Here's some information and corresponding math to help you out:
According to the Opioid conversion chart, orally ingesting 7.5 mg Hydromorphone is approximately equal to an oral dose of 20 mg of Oxycodone. Therefore, we can reason that Hydromorphone is 2.67 times stronger than Oxycodone when ingested orally.
However, different sources will provide different conversion rates. For instance, the Wikipedia article on Dilaudid (
http://en.wikipedia.org/wiki/Dilaudid) compares 30 mg of oral Morphine (~20 mg of Oxycodone) to 4-6 mg of oral Hydromorphone. By the way, I understand that some may not consider Wikipedia an accredited or reliable source, but for harm reduction purposes, I'm citing this article to support cautionary advice.
Using these conversions, we can calculate an equipotent dose of Hydromorphone. I'm only going to compare oral doses, as conversions for insufflated doses a) are not as readily available and b) tend to fluctuate due to variables such as individual bone structure, nasal contents, etc.
If we use the extremes of both conversion charts listed above...
30 mgs of Oxycodone is roughly equal 6 - 11.25 mgs of Hydromorphone.
60-80 mgs of Oxycodone is equal to approximately 12-30 mgs of Hydromorphone.
As you can see, the equipotent dose based on information you find online is subject to some debate. They are different opioids with different pharmacodynamic effects.
There are additional factors to consider: Hydromorphone has significantly greater bioavailability when insufflated compared to oral ingestion. Conversely, the bioavailability of oral Oxycodone is nearly twice that of an insufflated dose.
The bottom line is: Be fucking careful. Start your doses small and work your way up. Treat a new drug as a new drug, regardless of whether or not it belongs to a familiar class. Hydromorphone is fucking potent.