Egg weights
Serious recipes measure eggs by weight: convert “200 g of egg whites” into a number of eggs (and back) for every EU size class.
Egg size classes
In Europe eggs are sold by weight class (EC Regulation 589/2008), measured IN the shell: S under 53 g, M 53–63, L 63–73, XL 73 and up. A typical size-M egg weighs 58 g, about 10% of which is shell.
What whites and yolks weigh
On average the white is about 60% of the weight and the yolk 30%: for a size-M egg that's ~35 g of white and ~17 g of yolk. These are averages, for precision bakes (macarons, meringues) always weigh.
Why recipes weigh eggs
“3 eggs” can mean anywhere from 150 to 225 g in shell (about 135-200 g shelled) depending on size: in bakes where the liquid/solid balance matters, the difference shows. That's why professional baking lists grams.
Why recipes should weigh eggs
A small and an extra-large egg differ by twenty grams shelled, nearly a third. In a three-egg recipe the difference reaches sixty grams of liquid, which shows on a delicate mixture: a softer pastry, a sponge that rises differently, a custard that will not set.
It is why professional recipes give grams and not counts: «150 g of egg» is repeatable, «3 eggs» depends on what is in the fridge. For forgiving preparations (omelettes, meatballs) the count is enough; for pastry and leavened doughs the scales are better.
White and yolk do not scale together
Proportions change with size: in large eggs it is mainly the white that grows, while the yolk stays more stable. Two small eggs therefore hold more yolk (and more fat) than one large egg of the same total weight.
It matters in recipes where the two do different jobs: the white gives structure and lightness, the yolk richness and colour. A custard made with small eggs comes out richer, a meringue with large eggs whips up more. Where the recipe is delicate, weigh them separately.
Freshness, storage and the water test
Over time an egg loses water through the shell and its air pocket grows: that is why an old egg floats and a fresh one sinks. An egg standing upright on the bottom is still fine but should be cooked, one that floats should be thrown away.
On the shell there is a date and the producer's code, and the first digit gives the farming method (0 organic, 1 free range, 2 barn, 3 caged). Eggs keep better outside the fridge if they have never been in one: cold creates condensation when they come out, and condensation lets bacteria in.
Nearby tools
To adapt a recipe's quantities there is Scale a recipe, and for imperial units Kitchen measurements. If you are changing tin, look at Cake portions.