Quote:
Originally Posted by ayupbrewing
Exactly! Thats what I thought....not so easy retrospectively Thanks

Extract will have a specific and known gravity. If a known amount of that extract is dissolved in a known amount of water, the OG can be calculated very precisely, without any need to measure it.
When brewing allgrain, or simply just not knowing how much water and extract were used (which would be incredibly sloppy), there is another method. It requires taking a FLAT sample of the finished beer, and measuring the OG using a hydrometer AND the (uncorrected) OG using a refractometer, and then doing a bit of math.
People who use a refractometer frequently find the unknown variable of the *corrected* FG, by calculating it using two known variables  the OG and uncorrected FG. That equation can be rearranged to figure out a different unknown variable, the OG, if the two other variables are known... in this case, both the corrected and uncorrected FG (the hydrometer and the refractometer measurements, respectively).
Similarly, one could also rearrange the equation to determine the uncorrected FG if both the OG and corrected FG are known, but as you can probably tell, there's no real point to doing so.