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Controlling Your Incubator’s Humidity by

Monitoring Egg Weight Loss


By Linda Harrison


Depending on the humidity levels in the incubator and around the eggs, more or less moisture is lost through the shells by evaporation during the 21 days of incubation and hatching.  The suggested humidity range of 40% - 50% takes into fact many variables in the incubator as well as the individual differences in shell porosity between one egg and another laid by the same or different chickens. All eggshells are not equal.

Taking all of the possible mechanical and biological variables into account, it becomes a guessing game at best, and sometimes wrong guesses are made and eggs fail to hatch under the exact conditions that worked last time.

Many articles indicate that the most common reason for hatch failures is too much humidity... not enough moisture has evaporated out of the egg over the 21 days of incubation. Eggs that don't loose enough moisture produce chicks that are too large and are unable to maneuver inside the egg when trying to pip. The final air space created as moisture evaporates plays an important role in the survival of the emerging chick.  From the correct amount of air space, the chick gets its first breath of air and enough room to maneuver as it pips it's way around the shell.

Too little air and it may drown in it's own moisture or be unable to rotate within the egg or suffocate if it is unable to penetrate and break the membrane between it and the shell. The result could be a fully formed chick that dies in the shell. That's a sad moment.  Eggs that lose too much moisture (low humidity levels) produce small chicks with large air spaces. These chicks are often weak and may die before or during hatching.  Slightly lower humidity levels are more likely to be less disastrous than slightly higher humidity levels.

Since so much depends on a specific loss of moisture or in other words a precise loss of weight from an egg at day 21 for a successful hatch.  Weighing the egg at specific intervals can give a very good indication of the moisture loss at any point in the process and make it possible to monitor and correct humidity levels to achieve the correct weight loss.

Chicken eggs need to lose 13% moisture over 21 days.  Weigh all the eggs simultaneously on the first day, just before you put them into the incubator.  Calculate the average weight of the eggs (weight in grams) and plot this on a graph.  You will create an Ideal Weight Loss Line on a graph.  The ideal weight loss line will be a line plotted by joining the first point representing the first average weight calculated from the total weight of all of the eggs at day one (y-axis, weight in grams), with the second point, the ideal hatch weight, which is the average weight loss of all of the eggs at day 21 (x-axis representing the incubation period in days, 21 days).  The goal is to lose 13% weight over the 21-day period.

Here is a graph on which you can record your eggs’ weights.


By measuring actual average weights every three or four days, the actual weight loss can be plotted and compared to the ideal weight loss line. If the actual average readings from the eggs in the incubator falls away from the ideal weight loss line (above or below), small corrections can be made immediately.  An example: If the actual weight loss is greater than ideal (below the line), then the air has been too dry and humidity levels need to be increased to compensate.

For more accuracy, a digital scale should be used which can weigh in grams. Don't forget to subtract the weight of the container holding the eggs from the total weight when calculating the average egg weight.

Do not weigh the eggs from day 19 on and keep the incubator closed.  Raise the humidity to at least 65% and resist opening, and only do so very quickly with 6-8 hours between openings.  Humidity will fall dramatically when the incubator is opened.


Good luck hatching!


Editor’s note:  You can safely track your eggs’ weight if you:

  1. Weigh the rack into which you will place your eggs.
  2. Put your eggs in the rack.
  3. Weigh the rack and eggs together.
  4. The weight of all your eggs is

(the weight in 3) – (the weight in 1)

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