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.
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.
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.
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.
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
If you can imagine a cylindrical "donut," the outer diameter is the total width while the inner diameter is the width of the hole.
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.
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.
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".
If you need the inner diameter, measure the width of the lip.
REMEMBER: The lip runs around the outside, so you will have to double the width of it for your calculation.
Subtract twice the width of the lip from the outer diameter (OD), and you'll have the inner diameter (ID).
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.
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).
Take several measurements at different points along the pipe. Do this for increased accuracy.
Measure each string to the mark, and use the most common number. This measurement is the circumference of the pipe.
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.
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.