Method for estimating a number of accessing entities which are accessing a website, comprising the steps: - providing a first array of bits and a second array of bits, each array of bits having a number of S bits, wherein the first array of bits is associated with a first time interval and the second array of bits is associated with a second time interval such that for the first time interval the first array of bits is used and for the second time interval the second array of bits is used; - repeatedly determining (501, 506) that the website is accessed by an accessing entity during the first and second time intervals, wherein the method comprises in reaction to each access of the website by an accessing entity: - determining identification data comprising a characterizing number x dependent on the accessing entity, each accessing entity being characterized by a characterizing number x taken from the interval 0 to M-1, - calculating a hash bucket H(x) for the accessing entity by mapping (503, 508) the characterizing number x dependent on the accessing entity onto a value between 0 and S-1 according to a non-linear mapping function H(x), and - activating the bit corresponding to the determined hash bucket H(x) for the accessing entity in the first array of bits when the access of the website is determined during the first time interval and activating the bit corresponding to the determined hash bucket H(x) for the accessing entity in the second array of bits when the access of the website is determined during the second time interval, wherein the first array of bits allows to estimate the number of accessing entities of the website that accessed the website during the first time interval and the second array of bits allows to estimate the number of accessing entities of the website that accessed the website during the second time interval; - combining the first and second arrays of bits using a logical OR-operation between the bits of the arrays of bits having the same significance, the result being written to the first array of bits; - deleting the second array of bits; and - estimating the number of accessing entities of the website which accessed the website during the first and second time intervals based on the activated bits of the first array of bits. - providing a first array of bits and a second array of bits, each array of bits having a number of S bits, wherein the first array of bits is associated with a first time interval and the second array of bits is associated with a second time interval such that for the first time interval the first array of bits is used and for the second time interval the second array of bits is used; - repeatedly determining (501, 506) that the website is accessed by an accessing entity during the first and second time intervals, wherein the method comprises in reaction to each access of the website by an accessing entity: - determining identification data comprising a characterizing number x dependent on the accessing entity, each accessing entity being characterized by a characterizing number x taken from the interval 0 to M-1, - calculating a hash bucket H(x) for the accessing entity by mapping (503, 508) the characterizing number x dependent on the accessing entity onto a value between 0 and S-1 according to a non-linear mapping function H(x), and - activating the bit corresponding to the determined hash bucket H(x) for the accessing entity in the first array of bits when the access of the website is determined during the first time interval and activating the bit corresponding to the determined hash bucket H(x) for the accessing entity in the second array of bits when the access of the website is determined during the second time interval, - determining identification data comprising a characterizing number x dependent on the accessing entity, each accessing entity being characterized by a characterizing number x taken from the interval 0 to M-1, - calculating a hash bucket H(x) for the accessing entity by mapping (503, 508) the characterizing number x dependent on the accessing entity onto a value between 0 and S-1 according to a non-linear mapping function H(x), and - activating the bit corresponding to the determined hash bucket H(x) for the accessing entity in the first array of bits when the access of the website is determined during the first time interval and activating the bit corresponding to the determined hash bucket H(x) for the accessing entity in the second array of bits when the access of the website is determined during the second time interval, - combining the first and second arrays of bits using a logical OR-operation between the bits of the arrays of bits having the same significance, the result being written to the first array of bits; - deleting the second array of bits; and - estimating the number of accessing entities of the website which accessed the website during the first and second time intervals based on the activated bits of the first array of bits.