Sizu You have a recipe for 4 servings and ten people are coming. Doubling is easy. Multiplying by 2.5 with measuring cups is where people guess, then end up with a chili that is too thin or too salty. Here is the whole method, with a real recipe.
Step 1: find the scale factor
Divide what you want by what the recipe makes:
10 servings ÷ 4 servings = 2.5
That one number is the multiplier for every ingredient. If a recipe makes 6 and you need 9, it is 9 ÷ 6 = 1.5. If it makes 8 and you need 2, it is 2 ÷ 8 = 0.25.
Step 2: multiply each amount
Our red lentil chili serves 4:
| Ingredient | For 4 | × 2.5 for 10 |
|---|---|---|
| Red lentils | 1 1/2 cups | 3 3/4 cups |
| Olive oil | 2 tbsp | 5 tbsp |
| Onions | 1 | 2 1/2 (use 2 large, or 3 small) |
| Canned tomatoes | 14 oz | 35 oz |
| Water | 2 1/2 cups | 6 1/4 cups |
| Salt | 3/4 tsp | 1 7/8 tsp |
Two of these deserve a little arithmetic, because kitchen fractions trip people up.
Lentils: 1 1/2 is 3/2. Multiply 3/2 × 5/2 = 15/4 = 3 3/4 cups.
Salt: 3/4 × 5/2 = 15/8 = 1 7/8 tsp. Your spoon set probably has no 1/8 teaspoon, so that is “a little under 2 tsp.”
A shortcut that works with floury hands: ×2.5 means “double it, then add half of the original.” For the oil, 2 tbsp doubled is 4, plus half of 2 is 1, so 4 + 1 = 5 tbsp.
Step 3: know which amounts do not scale straight
Most ingredients go up in step with the number of people. A few do not, and a good cook adjusts them:
- Salt and strong spices. The straight number, 1 7/8 tsp, is a ceiling, not a target. Start with about 1 1/2 tsp, taste near the end, and add the rest if the pot needs it. You can add salt; you cannot take it out.
- Water or stock. A wider pot loses more liquid to steam, and a thicker or thinner chili is a matter of taste. Start with about 5 cups, then add up to the full 6 1/4 as the lentils soak it up.
- Cooking time. More food in one pot takes longer to come up to heat. The simmer time itself changes less than people expect, so go by how the lentils taste rather than the clock.
- The pot. 2.5 times the food needs about 2.5 times the room. If 4 servings filled a 3-litre pot about two-thirds full, 10 servings is about 5 litres of food and needs a pot of 7 to 8 litres. Check before you start chopping.
Odd amounts you cannot split
Scaling often lands on amounts you cannot measure or buy exactly, like 2 1/2 onions or 2.5 cans of tomatoes. Round to something you can actually do:
- Onions: use 2 large or 3 small. Vegetables are forgiving.
- Tomatoes: 35 oz is two 14 oz cans (28 oz) plus 7 oz from a third can. Open the third can, use half of it, and the leftover goes into tomorrow’s lunch.
- Spoons: 5 tbsp is 1/4 cup plus 1 tbsp, because 1/4 cup is 4 tbsp. Fewer scoops means fewer chances to lose count.
Step 4: write the new numbers down before you cook
Once you have the scaled list, write it on paper or a phone note. Do not do the multiplying in your head while measuring; the standard mistake is to double half the ingredients and forget the rest.
A useful check: your scaled amounts should look like the original times 2.5. Lentils went from 1 1/2 cups to 3 3/4, which is a bit more than double and a bit less than quadruple. If a scaled number looks wildly off from that, redo it.
Going down instead
The same method works in reverse. For 3 servings from the 4-serving chili the factor is 3 ÷ 4 = 0.75, so the lentils become 1 1/2 × 3/4 = 1 1/8 cups, and the oil 2 tbsp × 3/4 = 1 1/2 tbsp. Smaller batches in a big pan thicken faster, so watch the liquid.
Try it with your own numbers
The free Recipe Scaler scales by servings, gives amounts back as kitchen fractions, and holds salt and leaveners back for you. It comes from The Kitchen Math Workout, whose chapters cover kitchen fractions, conversions and recipe scaling. The book page says every recipe in it is plant-based.
Rule of thumb: factor = servings you want ÷ servings the recipe makes. Multiply everything by it, then go easy on salt and water and taste as you go.
This is arithmetic for planning a meal, not dietary or food-safety advice.