Scaling works for most ingredients, but leavening, salt and spices are often scaled a bit less in very large batches, and baking times change with pan size.
How to scale
Multiply every ingredient by the same factor: servings wanted ÷ servings in the recipe. Scaling a 4-serving recipe to 6 uses a factor of 1.5. Weights scale cleanly; for cups and spoons, the result is rounded to the nearest kitchen measure.
Things that do not scale simply
Eggs: round to the nearest whole egg, or beat and weigh (a large egg is about 50 g without the shell). Pans: a doubled cake needs a pan with about twice the area; use the cake pan converter. Baking time changes with depth, not with the factor, so start checking early.
Frequently asked questions
Can I scale baking recipes?
Usually by weight; check pan size and time.
Can I halve a recipe with 3 eggs?
Use 1 egg plus half of a beaten second egg, about 75 g of egg in total.
Do I double the salt?
For normal batches yes; for very large batches many cooks scale salt and spices slightly less and adjust to taste.