How to calculate TTR (types/token ratio)?

Sketch Engine does not display the type/token ratio but provides all data necessary for the calculation. They are on the corpus info page  (https://www.sketchengine.eu/guide/corpus-info-page/):

The LEXICON SIZES table displays the number of unique items in the corpus. 
The COUNTS table displays the total numbers found in the corpus. 

The most traditional calculation should be done using words from LEXICON SIZES and tokens from COUNTS like this: 

word ÷ tokens × 100 = TTR

Please, be aware that different systems use different definitions of tokens or words (types) and, therefore, the TTR may differ. 

See also: