I see recommendations to add anywhere from "1/2 teaspoon" to "1 table spoon" worth of gelatin as a fining agent. I'm surprised to see this, given that brewers on the internet tend to demand that hops, salts, and etc. be weighed on a gram scale.
So, how much gelatin in grams is required to clarify a beer? Does it depend on whether or not the beer is to be bottled instead of kegged?
It's not that critical as long as you stay between 1/2-to-1 tsp, whatever that weighs. Use a teaspoon measurer that's designed for measuring cooking ingredients (not an actual "teaspoon"). Such a teaspoon of salt will weigh 5g, but gelatin is lighter and so you will probably be in the 3-4g range.
As for bottling vs. kegging, it doesn't matter, since in either case the gelatin will take the haze particles with it down to the bottom of the carboy where they will stay.
Here is what I do:
1) Stir 1 tsp of Knox brand unflavored gelatin in 2/3 cup of distilled water. Most grocery stores will carry Knox gelatin. The gelatin sold at homebrew shops is no better and probably identical but a lot more expensive. Knox works perfectly (but it must be unflavored).
2) Microwave the gelatin mixture in 15-second increments to get up to 160F, stirring between increments. Be careful not to exceed 170F or you will have Jello instead. At 160F the gelatin will be sufficiently sanitized for addition to your beer.
3) Pour the gelatin mixture into your carboy.
4) Keep the carboy in the fridge until the beer is clear (approx. 2-3 days). The gelatin will not clear well if the carboy has not been kept cool.
The first respondent's estimate of the weight of a teaspoonful of Knox gelatin was correct. On a gram scale it was 3.15 gm. I found that 30 seconds to start and then 15 second bursts of microwaves on "high" resulted in a sufficiently fine incremental temperature rise to hit the desired range. Neither of the answers so far described whether swirling of the carboy was desired, but I figure it would be helpful in getting even distribution, so I did. I then put the carboy in my mini-frig.
FWIW, the Knox gelatin box I used says the contents are 28 gm and they are divided into 4 packages, so each packet is 7 grams and there was probably a teaspoonful left after measurement, so if you didn't have a gram scale or a teaspoon, using half of one of the packets would probably be accurate enough.
Take a look at this, the guy (Marshall Schott for Brulosophy) has documented pretty well his experiments related to gelatin usage for clarification and decrease of chill haze.
Step 2: Make gelatin solution. I followed my friend Brad’s method of combining 1/2 tsp Knox Unflavored Gelatin with 1/4 cup cool water then microwaving it in short (7 second) bursts until it reached 145-150°F, stirring with the end of a thermometer between each burst.
