I just found out today that some vendors carry stickers to calibrate the volume in your carboy.
I thought I would pass on how I solved this myself prior to finding this out.
I poured an accurate 1 gallon measurement of water into my carboy and marked the level on the glass with an extra fine sharpy marker. When I had completed the total volume as described above I broke out the Dremel tool and an abrasive bit and permanently marked the lines and corresponding gallons.
I was a little worried about the carboy being brittle so I practiced on some soda bottles first.
I thought I would pass on how I solved this myself prior to finding this out.
I poured an accurate 1 gallon measurement of water into my carboy and marked the level on the glass with an extra fine sharpy marker. When I had completed the total volume as described above I broke out the Dremel tool and an abrasive bit and permanently marked the lines and corresponding gallons.
I was a little worried about the carboy being brittle so I practiced on some soda bottles first.