The fluid ounces printed on a jar are its capacity to the brim. For accurate batches, measure the volume you'll actually fill.
- Put the empty, dry container on a kitchen scale and zero (tare) it.
- Fill it with water up to the line where you'll stop pouring wax, usually about ½ inch below the rim.
- Read the weight of the water in ounces or grams.
- Enter that number in the calculator as "oz of water (weighed)" or "grams of water (weighed)" and set the fill level to 100%, since you already measured to your fill line.
Dry the container completely before pouring wax; even a little water causes pitting and poor adhesion.
Why not just use the label?
Labels round numbers, measure to the brim, and describe volume rather than weight. Melted wax is lighter than water, so the label overstates how much wax fits. The calculator corrects for both, but weighing water is the most accurate method when you'll be making the same candle many times.