In order to measure the density and conversion of fermentable sugars while brewing, a refractometer can be used to measure the original gravity of the wort.
You can use a hydrometer also but this would require taking around 10 oz. of the boiling wort, letting it cool down so an accurate reading can be taken. If you take a reading as soon as you remove your boiling sample, your reading will not be correct. The sample requires some time to wait for it to cool so an accurate reading can be done. And by that time, the wort that is still boiling in the pot will be at a different gravity.
But by taking a small amount of the boiling wort (about a table-spoon which will cool quickly) you can get a fast reading and see if your wort is close to the original gravity. If it is, you can shut off the burner and start cooling the wort.
Below, is the conversion list from the BRIX to the Specific Gravity.