One factor for the whole recipe
Every quantity in a recipe is tied to the number of portions it makes. Change the portions and each quantity changes in the same proportion, so the whole job comes down to one number, the scale factor: the servings you want divided by the servings the recipe makes. A factor of 2 doubles everything, 0.5 halves it, 1.5 makes half as much again.
The tool works line by line. Enter Recipe serves and You want to serve, then paste the list into Ingredients, one per line. It shows the factor at the top (for example × 1.5) and rewrites each line:
- The amount at the start of the line is read whether it is written as a whole number (3), a fraction (3/4), a mixed number (1 1/2), a decimal (1.5 or 1,5) or a fraction character (½, 1½).
- Weights and metric volumes (g, kg, mg, ml, l, cl, dl, oz, lb) are shown as a decimal with three significant figures: 125 ml × 0.75 is 93.8 ml.
- Everything else, such as cups, tablespoons, teaspoons or a count of eggs, is shown to the nearest eighth, the smallest step most measuring sets handle, with the exact decimal in brackets whenever the result is not a whole number.
- A line that does not start with a number, such as “Zest of 1 lemon” or “Salt to taste”, is kept exactly as written. Adjust those yourself.
Formula
The factor is exact, but kitchen measures are not: cup and spoon amounts are rounded to the nearest 1/8 for display, and weights to three significant figures, which is finer than most kitchen scales read.
Worked examples
- Factor: 6 ÷ 4 = 1.5.
- 1 1/2 cups × 1.5 = 2.25, shown as 2 1/4 cups.
- 200 g × 1.5 = 300 g, and 250 ml × 1.5 = 375 ml.
- 3 eggs × 1.5 = 4.5 eggs; 3/4 tsp × 1.5 = 1.125, shown as 1 1/8 tsp.
- “Zest of 1 lemon” has no leading amount and stays unchanged; use the zest of one and a half lemons if you want to follow the factor.
- Factor: 3 ÷ 4 = 0.75.
- 1/2 cup × 0.75 = 0.375 cup, shown as 3/8 cup. Few measuring sets include a 3/8 cup, so convert it: with US measures a cup is 16 tablespoons, and 0.375 × 16 = 6 tbsp.
- 125 ml × 0.75 = 93.75 ml, shown as 93.8 ml; 94 ml or 90 ml is close enough for oil in most dishes.
- 2 eggs × 0.75 = 1.5, shown as 1 1/2 eggs.
- Factor: 2 ÷ 8 = 0.25.
- 2/3 cup × 0.25 = 0.1667 cup. The nearest eighth is 1/8 cup, which is 25% short, so the tool also prints the exact decimal. In spoons, 0.1667 cup is 2 2/3 tbsp, or 2 tbsp plus 2 tsp.
- 1 1/4 cups × 0.25 = 0.3125 cup, shown as 3/8 cup (0.3125). Measure 5 tbsp instead.
- 3 eggs × 0.25 = 3/4 of an egg (see below).
US cups and spoons for awkward results
When a scaled amount lands between the marks on your cups, switch to a smaller measure. In US customary measures, a cup is 8 fluid ounces and a tablespoon is half a fluid ounce, as NIST’s kitchen equivalency table shows, which gives these handy steps:
| Cup amount | Tablespoons | Teaspoons |
|---|---|---|
| 1 cup | 16 tbsp | 48 tsp |
| 1/2 cup | 8 tbsp | 24 tsp |
| 3/8 cup | 6 tbsp | 18 tsp |
| 1/4 cup | 4 tbsp | 12 tsp |
| 1/8 cup | 2 tbsp | 6 tsp |
| 1 tbsp | 1 tbsp | 3 tsp |
Cups and spoons are not the same size everywhere; a metric cup of 250 mL is used in Australia, New Zealand and Canada, for example. Scale in whatever units the recipe uses, and convert only at the end with the cooking measurement converter.
Eggs and other whole items
A fraction of an egg is easy once it is beaten. Crack the egg into a cup, whisk white and yolk together, and use the share you need, judged by volume or, more precisely, by weight on a kitchen scale. Half an egg is half of that mixture. If a fraction is close to a whole number (4 1/2 eggs in a cake for six), many cooks round to the nearest egg and adjust the liquid slightly.
What does not scale in a straight line
- Cooking time and temperature. These depend on the size and depth of what is in the pan more than on the total amount. A bigger batch in a bigger pan of the same depth may need about the same time; a deeper one needs longer. Check doneness rather than multiplying the time.
- Pan size. Pan area grows with the square of its width: a 24 cm round pan holds 1.44 times the area of a 20 cm pan, not 1.2 times. Choose a pan whose area matches the factor so the batter depth stays the same.
- Salt, spices and strong flavorings. People often find a straight multiplication too strong when scaling up a lot. Add the scaled amount in stages and taste as you go.
- Leavening. Baking powder, baking soda and yeast do not always behave proportionally in much larger or smaller batches. For big changes to a baking recipe, look for a version written for that size.
- Evaporation. How much liquid boils off depends mainly on the pot’s surface and the simmering time rather than on the quantity, so a resized soup or sauce can come out thicker or thinner than the original. Adjust the liquid at the end.
Weights scale more accurately than cups. If the recipe gives grams or ounces, keep using them; three significant figures is already finer than most kitchen scales display.
Sources
- Metric Kitchen: Cooking Measurement Equivalencies, National Institute of Standards and Technology.