Bonnie, to add to Wa'il's excellent info:
- Make sure you get the white 'stevia extract' powder, not the powdered green (dried whole) leaf;
- Now brand and Stevina brand (white extract) are both available out here on this coast, usually in the HFS;
- my recipe for making the liquid 'working solution', as provided by the Now folks (I think) is 1 tsp white powder to 3 tbsp filtered water, which I also keep in a 2 fl oz dropper or squeeze bottle.

1 tsp of this mixture is roughly equivalent to 1 cup of sugar in sweetness (so 1/3 tsp is supposed to be about that sweet too) BUT... since the strength of the white powder can vary quite a bit, as can the aftertaste (faintly licorice). If you use too much stevia, the aftertaste gets really strong. Always best to err on the side of caution and start with small measures.

The premixed liquid stevia is 0-calorie as long as no glycerine is used as the base (water is usually the base). The packets are most often 'fillered' with maltodextrin, just like the AS, so the packets have carbs. If the 'filler' is FOS, then it's about 85% soluble fiber filler so very low carb.

Hope this helps?