I've got some good news and some bad news.

The good news is the allowed blade length is 3 1/2 inches, which is fairly consistant with many sgians.

The bad news is that under Seattle Law, no fixed blade knife of any length is lawful.

"SMC 12A.14.080 Unlawful use of weapons.

It is unlawful for a person knowingly to:...B. Carry concealed or unconcealed on his or her person any dangerous knife..."

SMC 12A.14.010 defines a "Dangerous knife" as "any fixed-blade knife and any other knife having a blade more than three and one-half inches (3 1/2") in length."


That is the letter of Seattle Law.

However,
In practicality, it will depend a lot on the situation and the officer in question. As long as there are no complaints and no other extenuating circumstances, if an officer even bothers to talk to you he/she will likely just warn you to put it away.

**DISCLAIMER - I'm not advising you, or anyone else to break the law, nor should my comments be seen as "permission" to violate any Seattle ordinance.**