To measure basket filters, flatten filter and measure overall diameter of the circle (say 250mm) and the diameter of the inner base circle (say 84mm). Therefore the filter size is 250-84.
To measure a coil filter roll measure the width and diameter of an unused roll in millimetres. Popular sizes are stocked by Abbeychart including 89mm, 100mm and 110mm wide rolls.
To measure household cone filters, flatten the filter and measure the narrow base of the filter (say 50mm) and the length of the side of the
filter (say 148mm).