During incubation, yolk fat and to a lesser extent carbohydrates are used for energy supply. The burning of these energy sources requires oxygen and produces metabolic heat, carbon dioxide and metabolic water.

This water has to be evaporated from the egg, to create enough space in the air cell for the start of lung ventilation. Too limited moisture loss will give an increased risk of drowning of the embryo in the shell, too much moisture loss will dehydrate the embryo.

Due to the relative weight of oxygen, carbon dioxide and moisture damp, weight loss of the egg will be completely due to moisture loss, so for determining the moisture loss of egg, weighing of the egg is sufficient. he weight loss of an egg.

The optimum moisture loss until 20 days of incubation (chicken eggs) is approximately 12-14% of the initial egg weight.
The minimum average weight loss for a set of eggs is approximately 10%, below this level a reduction of hatchability can be expected
The maximum average weight loss is approximately 16%. However, being on average too high in weight loss will show less detrimental effects on hatchability then being too low.

Although 12 to 14% of weight loss is recognised to be optimum, the individual eggs can handle a huge variation of weight losses. The spread in weight loss between individual eggs can be as high as 3%. This means that if on average 12% weight loss is observed, individual eggs will range between 9 and 15%.

For individual eggs, a weight loss of less than 7-8% will create problems with hatchability. This is why on average the weight loss should not be below 10%, as that will prevent the individual eggs from loosing less then 7-8% moisture.
The same holds for the maximum weight loss. Individual weight losses of 18% and more will create problems with hatchability, chick quality and dehydration. With a spread of 3%, this means the average weight loss should not be higher then 15%.