How to convert Pixel to millimetor in MATLAB GUI

Hi all.

i am stucking with a problem please help me. i have a JPG image i contoured the image by imfreehand and then find the [X,Y] coordinates of each pixel inside the contour. these [X,Y] cooridnates are in pixel so i want in millimetor. So, how can i get these values in mm. i want when i press excelPushbutton. the coordinates should b be in mm not in pixel. any help is apprteciated in advance.

the code i used is given below

if true
function ExcelPushbutton_Callback(hObject, eventdata, handles)
[Y, X] = find(handles.maskedImage ~= 0);
data=[Y,X];
Z = repmat(0.18,[length(X),1]);
[FileName,PathName] = uiputfile('*.xls','Save data to spreadsheet');
Data=[X,Y,Z];
xlswrite([PathName '/' FileName],Data);

end

here Z is 0.18mm which is thickness of the image.