Measuring Pipe, Ring, Line or Fittings

QUESTION: What do pipes, rings, lines and fittings have in common?

Learn how to measure diameters and circumference.

ANSWER: They are all round and they get measured differently than a straight or flat item!

Depending on your purpose of measuring the pipe, will depend on what part needs to be measured. Use the following guidelines for measuring a pipe, ring, line or fitting.

Helpful tools:

RIGID TAPE MEASURE

Make sure that you have a rigid tape measure handy, even if you don't think you'll need it.

You'll probably want to get a rigid English tape measure for basic diameter measurements. If you need to, take a refresher on how to use one by  clicking here.

CLOTH TAPE MEASURE

A cloth tape measure is sometimes useful for finding the distance around round surfaces.

While we don't recommend using a cloth tape measure for anything straight, it is usually okay to use one to measure round surfaces by wrapping it around a pipe or fitting.

MAGIC MARKER AND STRING

A marker and some light-colored string work almost as well as a cloth tape measure.

If you don't have access to a cloth tape measure, that's okay too -- you can just use a marker and a piece of string. We recommend light-colored twine.

You can use masking tape instead of string, but it can bind up easily and tends to tear when you remove it.

PENCIL AND PAD OF PAPER

A pencil and a pad of paper are essential to any measuring activity!

A pencil is great for making non-permanent marks on the surface you're measuring. Also keep a pad of paper on hand if you want to take down measurements or do calculations


Core circular measurement concepts:

DIAMETER, RADIUS, & CIRCUMFERENCE

Measuring pipe, ring, line or fitting
  • Circumference – measurement of the distance around the circle
  • Diameter – measurement across the center of the circle. Measuring from one side to the other.
  • Radius – measurement halfway around the circle (1/2 of the diameter).

UNDERSTANDING O.D. & I.D.

With a cylindrical donut, outer and inner diameters are easy to understand.

If you can imagine a cylindrical "donut," the outer diameter is the total width while the inner diameter is the width of the hole.

OUTER DIAMETER (O.D.)

Outer Diameter Measure

Measure from the most outer part of the circle, through the center of the circle, to the most outer part of the other side of the circle.

INNER DIAMETER (I.D.)

Inner Diameter

Measure from the most inner part of the circle, through the center, to the most inner of part of the other side of the circle.


Measuring the diameter:

BASIC DIAMETER MEASUREMENT

You can use a rigid tape measure to easily find the outer diameter of any round surface.

A rigid tape measure is the easy way to find the outer diameter (OD) of any round pipe. In this example, the diameter of this is 7".

MEASURING THE LIP FOR INNER DIAMETER

Measure the width of the lip.

If you need the inner diameter, measure the width of the lip.

DOUBLE THE LIP WIDTH

Remember, the lip is on either side of the pipe.

REMEMBER: The lip runs around the outside, so you will have to double the width of it for your calculation.

CALCULATING INNER DIAMETER

Once you have the OD and the lips, you can calculate the ID.

Subtract twice the width of the lip from the outer diameter (OD), and you'll have the inner diameter (ID).


Finding the diameter of crushed pipe or when you can't remove the pipe:

WHEN YOU CAN'T MEASURE THE DIAMETER...

If you have a pipe that you can't remove or a dented pipe there are other ways to get the diameter.

If there's a pipe or other round structure that you don't want to remove, or if your pipe is crushed, there is another way to get the diameter.

1. WRAP A LINE AROUND THE PIPE

Use a cloth tape or string to wrap around the pipe, and mark where the end intersects.

In our example we are using the string and marker, but this is easier if you use a cloth tape measure. Wrap a line around pipe and mark where it intersects the end (or mark both ends). Be sure to keep the line as tight to the pipe as possible (keep pressure on the dents).

2. MARK MULTIPLES FOR ACCURACY

Mark multiple cables to get more accurate measurements.

Take several measurements at different points along the pipe. Do this for increased accuracy.

3. MEASURE EACH STRING

Measure each string to the marks. Jot down the measurements.

Measure each string to the mark, and use the most common number. This measurement is the circumference of the pipe.

4. CONVERT ENGLISH TO DECIMAL

Convert fractional inches to decimals.

In order to do the next step, you will have to convert any fractional inches to decimal numbers. You can use the Decimal Conversion Sheet on this page to do this.

5. TIME FOR SOME MATH (USE A CALCUALTOR)

Divide the number by "pi." This will give you the diameter.

After you have the decimal number, divide it by "pi" (for simplicity's sake, just use 3.14). This will give you a decimal number. Round it to the nearest increment. Most stove and fireplace pipes use whole number diameters, so it's safe to assume that our example's is 4".

 

 

 

 

 

Use this table to convert inch fractions into decimal numbers.

Decimal conversion

 Inch Sizes   Decimal 
1/16 0.0625
1/8 0.125
3/16 0.1875
1/4 0.25
5/16 0.3125
3/8 0.375
7/16 0.4375
1/2 0.5
9/16 0.5625
5/8 0.625
11/16 0.6875
3/4 0.75
13/16 0.8125
7/8 0.875
15/16 0.9375
 

Examples of products that can benefit from diameter measurement:

Fire bowls, stove and chimney pipe, the exhausts of furnaces, stoves, and prefabricated fireplaces, and starter pipe.

 

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