Washington Revised Code Title 82. Excise Taxes § 82.08.054. Computation of tax due

Sellers shall compute the tax due under this chapter and chapters 82.12 and 82.14 RCW by carrying the computation to the third decimal place and rounding to a whole cent using a method that rounds up to the next cent whenever the third decimal place is greater than four. Sellers may elect to compute the tax due on a transaction on an item or an invoice basis. This rounding rule shall be applied to the aggregated state and local taxes.