predicting beers mouthfeel/body

Is there a formula (or rule of thumb) for predicting mouth feel/body or is it a trial and error thing? I've brewed a couple of batches already and would like to try to make my first extract recipe. I want a stout, so I'd like a way to create a heavy bodied beer. I've heard that you should add malto dextrin for a heavier body, but never really how much should be added. Any suggested reading? And yes I've already read "How to brew" by John Palmer.

from what i've learned so far, it seems that the more extract per gallon will raise the body of the product.

1 pound of extract to 1 gallon of water will yeild a more normal body

whereas 1.5 to 2 pounds of extract to 1 gallon of water will yeild a heavier body.

That will certainly give you a higher gravity reading, but a hight gravity wont' give your beer a thicker body (at least I don't think it will)

Well it to a certain extent as you start maxing out the yeast's attenuation range. Thicker body is acheived by using less fermentable ingredients. That can be certain extracts like Laglander or by steaping grains like Carapils and Caramel malts. Don't use any dextrose in the brew either.

i just bottled a stout that has great mouthfeel. i don't think there are formula, exactly, but certain additions add to body (malto dextrin) and if you can use some flaked barley + roasted barley it will really add to body. i don't know if steeping these ingredients is enough or if you need to use a partial mash approach (i did a partial mash).

I have not brewed a stout yet, but I believe that you can add lactose to improve the body and mouthfeel.

The thing about adding extra extract to increase mouthfeel is that the type of extract you use is important in determining the way a beer will feel. Some portion of the extract from each bag or can is unfermentable, there is more unfermentable material in the darker extracts and certain liquid extracts like John Bull are known for not being highly fermentable. Throwing extra light malt extract into the beer isn't going to do as much for your mothfeel, but it will increase your abv as most of the weight of the extra light malt extract is fermentable. The only way you are going to figure out what extracts give you the type of mouthfeel you are looking for is to experiment with the different brands.

Seeing as how you are brewing a stout, if you are not happy with the mouthfeel of your current recipe and it calls for using Light malt extract, you could always swap the light extract out for an amber extract or a dark extract. Adding malto dextrine will work, it is an unfermentable, but remember a little 4 - 8 oz will go a long way. Lactose is an unfermentable sugar it will add a little to the mouthfeel, not as much as the malto dextrine, but the main reason people use this is to add some extra sweetness to a beer.

As was mentioned earlier the best thing to do is to steep some specialty grains that don't have active enzymes i.e. carmel malt, chocolate malt, black patent malt, and then brew, but again you need to realize a small amount of these grains will go a long way.