Tinker 
How to use it
Type the number of servings the original recipe makes and how many you want. Then enter one ingredient amount in any unit. The new amount comes back in that same unit, so you can work through the ingredient list one line at a time using the same factor.
The formula
Scaling factor = desired servings ÷ original servings. New amount = original amount × scaling factor. Each ingredient gets multiplied by the same factor.
Worked example
A soup serves 4 and you need to feed 6. The factor is 6 ÷ 4 = 1.5. If the recipe calls for 2 cups of stock, you need 2 × 1.5 = 3 cups. For 300 g of lentils, you need 450 g.
Practical tips
Weighing ingredients in grams makes scaling far more accurate than using cups. Eggs can be beaten and measured by weight, at about 50 g per large egg. Seasoning, chilli and baking powder don't always scale in a straight line, so add about three quarters of the scaled amount and adjust to taste. Cooking time depends on pan depth more than on quantity, so these timings are estimates. Check food early.
Frequently asked questions
Does cooking time change when I double a recipe?
Often only slightly. A deeper pan or a bigger roast takes longer, but a wider pan at the same depth cooks in about the same time.
How do I halve an egg?
Beat the egg and use about half by weight, roughly 25 g of a large egg.
Can I scale baking recipes?
Yes, especially by weight. Pan size has to match the new volume, and leavening may need small adjustments for very large batches.
What if my recipe gives a yield instead of servings?
Use any count, such as 24 cookies to 36 cookies. The ratio works the same way.