The practical rule is to use weight: use the same total number of grams of powdered gelatin as the sheet gelatin called for in the recipe. Converting by sheet count is unreliable because sheet gelatin is made in different bloom strengths, and sheets do not weigh the same across brands. Read the package’s gram information, weigh the sheets, and measure the same weight of powdered gelatin.
The two forms are the same substance; one is dried in thin sheets, and the other is made into powder. Setting power depends on weight, not form. So the conversion rule is simple: use the same number of grams of powdered gelatin as the number of grams specified for sheet gelatin. The same rule works in reverse. Confusion arises because most recipes give a sheet count instead of a weight in grams. The same confusion appears on powdered gelatin packages; some brands list servings instead of grams.
Why is counting sheets unreliable? Sheet gelatin is made in different bloom strengths, known by names such as bronze, silver, and gold. As the strength changes, both the sheet’s weight and its setting power change. One brand’s sheet may weigh around 2 grams, while another brand’s may be lighter. If a recipe lists only a sheet count, the bloom strength is unknown, and the result will vary each time. So if a recipe works well, note which brand you used and do not switch it next time.
The right approach is to read the package. It usually says how many sheets will set how many liters of liquid; this information is much more useful than a sheet count. If weighing sheets, use a precise kitchen scale, because the amounts involved are only a few grams, and a coarse scale will not read accurately in this range. Weigh powdered gelatin on the same scale; a spoonful of powder varies each time. Do not weigh without taring the scale; with small amounts, an error of one or two grams can change the result completely.
The water calculation works differently for the two forms, and this is where they get confused. Bloom sheet gelatin in plenty of cold water, then squeeze it and discard the excess water it absorbed; only the gelatin itself goes into the recipe. Bloom powdered gelatin in a measured amount of water and add it to the mixture with the water it absorbed. If this difference is overlooked, extra liquid goes into the recipe and the mixture will be softer than expected. Do not squeeze sheet gelatin too hard; a crushed sheet breaks apart and some of it stays in your palm.
There is a simple way to check the result before pouring the mixture into molds. Take a spoonful into a small bowl and chill it in the refrigerator for a few minutes; if that sample sets, the rest will set too. If it does not, you can add a little more gelatin while the mixture is still warm. Adding gelatin to a set mixture does not work; the mixture needs to be reheated. Put the test bowl on the coldest shelf in the refrigerator so you can check the result in a few minutes.