According to Modernist Pantry and thanks to SAJ14SAJ:
You can successfully substitute sheet gelatin for powdered gelatin in
any recipe by using the following scaling.
1 (0.25 oz.) envelope granulated gelatin = 1 tablespoon powdered
gelatin = 3 sheets leaf gelatin.
and a bit extra info from Modernist Cuisine
Gelatins are measured by what is called their Bloom strength (usually labeled as bronze, silver, gold, or platinum). Knox brand has a bloom strength of 225. If you are making a recipe (not just one of ours, but any recipe), be it a Jell-O wreath or a pâté, you can use a different Bloom strength than what the recipe calls for, but you’ll have to do a little math. You can convert the recipe to use whatever gelatin you have on hand if you know the weight (MA) and Bloom strength (BA).
For gelatin A, you can find the equivalent weight of gelatin B (MB) with a Bloom strength of BB by using the formula MB = MA × BA ÷ BB. For example, if a recipe calls for 2.6 g of Knox gelatin, you could use 3.7 g of silver gelatin, which has a Bloom strength of 160 (2.6 × 225 ÷ 160 = 3.7). To make a vegetarian panna cotta, we substitute 0.8 g agar and 0.65 g xanthan for the 4.3 g gelatin the recipe normally calls for.