Bottles and jars are listed as to the "fluid ounces" they will hold. Unfortunately, when making soap and other products, one is taught to do everything in "weight ounces". For a heavy product like lotion or shower gel, 2oz in weight will occur before filling a 2oz bottle. What I've found useful is to weigh a measuring cup, tare to zero, pour my product into the measuring cup to desired fluid ounces, check weight, add correct percentage of FO. I don't go super strong or heavy on any of my FO, so it is a minimal increase in the volume. Most bottles will hold a smidge more than the advertised amount, also. I still get extra sometimes, but my daughters snatch it up quickly, or I use it.