A candle will burn down perfectly if:

The wax melts completely up to the rim

The wick does not turn over too early

The candle does not produce soot.

It has always required individual experiments to fulfil those mentioned parameters perfectly.
The indicated sizes are only guide values. Only the candle producer is responsible for a good burning down of the candle. Anyway, if you only know how thick the wick is you won’t be able to characterize the burning behaviour. 

The flat wick is the most used wick. It consists of three braided threads with the indicated number of thin threads. It doesn’t matter which end of the wick looks up and which one down. It is suitable for all kinds of wax.

Paraffin and stearin mixture and gel candles 15 to 25 mm, flat wick 3x7
Paraffin and stearin mixture and gel candles 20 to 60 mm, flat wick 3x9.
                 Good standard wick with a great application spectrum.
Paraffin and stearin mixture and gel candles 40 to 60 mm, flat wick 3x12
                 Gel candles with thicker wicks tend to produce soot.
Candles made of a mixture of paraffin and stearin about 60 mm, flat wick 3x15
Candles made of a mixture of paraffin and stearin from 60 mm, flat wick 3x18
Candles made of a mixture of paraffin and stearin from 70 mm, flat wick 3x21

For thick candles or lightly polluted wax you should chose a flat wick 3x27.

The round wick is also braided but it has a round cross section. The direction of rotation is marked on the wick and must be respected. If the wick is inserted correctly the thin threads will spread outwards while the candle burns down. If it is inserted the other way around the thin threads will spread inwards and the candle does not burn well. A round wick is more stable than a flat wick. As soon as the label has been removed you cannot determine the direction of rotation.
 

Mixture of beeswax and stearin to 25 mm round wick 1
Mixture of beeswax and stearin to 30 mm round wick 2
Mixture of beeswax and stearin to 35 mm round wick 3
Mixture of beeswax and stearin to 40 mm round wick 4

For oil lamps all the round wicks (up to number 20) are suitable. Fibre glass wicks hardly wear off.

 

A torch wick is made of cotton – like the other wicks as well. It is braided quite grossly and produces a huge, wind-resistant flame. It should only be used outdoors.
 

For garden torches in a flaming platter which is 10cm in size you only need one torch wick. If the flaming platter is bigger you’ll need more torch wicks which you insert with 15cm distance between the wicks. You can also bundle a few wicks. This will produce a very huge flame but is not dangerous.

Fibre glass wicks hardly wear off when they burn down. Consequently it is necessary that enough liquid material (hot wax or lamp oil) flows constantly. These wicks are very suitable for oil lamps. If you made a candle with it, the wax level would sink while the wick would stay where it was. As a result the candle would produce a lot of soot. There are some methods where a fibre glass wick keeps burning due to constantly melting wax. Especially for those wicks it is very important to use clean wax/oil since pollutants can accumulate over a long period of time. A wick made of cotton burns up and with it the dust particles burn up as well.