There is no set rule for when a double wick should be used. However, a general guideline is that candles with a diameter of 4" or larger may need a double wick (or triple if preferred). The reason for using a double wick in larger diameter candles is to achieve a full burn pool, which can be challenging with only a single wick. There are large wicks designed for wider diameter candles. However, larger wicks are more prone to mushrooming, soot, and smoke. For this reason, a double wick may be a better alternative.
WHEN SHOULD I DOUBLE WICK? When Containers are Larger than 9cm - When you have a larger container with a diameter of 9cm or more, it is often more beneficial to use two smaller wicks to achieve an even and measured burn.
When the Container is Irregularly Shaped - Double wicking is also useful when the container you use has an irregular shape, such as an oval or square where the diameter of one wick will not reach all sides of the container.
HOW DO I CHOOSE MY WICK SIZE WHEN DOUBLE WICKING? To double wick, the rule of thumb is to divide the diameter of the candle in half and choose a wick size that would sufficiently melt "each half". For example, if your candle diameter is 15cm, you would use a wick that is suitable for a 7.5cm burn diameter (i.e. the UC 3.000).