Green’s Dictionary of Slang

(US) to think, to opine.

c.1806		Z.M. Pike Sources of the Mississippi II 152: We had reason to calculate, that they had good guides [DA].
1815		D. Humphreys Yankey in England 104: Calculate, used frequently in an improper sense, as reckon, guess.
1834		Marryat Peter Simple (1911) 350: ‘Well, captain,’ said he, ‘so you met with a squall?’ ‘I calculate not.’.
1841		A.M. Maxwell Run Through the United States II 81: Now, I ‘calculate,’ you will be sufficiently sick of ‘what I said,’ and ‘what they said,’ and so on.
1852		Leics. Mercury 26 June 3/5: Ten persons left this town for Australia on Tuesday last. One young man who has been brought up ‘on the carpet’, has taken a pickaxe and a spade, which he ‘calculates’ will be service to him.
1865		M. MacFie Vancouver Island and British Columbia 415: The slang in vogue in the mining regions is imported mainly from California, and is often as expressive as it is original. ‘Guessing’ and ‘calculating’ are exercises of perpetual occurrence.
1873		Sl. Dict. 107: Calculate a word much in use among the inhabitants of the Western States U.S., as ‘I calculate you are a stranger here.’ New Englanders use the word ‘guess’ instead of calculate, while the Virginians prefer to say ‘reckon’.