Calculating pipe length with fittings, also known as fitting allowance, is an essential part of putting in plumbing of any kind. However, in most cases, for residential applications and DIY, you won’t ever have to do an exact calculation. In fact, it’s difficult to find actual measurements for pipe that you’d need to do these calculations outside of trade manuals and manufacturer’s specification sheets.
Therefore, this guide will cover both how to roughly estimate the right pipe lengths with fittings as well as how to calculate it properly.
How to Calculate Pipe Length with Fittings
Calculating the right length to cut a pipe is relatively easy. In most cases, all you’ll need is a tape measure and a rough understanding of what kind of fittings you’re using. However, these measurements vary significantly depending on the pipe. E.g., if you’re using PEX fittings or PVC.
This guide covers how to calculate pipe length with fittings, with both the professional method and the more accessible DIY option. However, we’ll start with the DIY method. This method allows you to approximately get the right measurements, without acquiring a specification or fitting size chart from the manufacturer. It’s more than good enough for any soft pipe such as PVC, copper, or PEX.
What You’ll Need:
- Measuring Tape
- Pen/pencil and paper
Most of this work is relatively easy. It’s also a good idea to cut as you go, so you can more easily calculate the distance of the next fitting with a physical marker in place. However, you should never glue pipes into place until you’ve cut everything. Otherwise, learning how to measure pipe length with fittings is relatively easy.
1. Draw out your pipes
The first step to figuring how much pipe you need is to draw out the rough layout of the pipe you want. That includes any corners, angles, and where you might need fitting connectors based on the length of the pipe. Here, a simple line sketch will do. You want to see the rough shape of what you’re cutting. You can then roughly measure out what those measurements should be.
You’ll then want to measure out and roughly mark the wall or space with a chalk or soft pencil.
2. Start from the wall
Start your measurements from the least mobile portion of your pipes. Attach the first fitting to get started and then measure from the center of that fitting to where the next should go. To make this easier, you can actually insert the pipe you’re cutting into the fitting. Have someone hold it so it doesn’t put too much pressure on the fitting. Then, measure to where you want the next fitting. Use a marker to mark the pipe directly. Mark the measurement down on your drawing. Then, cut the length of pipe and insert it into the fitting. Don’t glue anything yet.
3. Attach your next fitting
Attach the fitting at the first corner or section. Measure from the center of that fitting to the space where you need the next one. Then, cut that pipe. You’ll want to mark your measurements on your drawing as you go so you can double check before you actually cut the pipe.
Keep going until you reach the wall. You’ll also want to stop and double check that the angle, length, and level correspond with what you need for the pipes. E.g., if you’re running vent pipe, is it at least 45-degrees from horizontal?
4. Do a final check
Double check that the fittings all line up and that there’s no pressure on the pipe at any point. Make sure everything fits together well and that your pipes fit fully into the fittings without creating stress points. Then, use sandpaper and a utility knife to clean up the pipe cuts to remove any sharp edges. From there, you can glue everything together.
Is There a Standard Allowance for Pipe Fittings?
There is no standard allowance for pipe fittings. However, you can use a rough rule of thumb to approximate these measurements. Here, the allowance for a fitting is usually about the size of the diameter of the pipe. In addition, the extra reach, or the part of the fitting the pipe doesn’t fit into is usually about 2/3rds of the diameter of the pipe.
So, if you’re running a 2” shower drain, your pipe fittings likely need a 2” allowance plus an additional 1.75” for the extra reach. If you’re using a ½” pipe, you’d want 7/16th‘s per reach or about ½” per extra reach for a ¾” pipe.
Unfortunately, this is not a precise measurement because there are no exact nationwide standards for the reach, allowance, or even external diameter for most PVC pipes. Instead, these measurements fall roughly into the same guidelines. National standards mandate the inside diameter of the pipe and the fittings. Otherwise, it’s up to the manufacturer. This means that if you buy fittings from two different manufacturers, there can be small differences in the external diameter, the fitting allowance, and in the pipe reach.
That’s why we recommend using the fittings and measuring from those fittings, with the pipe pressed into the fitting for the most accurate measurement. Otherwise, you’re always using some guesswork, unless you use the professional fitting allowance calculation.
In addition, threaded pipe fitting allowances can be a little different. That’s because not all threaded pipes fit fully into each other, especially if you’re using Teflon tape. Those micro differences shouldn’t make too much difference, but they can.
In addition, if you’re using PVC or ABS pipe, the external diameters are significantly different than if you’re using copper pipes. For example, gas lines use thin, copper lines but the fittings can be quite bulky. So, they might have more reach than a PVC pipe. It’s always important ot measure the actual fitting, minus the allowance.
Using Fitting Allowance to Calculate Pipe Length
If you’re using fitting allowance, you can get exact measurements and cut your pipe and fittings to perfect size. However, you’ll need access to the fitting spec sheet from the specific manufacturer.
Every pipe fitting size chart includes two measurements for the fitting. These include:
- Face-to-Center (Throw) – The distance between the start of the pipe and the center of the pipe (available in any pipe fitting size chart). If you don’t have the fitting chart, you can roughly measure these with a tape measure. Here, you’ll want to insert the measuring tape into the pipe and stop it at the center.
- Thread Engagement / Engagement (only for threaded pipes) – The depth the pipe is expected to insert into the fitting. You need the manufacturer’s specification chart for this because it varies slightly per manufacturer. Again, if you don’t have a specifications chart, you can generally take this measurement with a tape measure.
To get the length of a pipe with fitting allowance, you also need:
- End-to-end – the length of pipe between two fittings
So, a pipe length with fitting allowance is:
End-to-End measurement + (Face-to-Center measurement minus Engagement measurement)
If you know the measurements of your space, the Throw, the engagement, and the end-to-end measurements, you can easily fully calculate the length of your pipes and pre-cut them without ever putting the pipe into place.
Frequently Asked Questions
If you’re putting in plumbing or gas lines, you likely have questions. This FAQ should help.
What is an equivalent length of pipe?
An equivalent length of pipe is a length in the same size as a fitting, giving rise to the same pressure drop.
How do you calculate pipe length?
Determine where the fittings should be and then measure from the center of each fitting. It’s also an option to insert your pipe to the first fitting, then mark where you want the new fitting to start on the pipe. You can then add the allowance, usually roughly the diameter of the pipe and cut the pipe there.
Does pipe length include thread?
If you’re buying threaded pipe, the stated length of the pipe includes the full length. This includes the pipe nipple with threads.
What length does PVC pipe come in?
PVC pipes come in lengths from about 1 foot to well over 20 feet sections. If you’re buying it from a hardware store, you’ll normally purchase it in 6, 8, or 10-foot sections.
How do you calculate pipe bend length?
In most cases, you want to take your U-bend or corner bend and measure it end to end. The formula for a correct calculation is based on the basic formula for a circle, 2πR. Here, π is a constant 3.14 and R is the radius of the circle. So, if your pipe bend is ¼ of the bend around the circle, you’d want ¼ x 2πR and so on. Of course, you’ll still have to add in allowance for the pipe that fits into the bends as well.
Calculating pipe length with fittings can range from a quick and rough measurement to a very precise calculation. The steps you take should often depend on the pipe, pipe material, and the pressure requirements for the system. Here, the most important step is to make sure you double check measurements, ensure that you’re leaving enough allowance, and that you double check everything to ensure there’s no pressure on the pipes before you glue everything together.