I usually weigh the mice to get a general idea of their weight.... especially if they look big or small... but I am pretty good at 'eyeballing' their weight now everyone follows Adam's advice.....I am going with his advice on the appropriate number of mice to feed my female, since I dont feed rats This number works great....I have been alternating between 3 and 4 every week....I want to put on that 100 g per month...but I dont want to cause a fast