The length of the text is given by the number of characters in the text, but there are few exception, While for Premium voice the number of credits needed is higher then the length of the text, see theĬorresponding section in the FAQ to learn more about Premium voices.
In short, when using normal voices the number of credits needed is the same as the length of the text, Length of the text and the type of voice chosen. Of charge, while the price for downloading a text as a sound file has a price that is depending on the The acapela-box service is priced using a model based on prepaid credits.