The Collaborative International Dictionary of English v.0.48:

Reckon \Reck"on\, v. i.
   1. To make an enumeration or computation; to engage in
      numbering or computing. --Shak.
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   2. To come to an accounting; to make up accounts; to settle;
      to examine and strike the balance of debt and credit; to
      adjust relations of desert or penalty.
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            "Parfay," sayst thou, "sometime he reckon shall."
      --Chaucer.
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   To reckon for, to answer for; to pay the account for. "If
      they fail in their bounden duty, they shall reckon for it
      one day." --Bp. Sanderson.

   To reckon on To reckon upon, to count or depend on; to
      include as a factor within one's considerations.

   To reckon with,
      (a) to settle accounts or claims with; -- used literally
          or figuratively.
      (b) to include as a factor in one's plans or calculations;
          to anticipate.
      (c) to deal with; to handle; as, I have to reckon with
          raising three children as well as doing my job.
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                After a long time the lord of those servants
                cometh, and reckoneth with them.  --Matt. xxv.
                                                  19.
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   To reckon without one's host, to ignore in a calculation or
      arrangement the person whose assent is essential; hence,
      to reckon erroneously.
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