A multi charged particle beam writing apparatus includes a unit to calculate a predetermined function expression by a correction coefficient that corrects an open area of each opening for forming a beam group, wherein the predetermined function expression minimizes a sum of squared values of all the beam groups, where each of the squared values is calculated by squaring a difference between a current value measured of the beam group and a sum of products for the beam group, where each of the products is obtained by multiplying a corrected open area by the predetermined function expression; a unit to calculate the correction coefficient that corrects the open area by using the predetermined function expression and the current value of the beam, for each beam; and a unit to calculate a current density of each of the multiple beams by the predetermined function expression.