Third-person singular simple present indicative form of reckon out

(transitive) To figure out; determine by calculation, computation, or logic; make out / (transitive) To count out

Alternative spelling of ready reckoner

(historical) A very early mechanical calculator (completed in 1694) invented by Gottfried Wilhelm Leibniz.

plural of stepped reckoner

Borrowed money should be paid back as soon as possible.

(now idiomatic, rare) To calculate one's bill without checking with the host or landlord; hence, to come to false conclusions, to miscalculate.