Generally speaking there IS a conversion between the two. However, PHRF is a rating that is called Time-On-Distance...AND it is adjusted to what region of the country you are in...

To convert from Portsmouth to PHRF:

PHRF = (Portsmouth - 55) * 6

To convert from PHRF to Portsmouth:

Portsmouth = (PHRF / 6) + 55

Remember, these will give you a ballpark estimate, NOT an actual number as PHRF varies greatly...I sail on an Olson 34 which has a PHRF here of 108. But out in California, it's rating ranges from 105 to 112. Using the conversion above, I come up with a PHRF of 123 for a boat that should be a 108 PHRF...