When purchasing irrigation compression fittings, there are a couple of key steps to ensure that you select the correct size. First, you will need to measure the outer diameter (OD) of your pipe or tubing. You can use a caliper or ruler for this. Measurements for most household irrigation compression fittings range from 1/8-inch to 3/4-inch in diameter. You’ll also want to choose the compression fitting size that matches the outer diameter of your pipe. For example, if you have tubing with an OD of ¼ inch, you’ll need a 1/4-inch compression fitting.