Recipe Scaler

Enter how many servings the recipe was written for, how many you want to make, and one ingredient amount. The scale factor and the resized quantity update as you type, and the unit you enter — grams, milliliters, cups, or anything else — carries through to the result. Read the factor off the top and apply it to the rest of the list, or work through the ingredients one line at a time.

The arithmetic of proportion

Two numbers do the work. The factor is the ratio of the servings you want to the servings the recipe assumes:

factor = target servings ÷ original servings

Every ingredient then multiplies by that same factor:

scaled amount = amount × factor

Take the defaults: a recipe written for 4 that you want to stretch to 6, with 250 g of flour. The factor is 6 ÷ 4 = 1.5, so the flour becomes 250 × 1.5 = 375 g. That same 1.5 turns 100 g of butter into 150 g, 2 eggs into 3, and 200 ml of milk into 300 ml.

One factor governs the whole list because the relationship is linear and passes through zero: with no servings you need no ingredients, and each extra portion adds the same fixed amount of every item. This is direct proportion, among the oldest tools in practical arithmetic. The scaled amount is to the target servings as the original amount is to the original servings, and solving for that unknown fourth value is the rule of three. Indian mathematicians knew it as trairashika, the method of three quantities; it is set out in Aryabhata's Aryabhatiya of 499 CE, travelled west through Arabic mathematics, and became the workhorse of European commercial arithmetic. Cocker's Arithmetick, published posthumously in 1678 and reprinted for British schoolrooms across more than a century, drills it with a cloth-pricing problem: if so many yards cost so much, what does a different length cost. Scaling a recipe is that same procedure applied to a shopping list.

From "butter the size of an egg" to the level cup

Direct proportion only helps if the starting amounts are actual numbers, and for most of cooking's history they were not. Before the late nineteenth century, English-language recipes rarely gave exact quantities. Cooks were told to add a cupful of one thing, butter the size of a walnut, a handful of another, and salt to taste or sufficient. A recipe written that way cannot really be scaled, because the base amounts are gestures rather than measures. Reliable scaling needs a fixed unit to multiply.

That unit arrived with Fannie Merritt Farmer. As principal of the Boston Cooking School she took on the revision of the school's textbook, Mrs. Lincoln's Boston Cook Book of 1884 by Mary Johnson Bailey Lincoln, and the rewrite grew into The Boston Cooking-School Cook Book, published in 1896 by Little, Brown and Company. The publisher had so little faith in it that Farmer covered the cost of the first printing of 3,000 copies herself and kept the copyright. The book insisted that a spoon or cup be filled and leveled off rather than heaped or scanted, and it stated ingredients in those standard, repeatable units. Farmer became known as the mother of level measurements, and the level teaspoon, tablespoon and cup that American kitchens still use owe their currency to her campaign for a more exact, scientific approach to cooking. More than 360,000 copies had sold across twenty-one printings by the time she died in 1915, and the title passed four million by 1990, which is how the standardized recipe became ordinary. Once an amount is a definite 250 g or one level cup, multiplying it by 1.5 has a clear meaning, which is what makes any scaler possible.

Weight, volume and the metric cup

Standard units are not identical everywhere. The cup behind American recipes is the US customary cup of exactly 236.5882 ml, which is eight US fluid ounces. US nutrition labels use a different figure, the legal cup of exactly 240 ml set by FDA regulation, so the same word carries two values depending on where it is printed. The metric cup of 250 ml belongs to Australia, New Zealand, Canada and other Commonwealth countries that metricated; Australia settled on that round figure during the changeover launched by its Metric Conversion Act of 1970, and fixed its tablespoon at 20 ml at the same time, four teaspoons rather than the three used almost everywhere else. Britain never standardized a culinary cup at all, so a British recipe written in cups is either borrowing the American one or being loose. The metric cup runs about 13 ml above the US customary cup, minor on one line and compounding across a batch, so a recipe's country of origin matters before you scale it.

