Capsules are generally half a gram each, if you pack the 000 capsules yourself at home with a pill filler you can get almost a gram in there, I think most pre-filled capsules are 00.
I heard someone say 4-6 grams of kratom for 15mg of morphine, that seems like a lot of kratom to me, especially if we are talking MS contin, which will retain most time release effects even after broken.
It's hard to compare, they are different animals, the level of kratom that will SUBSTITUTE for a certain dose of morphine is quite a bit lower than the level that will provide the same subjective effects or buzz.
You don't ever want to take more than 12-15 grams of kratom at a time, or an ounce a day. If you are using it to kick a habit 36-45g a day broken up into 3 doses is the max, there is just no point in taking more than that. At these high levels it can easily substitute for a 120-150mg a day oxycodone habit, you won't be satisfied, you won't get high, but you will feel fine.
As far as using it for a buzz or painkilling effects, 5 grams seems to be the sweet spot, dose every 2-3 hours if you need to(if you are trying to switch from a mild opiate habit I mean it's better to do that than take 10-12 grams every 4 hours), if you've got a tolerance 10-12 grams, but after 8 grams you can start to feel some negative side effects.
If you don't have a tolerance as little as 2 grams can provide a nice buzz or some pain killing effects, I would say 2-3 grams is about equal to 10mg oxycodone in painkilling effects, the buzz from kratom will never be as euphoric as morphinan opioids but it'll do the job just fine,
If you are a chronic pain patient, and your meds haven't gotten too out of hand, I would DEFINITELY say give Kratom a try, because it is self-limiting, and you can re-dose as much as you need and take it for as long as you want, you WILL have WDs from it if you take high doses for a long time, but they are MUCH more milkd than opioids WDs and over fairly quickly...tapering is possible but not easy because of its short duration of action.
I assume because most of its alkaloids are partial agonists with g-protein pathway selectivity that tolerance rises comparatively slowly, and is also why WDs are easier, you can not lethally overdose on it alone, no matter how hard you try, and if you take too much you will not only get nauseous but can suffer other side effects like sweating and anxiety because it has adrenergic effects, it is known to provide a high degree of energy at certain doses probably due to these adrenergic effects, adrenergic overdose is not pleasant.
If you are looking for an opiate like effects you want red strains.
For a more milkd effect, or if you have had problems with anxiety, a white strain.
A green strain is known for giving energy, but can tend to have to much adrenergic action in those who abuse it.