

FAQs

Question: Are rats capable of chewing through PEX?

Answer: Yes they most definitely are. Rats and mice can chew through most things including plastic PEX pipes.

Question: Will my PEX pipes get flimsy over time?

Answer: Yes, PEX will get flimsy over time. Especially if it’s kept in sunlight, it might crack under pressure.

Question: Does PEX have better water pressure than CPVC?

Answer: Yes PEX has a higher water pressure than CPVC pipes. This is because the pipe fits around the outside of the fitting. The internal diameter is reduced which in turn increases the pressure.

Conclusion

Now you know what size PEX for the bathroom you should use. We hope this guide came in handy. You are now equipped with the knowledge to tackle issues regarding PEX size.

Till then, stay safe and happy with plumbing!

Richard Allen