Find the factor from the portions
Divide the desired number of portions by the original number. Moving from four portions to two gives a factor of one-half. Moving from four to six gives a factor of one-and-a-half. Keep the original yield visible so every adjustment starts from the same base.
Carry the factor through the ingredient list
Multiply each relevant ingredient quantity by the factor and keep its unit attached. Write the adjusted list before preparing the dish. If a quantity is awkward to measure, use a suitable reliable recipe or method rather than making an unexplained substitution.
Common misunderstanding: every number in the recipe scales
Portion arithmetic does not tell you to halve the oven temperature or double the cooking time. Equipment size, thickness, heating, and the recipe’s method affect preparation. Review those decisions using the recipe’s guidance rather than treating them as ingredient proportions.
Worked example
Four portions become two
For an arithmetic example, a four-portion recipe lists 200 g of one ingredient and 100 mL of another. The factor for two portions is 2 ÷ 4 = 1/2. The adjusted quantities are 100 g and 50 mL. This changes the ingredient amounts; it does not establish a new cooking time.