From BiblePortal Wikipedia

Vine's Expository Dictionary of OT Words [1]

A. Verb.

Yâchaś— ( יָחַשׂ , Strong'S #3187), “to reckon (according to race or family).” In Aramaic, yâchaś appears in the Targumim for the Hebrew mishpachah —(“family”) and toledot (“genealogy or generations”). This word occurs about 20 times in the Old Testament.

In 1 Chron. 5:17 yâchaś means “reckoned by genealogies”: “All these were reckoned by genealogies in the days of Jothan King of Judah …” (cf. 1 Chron. 7:5). A similar use is found in Ezra 2:62: “These sought their register among those that were reckoned by genealogy, but they were not found …” (NASB, “searched among their ancestral registration”).

The Septuagint renders yâchaś variously: ogdoekonta (“genealogy … to be reckoned”); arithmos (“member of them; father their genealogy”); paratoxin (“member throughout the genealogy”); sunodias (“reckoned by genealogy”).

B. Noun.

Yachaś ( יַחַשׂ , Strong'S #3188), “genealogy.” This word appears in the infinitive form as a noun to indicate a register or table of genealogy: “And the number throughout the genealogy of them that were apt to the war, and to battle was twenty and six thousand men” (1 Chron. 7:40; cf. 2 Chron. 31:18). Another rendering concerning the acts of Rehoboam, recorded in the histories of Shemaiah (2 Chron. 12:15), meant that the particulars were related in a genealogical table.

Webster's Dictionary [2]

(1): ( v. t.) To conclude, as by an enumeration and balancing of chances; hence, to think; to suppose; - followed by an objective clause; as, I reckon he won't try that again.

(2): ( v. i.) 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.

(3): ( v. i.) To make an enumeration or computation; to engage in numbering or computing.

(4): ( v. t.) To charge, attribute, or adjudge to one, as having a certain quality or value.

(5): ( v. t.) To count as in a number, rank, or series; to estimate by rank or quality; to place by estimation; to account; to esteem; to repute.

(6): ( v. t.) To count; to enumerate; to number; also, to compute; to calculate.

King James Dictionary [3]

Reckon rek'n. L. rego, rectus, whence regnum, regno, Eng. to reign and right.

1. To count to number that is, to tell the particulars.

The priest shall reckon to him the money, according to the years that remain, even to the year of jubilee, and it shall be abated.  Leviticus 27 .

I reckoned above two hundred and fifty on the outside of the church.

2. To esteem to account to repute.  Romans 8 .

For him I reckon not in high estate.

3. To repute to set in the number or rank of.

He was reckoned among the transgressors.  Luke 22 .

4. To assign in an account.  Romans 4. 5. to compute to calculate.