Wash. Rev. Code § 43.62.030
The office of financial management shall annually as of April 1st, determine the populations of all cities and towns of the state; and on or before July 1st of each year, shall file with the secretary of state a certificate showing its determination as to the populations of cities and towns of the state. A copy of such certificate shall be forwarded by the agency to each state official or department responsible for making allocations or payments, and on and after January 1st next following the date when such certificate or certificates are filed, the population determination shown in such certificate or certificates shall be used as the basis for the allocation and payment of state funds, to cities and towns until the next January 1st following the filing of successive certificates by the agency: PROVIDED, That whenever territory is annexed to a city or town, the population of the annexed territory shall be added to the population of the annexing city or town upon the effective date of the annexation as specified in the relevant ordinance, and upon approval of the agency as provided in RCW 35.13.260, as now or hereafter amended, a revised certificate reflecting the determination of the population as increased from such annexation shall be forwarded by the agency to each state official or department responsible for making allocations or payments, and upon and after the date of the commencement of the next quarterly period, the population determination indicated in such revised certificate shall be used as the basis for allocation and payment of state funds to such city or town until the next annual population determination becomes effective: PROVIDED FURTHER, That whenever any city or town becomes incorporated subsequent to the determination of such population, the populations of such cities and towns as shown in the records of incorporation filed with the secretary of state shall be used in determining the amount of allocation and payments, and the agency shall so notify the proper state officials or departments, and such cities and towns shall be entitled to participate in allocations thereafter made: PROVIDED FURTHER, That in case any incorporated city or town disincorporates subsequent to the filing of such certificate or certificates, the agency shall promptly notify the proper state officials or departments thereof, and such cities and towns shall cease to participate in allocations thereafter made, and all credit accrued to such incorporated city or town shall be distributed to the credit of the remaining cities and towns. The secretary of state shall promptly notify the agency of the incorporation of each new city and town and of the disincorporation of any cities or towns.
For the purposes of this section, each quarterly period shall commence on the first day of the months of January, April, July, and October. Whenever a revised certificate due to an annexation is forwarded by the agency thirty days or less prior to the commencement of the next quarterly period, the population of the annexed territory shall not be considered until the commencement of the following quarterly period.
Armed forces shipboard population, on-base naval group quarter population, and military dependents living in housing under United States navy jurisdiction, shall be determined quarterly by the office of financial management on the first days of January, April, July, and October. These counts shall be used to increase or decrease the armed forces component of the resident population determinations in the cities of Bremerton and Everett for the purpose of allocating state revenues according to this section. Counts on the first day of the quarterly periods commencing with January, April, July, and October shall be used to adjust the total population for the following quarter, in the same manner adjustments are made for population changes due to annexation as specified in RCW 35.13.260 and 35A.14.700.
Population determinations made under this section shall include only those persons who meet resident population criteria as defined by the federal bureau of the census.
RCW 43.62.030
Determination of population of area annexed to city: RCW 35.13.260.