I don't have any substantial experience with Kratom. I've taken what I would call "gas station" quality Kratom once or twice, so I have no personal, worthwhile knowledge.
I've done some light reading on the subject that doesn't really give us great, specific information that one could take to the bank, but it does give us a good place to start.
Mitragynine and its analog 7-Hydroxymitragynine are the primary, active alkaloids found in Kratom leaf. Mitragynine itself is reported to be 66 times less potent than Morphine, while 7-Hydroxymitragynine is reported to be 5 times more potent than Morphine. Now, the level of 7-Hydroxymitragynine seems to be relatively constant from variety to variety at 2%-3%. The shitty thing is, the levels of Mitragynine itself tend to vary widely, from 12%-60% of all alkaloids. This is a wide range.
However, for sake of providing us with a starting point from which to hopefully branch out, let's start with a Kratom leaf that we will say is right in the middle in regards to Mitragynine content at 35%. We will use one gram of Kratom leaf for our example.
35% of one gram will equal 350mg Mitragynine
2% of one gram will be 20mg 7-Hydroxymitragynine will be 20mg 7-Hydroxymitragynine
We will start by standardizing this to Morphine equivalents. 20mg x 5 = 100mg while 350/66 = 5
So, we are now left with this number of 105 Morphine Milligram Equivalents or MMM's. To me, this number seems a little bit high. Now, the literature I was reading was conducting experiments using Subcutaneous Injections. The majority of Opioid analogs that I'm familiar with follow a pretty disproportionate bioavailability relative to route of administration, with oral administration often being only 1/3 the potency of an injection administration.
We're really making a lot of guesses at this point, but with this information in mind, I think you could say certain, average Kratom could be as potent as a 30mg oral dose of Morphine or 20mg Oxycodone. That seems a little bit high for me, but I still think these numbers give us something good to start with.