Thickness tolerance is the acceptance range that defines how far a film may deviate from its target thickness. Blown film is never exactly the same thickness at every point; small differences in the die gap, cooling and haul-off conditions create thinner and thicker areas in the film.
Tolerance is something the buyer and the producer should agree on in advance. Looking only at the average thickness can be misleading; the thin areas of a film whose average seems adequate may be where tearing starts. For this reason, quality control takes measurements at several points across the width of the film. The appropriate range is determined by the film type, the intended use and the production equipment.