Eggs Calculator
Two and a half eggs a head for scrambled, which is more than most people guess.
The formula
quantity = per-serving amount * servings
Most recipes scale linearly, and a few do not
Flour, liquid and fat scale straight up with the serving count. What does not is anything governed by surface area or by time: seasoning needs less than proportionally in a large batch, leavening rather more, and cooking times barely move at all because a doubled pan is wider rather than deeper.
Eggs are the awkward one
They come in indivisible units of about 50 g, so a recipe scaled to two and a half eggs has to round, and rounding an egg is a 20 % change in the binding. Beating an egg and weighing out part of it is the honest fix; rounding up and taking a little liquid out is the practical one.
Scaling down is harder than scaling up
Halving a cake recipe leaves quantities that are hard to weigh and a batter too shallow for the tin, and the crust-to-crumb ratio changes. Small batches of anything baked are simply less forgiving, which is why professional recipes are written large and domestic ones rarely go below four servings.
Recipes are starting points
Ovens run hot or cold by 20 degrees, flour absorbs water differently by season and brand, and the same joint of meat cooks at a different rate depending on its shape rather than its weight. Everything here is a planning figure that gets you close enough to start, and the thermometer, the scales and your own notes from last time are what finish the job.