.5。 In writing instruments, the diameter of the refill is usually measured in millimeters. .5 mm is less than .7 mm, so the .5 mm refill is thinner than the .7 mm refill. This difference makes the .5 mm refill suitable for occasions that need careful writing or drawing, such as calligraphy, painting, engineering drawing and so on. The .7 mm refill is often used for general writing, signature and so on.