I have some chicken breasts which are frozen. They say no added water.

If they weigh 100g, what do you think that would translate to as a defrosted weight?

The calories on the packet say, calories per 100g (oven cooked), so they are taking them as defrosted and then cooked so they will be much less dense as there will be less water in them.

Any ideas?

(am cooking them from frozen which is why I cant weigh them at their defrosted weight)

If you're worrying about the calorie content of plain chicken, you're worrying too much. It's what you serve 'with' the chicken that you have to watch.

If you need a rough idea, a chicken kiev from M&S ( served with a garlic sauce and breadcrumbs ) has about 350 cals but that includes the buttery garlic sauce, so the plain chicken on its own will be about 200 cals, not a lot.