Any time you want or need to replace your well cap, the prospect of measuring your well casing to ensure you’re buying the right size cap can feel overwhelming if you’ve never done it before. It’s not quite as straightforward as some people might think.
In this blog, we’re going to cover the process for measuring your well cap and choosing the proper one based on what you find.
About Well Caps
As opposed to well seals, well caps are measured based on the well casing Outer Diameter (OD) because the cap sits on the outside of the well casing. We’ll keep this in mind when measuring.
Fun fact: Well casings are sized nominally by their Inner Diameter 12” or smaller. When measuring well casings 14” or larger, the name is controlled by the Outside Diameter of the casing.
How do I measure the well casing to choose my well cap?
If you happen to have a ruler (as opposed to a tape measure), this can sometimes make the process of measuring your well casing easier because the metal hook on a tape measure can sometimes prove problematic. However, if all you have available is a tape measure, you will still be able to measure your well effectively.
Here’s the process you need to follow to measure your well cap.
Step 1:
Hook the end of the tape measure on one side of the well casing. If using a ruler, make sure the zero point is on the outside edge of the casing.
Step 2:
Bring the tape measure (or ruler) across the well casing to the approximate centre point. In order to locate the exact centre on the other side of the casing, pivot your tape measure or ruler until you find the largest measurement of the casing.
Step 3: 
Measuring to the nearest 1/16th of an inch, jot down the number you’ve got on your tape measure. This number will correspond to the proper well cap size in the chart below.
An Item of Note:
As stated earlier, the well casing size is controlled by the Inside Diameter (ID) up to 12”, and any measurement 14” or larger, it is controlled by the Outside Diameter (OD).
Well Cap VS Well Seal
As briefly touched on earlier, a well cap is not the same thing as a well seal.
A well cap, as described in this blog, is intended to cover the outside of the casing, and has different 
features and benefits than a well seal. A well seal, as opposed to a well cap, sits inside the well casing, again, with different features and benefits.
It is important to understand what you have currently and what you are intending to replace it with. If in doubt, stick with the one that is currently installed on your well. Alternatively, talk to your local experts for advice on best practice given your circumstances.
In Conclusion
Measuring a well casing can seem confusing at first, but by following this very simple step-by-step guide and referring to the included chart, it should be very easy for even the least technically inclined person to select the proper well cap size for their well.
Have further questions about this subject?
Head over to Boshart's Knowledge Base: technical product information, guidelines, and more.
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