As everyone learns in physics class, Hooke's law states that a spring's force is proportional to the distance it is stretched/compressed. The constant of proportionality is usually referred to as $k$.

I am currently working on a project for which I need a spring and I found online that I should probably use a "K25" spring for my application. I assumed that this K is the same $k$ that is commonly used to represent the spring constant.

However, when I actually started shopping for said spring I have found that it seems to be different. Springs are listed with both a "rate constant" and a K-something value. See this catalog list* from McMaster (the K-value is at the end of the part number, i.e. 9637K25).

The two numbers don't seem to be related. For instance there are two K25 springs, one has a rate constant of 28.53 and the other has a rate constant of 83.

So what's the difference between these two numbers, what do they represent physically, and what is their relation to the spring constant $k$ from Hooke's Law?

*For some reason McMaster's website doesn't allow me to link to a subsection of springs, I am looking in the "Cut-to-Length Compression Springs" category.

  • $\begingroup$ The McMaster rate constant in these tables is: (free length)*(spring constant). See this thread and this thread. $\endgroup$
    – atom44
    Oct 9, 2016 at 9:06
  • $\begingroup$ do you have some reason to think the number after the k in the part number is anything other than the manufacturers identifier number. I'm inclined to think if the "k value" had a technical meaning it would appear in the spec table, not just in the part number. $\endgroup$
    – agentp
    Oct 24, 2017 at 15:44

2 Answers 2


With McMaster, the part number usually is more related to where it is located in the warehouse or page number (or supplier's catalog item) then anything else. They may list it as "Aisle 96, Column 37, Shelf K, Bin 25" as an example. Other manufacturers and distributors use other part numbers. It has very little, if anything to do with the part's performance typically.

That said, sometimes the part number does correspond to the performance, and it does help. Spears Plastics will list their schedule 40 pipe with a 4 in front, and an 8 in front of schedule 80 fittings. They put a C in every piece for CPVC fittings instead of PVC. And they have a -XXX code at the end, 010 for 1", 020 for 2", etc.


The k number is the rate constant given in pounds per inch, which up to a multiplying factor should be the k in Hooke's law.

  • 1
    $\begingroup$ This doesn't explain what the other K is, which seems to be the point of the question. $\endgroup$
    – JMac
    Oct 24, 2017 at 15:10
  • $\begingroup$ Welcome to Engineering Stack Exchange! While this is a useful answer on springs, it doesn't address the actual question. A better answer would address the specific needs of the question. Please review and see if you can answer this particular question. Thanks! $\endgroup$
    – Mark
    Oct 24, 2017 at 18:33

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