Ingredient scale calculator
Ingredients scale; cooking does not. Doubling a cake batter does not double the baking time — it adds perhaps a third, and the tin size matters more than the clock. Doubling a stew barely changes the simmer. Salt and strong spices often want slightly less than the factor suggests, and yeast in a large batch works faster because the dough holds its own heat. Scale the quantities here, then use judgement on the method.
Scaling factor is target servings divided by original servings. Going from 4 to 6 is a factor of 1.5, so 250 g becomes 375 g. Apply the same factor to every ingredient, then adjust the method by judgement.
How to scale a recipe
Pan size is the constraint people forget. Doubling a cake and baking it in the same tin gives a batter twice as deep, which burns at the edge before the middle sets. Two tins is almost always the answer for cakes; for a traybake, a tin with double the area. For anything cooked in a pan on the hob, doubling the quantity in the same pan means the food steams rather than browns, because the surface area available for evaporation has not changed.
Questions
Multiply every ingredient by two. Cooking time increases far less, and pan size usually has to change.