Volume also scales awkwardly. Multiply three quarters of a cup by 1.5 and you get 1.125 cups, which no measuring cup marks. Recipes given by weight sidestep the problem: 180 g simply becomes 270 g, and a kitchen scale reads whatever figure the factor produces. If your recipe uses cups, converting to grams first with the cups to grams converter keeps the scaled numbers practical. Professional bakers go further and write formulas as baker's percentages, with flour fixed at 100 percent and every other ingredient stated as a share of the flour weight, so a batch of any size follows from one set of ratios.

Ingredients that ignore the factor

Straight multiplication works for flour, sugar, liquids and fats, but a few ingredients do not follow the math. Salt, spices and strong flavorings are perceived on a curve — double the chili in a stew and it reads as more than twice as hot — so start these near 75 to 80 percent of the scaled figure and adjust by taste. Leavening is chemistry rather than seasoning: baking powder, baking soda and yeast release gas in proportion to the batter, but too much collapses the crumb and leaves a metallic or over-yeasted aftertaste. Scaling a cake by 1.5 takes 6 g of baking powder to 9 g on paper, yet closer to 7 g gives a better rise.

Halving, thirds and the awkward egg

Scaling down meets the same units from the other side. Half an egg has no clean form, so beat one whole egg, weigh it — a large egg is roughly 50 g out of its shell — and use half by weight, keeping the rest. Factors such as one third leave odd figures on nearly every line, and very small batches suffer most, because a pinch of salt or a single gram of leavening is hard to divide accurately and easy to misjudge. The tool reports the exact scaled amount to two decimals; rounding to what your spoons and scale can actually read is the cook's job, and it matters least for bulk ingredients and most for the potent ones.

Pans, depth and timing

More batter needs a bigger tin, but pan capacity does not grow in neat 1.5 steps, and the depth of the batter matters as much as the area. Matching the scaled volume to a pan of the right size is its own small calculation — the baking pan converter handles it. Baking time is governed by how far heat has to travel to the center, not by total volume, so it scales far less than the amount of batter. Keep the depth close to the original and the timing barely moves; pour the same mixture into a deeper pan and it bakes slower, often needing the oven dropped 10 to 15 °C, while spreading it thinner shortens the time. Start checking for doneness a few minutes before the original recipe says.

Frequently asked questions

How do I scale a recipe from 4 to 6 servings?

Divide the servings you want by the servings the recipe was written for: 6 ÷ 4 = 1.5. Multiply every ingredient by that 1.5, so 250 g of flour becomes 375 g and 2 eggs become 3. This calculator does the division and one multiplication for you, and you reuse the same factor for the rest of the list.

Do I double the spices and salt when I double a recipe?

No. Salt, chili, garlic and strong spices are tasted on a curve, so twice the amount reads as more than twice as strong. Start these at roughly 75 to 80 percent of the scaled figure — for a doubled dish that means about 1.6 times the original spice — then adjust to taste before serving.

How much baking powder do I use when scaling a cake by 1.5?

Less than the strict multiple. A cake using 6 g of baking powder scaled by 1.5 works out to 9 g on paper, but leavening overshoots quickly, so hold it closer to 7 g. Too much baking powder collapses the crumb and leaves a soapy, metallic aftertaste, which is far harder to fix than an under-risen cake.

Does baking time change when I scale a recipe up?

Much less than the volume does. Doubling a batter does not double the bake time, because heat still travels the same distance if the depth stays similar. Keep the batter depth close to the original, start checking a few minutes early, and drop the oven 10 to 15 degrees Celsius if you switch to a deeper pan.

Can I scale a recipe that measures in cups?

Yes, though volume measures scale awkwardly: three quarters of a cup times 1.5 is 1.125 cups, which no cup marks. Converting to grams first keeps the scaled numbers usable, since 180 g simply becomes 270 g. Remember a US cup is 236.6 ml while the metric cup used in Australia and New Zealand is 250 ml, and Britain never standardized a culinary cup at all.