5.2 Scaling and converting a recipe

Step 1: find the conversion factor

The conversion factor is the one number used to change every amount in the recipe. Divide the number of servings you need by the number of servings the recipe makes now.

• What you want ÷ what you have = conversion factor.
• Example: a recipe makes 10 servings and you need 6. 6 ÷ 10 = 0.6.

Step 2: multiply every ingredient

Each ingredient amount is multiplied by the conversion factor. If the 10-serving recipe needs 2 quarts of chicken stock, the 6-serving version needs 2 × 0.6 = 1.2 quarts.

Step 3: turn the answer into a usable measure

The result is often a decimal. Some decimals are easy, because they match a common fraction:

• 0.25 is a quarter.
• 0.33 is a third.
• 0.50 is a half.
• 0.66 is two thirds.
• 0.75 is three quarters.

Other decimals, like the 0.2 in 1.2 quarts, are harder. You can judge the amount by eye, which is fine for much everyday cooking. In baking, though, exact amounts matter more, and guessing can ruin the result.

The more careful route is to convert to a smaller unit. A conversion chart shows 32 ounces in a quart, so 1.2 quarts is 32 × 1.2 = 38.4 ounces, or about 38. That is still an awkward amount. There are 8 ounces in a cup, and 38 ÷ 8 = 4.75, so the stock comes to roughly 4¾ cups, which is easy to measure. Most of the world uses metric units, so this kind of conversion is mainly needed in the U.S.

Do not expect every ingredient to need this much work. Many results land close to an easy fraction, and a half cup or two-thirds cup measure will do.

When the portion size changes too

Sometimes the number of guests and the size of each serving both change. A standardized recipe is one that reliably gives a known amount of food at a known quality, so it is the right starting point. In that case, compare total yields instead of numbers of servings.

Take a caterer asked to serve 284 guests a 6-ounce serving of jambalaya each, working from a standardized recipe for 100 servings of 4 ounces:

• Work out the total amount needed: 284 × 6 = 1,704 ounces. Work out the total the recipe makes: 100 × 4 = 400 ounces.
• Divide the amount needed by the amount the recipe makes: 1,704 ÷ 400 = 4.26. That is the conversion factor.
• Multiply every ingredient by 4.26, then cook following the recipe's own method.

Done this way, the kitchen neither runs short nor makes far too much.