The steam required for steam tracing is calculated on the basis of the amount of heat transfer that is required or the temperature that you want to maintain with the help of steam. It also depends on the pipe material through which the steam is passing,the condition of the steam that whether it is saturated or superheated etc. Overall heat transfer cofficient is calculated considering all the factors like fouling or scaling,inside and outside dia. , surface area etc. After calculating the amount of heat to be transferred, the requirement of rate of steam is calculated. Generally, Low pressure steam is used as it has got more latent/Sensible heat as compared to MP or Superheated steam.