Although different waxes have varying densities we have put together a quick formula based on diameter and height to calculate how much wax you will need.
Here are the weights per inch of a round candle 2 inch diameter = 38 grams or 1.35 ounces per inch, 3 inch diameter = 100 grams or 3.5 ounces per inch, 4 inch diameter = 165 grams or 5.8 ounces per inch
So say you have a 3 x 6.5 inch round pillar mold and you need to know how much wax you will need to make a candle.
You would use the 3 inch diameter rate per inch (3.5 ounces) times the height (6.5 inches).
3.5 x 6.5 = 22.75 ounces of wax to make the candle.