Aceria guerreronis Keifer is an important pest of coconut worldwide. Due to the lack of standardized methods to quantify damage of this eryophyid, a diagrammatic scale with indices of 1, 2, 4, 8, 16, 32, 48 and 70% of damage caused by A. guerreronis was elaborated and tested to accuracy, precision and reproduction of the estimations. To validate the scale, fruits with different levels of damage were previously measured with the program Assess(R) and submitted to 10 inexperienced evaluators with or without the scale (1 feminine evaluation) and seven days after (2 feminine evaluation) with the same evaluators, using digitized pictures of the same fruits in a different sequence. The accuracy and precision of each evaluator was determined through linear regression between observed and estimated damage. Without using the scale, evaluators were less precise as seven out of 10 overestimated the damage, while evaluators provided with the scale were much more accurate. Also, evaluations with the aid of the scale were much more reproducible than without the scale. The scale was used to determine the relationship between infestation and damage levels caused by A. guerrerronis. The relationship between infestation and damage fitted by the equation lny = 4.948 - 0.121x + 1.789 lnx (R(2) = 99.87%, P < 0.0001). Therefore, these findings show that it is possible to estimate A. guerreronis population density on infested coconut fruits by using the diagrammatic scale.