This is a washable Dyson vacuum cleaner style pre-motor filter for the barrel Dyson DC05 and the barrel style Dyson DC08 and the Dyson DC19 and the Dyson DC20 and the Dyson DC21 and the Dyson DC29
it's 155mm in diameter and 29mm thick.
It also suits the upright style Dyson DC14 with a clutch, circa 2009
It's often used with Or the upgraded filter