In the counting grid, all butterfly species occurring in the Netherlands are counted every week on fixed routes throughout the country. From 1 May to 1 October, species and numbers are recorded on a weekly basis. Other routes focus on only one species (mainly rare species) and the rare species is only counted during the period these butterflies are most active. Annually, more than 400 routes are counted.
All nature statistics, also the counting grid Butterflies, are funded by the Ministry of Economic Affairs.
The trend index of all butterflies is defined as the average index of 51 butterfly species. The trend index of grassland butterflies is based upon 13 species.