Going from Thai to Mandarin Chinese (普通话) has a composite linguistic distance of 69% (heuristic estimate). Below is the dimension-by-dimension breakdown.
Shared vocabulary, cognates, and loanwords.
Morphology, syntax, word order, cases.
Shared sounds and phoneme inventory.
Thai → Chinese
Tai-Kadai ↔ Sino-Tibetan
The 5 languages with the lowest linguistic distance from Thai:
A quick 3-question check between Thai and Mandarin Chinese. No signup.