How Much of Naruto Is Filler?
Filler is the tax a weekly anime pays for catching up to a monthly-paced manga. Naruto's manga published about four chapters a month while the anime consumed them faster, so Studio Pierrot regularly paused the adaptation and aired anime-original stories, written for television, until the manga had rebuilt its lead. Fans call those stretches filler, and Naruto became the textbook example.
By the count kept in fan-maintained episode guides, which classify every episode as manga canon, mixed or anime-original, about 90 of the original series' 220 episodes are anime-original, roughly 41 percent. Shippuden lands at almost the same share: about 203 of its 500 episodes, also roughly 41 percent. The classification is a community consensus rather than an official label, but it is the count the fandom's watch guides are built on.
The extreme case sits at the end of the original series. After the manga's Part I climax around episode 135, the show aired anime-original material almost without interruption until episode 219, holding the time slot for nearly two years while the manga's Part II accumulated chapters. When Shippuden finally premiered in February 2007, it was adapting a story its television audience had been waiting two years to see.
Why the share is so high, and why it is not all equal
Not all filler is equal, and the fandom's later judgment has softened on some of it. Shippuden's anime-original arcs include expansions the manga skipped, such as backstory for the tailed beasts' hosts, and the closing arcs adapted the Hiden novels outright. Watch guides now sort filler into skip, mixed and worth watching, which is why the Library treats the filler map as a viewing document rather than a verdict.
Filler and the physical releases
The filler years shaped the physical releases. English DVD singles and box sets from the long 2006 to 2007 stretch are almost entirely anime-original episodes, so a complete DVD shelf holds far more television-only Naruto than a manga shelf holds story. For collectors, episode guides double as contents maps: they are how you know which arcs a given box actually contains.