Source : Webster's Revised Unabridged Dictionary (1913)

Reckoner \Reck"on*er\, n.
   One who reckons or computes; also, a book of calculation,
   tables, etc., to assist in reckoning.

         Reckoners without their host must reckon twice.
                                                  --Camden.

Source : WordNet®

reckoner
     n 1: an expert at calculation (or at operating calculating
          machines) [syn: {calculator}, {figurer}, {estimator}, {computer}]
     2: a handbook of tables used to facilitate computation [syn: {ready
        reckoner}]