I was a Weight Watchers leader in Australia and on average for my female members a dress size was 6kg (13lb) and for male members a belt knotch was 4kg (9lb).
However depending on your body fat distribution and where you lose from first this can vary. My daughter recently lost 5kg and dropped 2 sizes in her work pants. When I lost weight a dress size in the beginning was about 7.5kg but as I got down it was more like 5kg.
I think it would depend on how much you weigh for a start, for example, if you were very large, then 3 kg's probably wouldn't change your dress size, but if you were tiny to start off with, then 3 kg's could take you down more. Also, it could depend on your proportions as someone else said. I have lost 2 stone and have only gone down 1 size - the weight went from my b00bs and tummy, but not much from my hips and thighs... :(
At a diet class the instructor always said you would drop a dress size with every half stone you lost - I only dropped one dress size after losing 1.5 stone - hmphh.
<em>edited by jillp on 06/02/2011</em>
Do you mean, if you are a size 12 then how much should you - or the dress - weigh?
If its how much the person should weigh, then you can't calculate the dress size on that because its the proportions that count.
Otherwise a dress may weight one or two kgs?