The three main elements needed for incubating chicken eggs are heat, humidity, and turning. All incubators should have an automatic method of controlling the temperature. And you can often choose incubators that have an automatic turning system as well. However, humidity features in different incubators vary and can range from fully automatic to fully manual. Keeping the humidity at the correct level for incubation and during the hatch can be tricky. While I won’t go into all the details on how you can control humidity in the incubator (that will be for a different post), I do want to cover the different methods you can use to monitor humidity in the incubator. Learning how to monitor humidity in an incubator is important so you can know if the humidity levels are staying stable enough throughout incubation.

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Why Humidity is Important

Humidity is the amount of moisture that is in the air. When air is dry, it has low humidity and will absorb moisture from the surroundings through evaporation. Eggs contain moisture. The longer they are stored, the more moisture evaporates through the pores in the eggshell. During incubation, you want the moisture to evaporate at the right rate in order for the embryo to develop and hatch properly.

When a mother hen sits on her clutch, she will pluck feathers from her breast and underbelly to expose her moist skin. The moisture from her skin prevents the eggs from losing too much moisture too quickly while they incubate. When using an incubator, we have to rely on other methods to control and monitor humidity levels during incubation.

The humidity levels in the incubator need to be maintained at a fairly constant level to prevent the eggs from drying out (humidity too low) or from not loosing enough moisture (humidity to high).

What happens when humidity is too low…

• the embryo will stick to the shell membrane within the egg
• hatchlings may or may not pip or fully hatch
• the egg’s shell membrane will dry out
• dried out shell membranes can inhibit proper hatching and trap the chick inside the egg
• chicks that do hatch will be small & weak
• twisted neck, crooked toes, or other muscular deformities may occur from low hatching humidity

When the humidity in the incubator is too low, the eggs lose too much moisture. Low humidity can be a problem when the water supply in the incubator is not adequately filled, the incubator is not filled to capacity (the more eggs, the more moisture evaporates into the air), or frequent opening and closing of the incubator.

What happens when humidity to too high…

• embryos grow too large
• embryos can’t move into proper hatching position
• the air cell is too small and won’t provide sufficient oxygen for the chick before it hatches
• the eggshell membrane can get too rubbery and be hard for the chick to break through
• results in soft, large hatchlings that often die shortly after hatch (mushy chick syndrome)

When humidity levels in the incubator are too high, not enough moisture evaporates from the eggs during incubation. High humidity levels in the incubator can result from insufficient incubator ventilation, high ambient humidity in the incubating room, or supplying too much water surface area in the incubator.

One apparent way to see that the humidity is too high in the incubator is when you see moisture building up on the viewing window of the incubator.

What Humidity Levels are Ideal for Incubation?

You don’t want the humidity too high, but you don’t want it too low. So what is the perfect humidity level for incubating chicken eggs? There are several factors that determine what the ideal humidity level is for a clutch of eggs.

One thing to keep in mind is that as the temperature goes up, relative humidity goes down. This is important to consider when you are using a forced draft versus a still air incubators. Still air incubators need to be operated at high temperatures since they don’t have a fan to circulate air and make for even heating throughout the incubator. Often times still air incubators need to be maintained at higher humidity levels than forced draft incubators to ensure proper embryo development.

You should also consider the size of the eggs you are incubating and how many eggs you can fit in your incubator. Small eggs need higher humidity levels since the rate of moisture evaporation needs to be slower than for large eggs over the span of incubation. If your incubator holds a lot of eggs, the humidity can be maintained at a lower level since the more eggs that are incubating, the more moisture they contribute to the air.

In general, humidity levels should be maintained between 45-55% relative humidity for days 1-18 of incubation. However, keep in mind that different factors will affect whether those levels need to increase or decrease.

Factors that Affect Humidity:

• egg size– Small eggs need to loose moisture at a slower rate than large eggs.
• shell porosity– Eggs with porous shells will loose moisture faster, which can often be a problem with eggs laid during the hot summer months.
• elevation– The higher in elevation you are, the drier the air will naturally be, which means low ambient humidity levels.
• storage length & conditions- The longer hatching eggs are stored, the more moisture evaporates prior to incubation. Minimizing evaporation during storage is essential, especially when storing hatching eggs for longer than 7 days.
• weather– Weather patterns can cause ambient humidity levels to fluctuate.
• incubation temperature– The higher the incubation temperature, the lower the humidity will naturally be in the incubator.
• air speed/fluctuations– Air fluctuations (like opening and closing the incubator or drafts) can cause humidity levels to fluctuate too.
• shell thickness– Thin eggshells will loose moisture faster than normal eggshells. Thin eggshells can become a problem when collecting eggs from older hens who often lay thinner-shelled eggs.

Ideal Humidity for the Hatch

Humidity levels should be increased for the last 3 days of incubation, which is considered the hatching period. For days 19-21 of incubation, the humidity levels should be increased about 10% in preparation for hatching. The ideal range is between 65-70% relative humidity for the hatch.

The air needs to more moist when chicks are hatching in order to prevent the shell membranes from drying out when a chick pips through the eggshell. If the shell membrane dries out before the chick fully hatches, it will become stuck inside the egg and will not be able to hatch on its own. As chicks hatch, they will naturally add more moisture to the air as well.

High humidity is important during the hatch, but you don’t want it too high that the hatched chicks can’t ‘fluff out’. Fluffing out is the term used to refer how a chick’s down feathers dry after it hatches and gives it a more fluffy, chick-like appearance. Hatching humidity that is too high won’t allow the chick’s down feathers to dry out fast enough and can result in chilled hatchlings.

4 Ways to Monitor Humidity

Now that you have a rough idea of what humidity level to maintain in the incubator during incubation and at hatch time, but how do you monitor humidity levels in the incubator? There are 4 main ways you can keep track of humidity levels in the incubator: a digital hydrometer, weighing the hatching eggs, using wet bulb readings, or monitoring egg air cell sizes.

#1. Digital Hygrometer

The most common way to monitor humidity levels in the incubator is by using a digital hygrometer. Digital hygrometers measure the percent of relative humidity within the incubator. The percent of relative humidity is derived from the ratio of the partial pressure of water vapor to the saturated vapor pressure of water at a given temperature (but you probably didn’t need to know that).

Many high-end incubators have a built-in digital hygrometer that is connected to a digital display menu where you can see what the humidity level is without having to open the incubator. If your incubator does not have a built-in digital hygrometer, you may consider getting a small hygrometer to place inside the incubator to help monitor humidity levels. An additional add-in hygrometer can also be helpful for double checking built-in digital hygrometer systems.

Small add-in hygrometers should be placed in a secure spot within the incubator of securely fastened to a wall or window the incubator. Ideally place the hygrometer in a location where you can see it without having to open the incubator (which would decrease the incubator’s temperature and humidity). Make sure the hygrometer won’t get bumped or inhibit the eggs when they are turned.

A digital hygrometer will help you monitor the relative humidity levels within the incubator throughout incubation and at hatch time.

#2. Weigh Hatching Eggs

One way to monitor humidity levels is to keep track of how much weight the hatching eggs loose throughout incubation. As moisture evaporates from the eggs, they loose weight. At proper humidity levels, hatching eggs will loose 12-14% of their original weight between days 1-18 of incubation.

In order to weigh your hatching eggs, you need a small scale and a little bit of math skills. Since you will be working with small weight changes, it is best to use a scale that can provide you with the weight of an egg in grams versus ounces. You will also want to make sure the scale is correctly calibrated so that it gives you accurate readings.

It is best to weigh the eggs each day that you candle the eggs during incubation. This usually means weighing the eggs prior to placing them in the incubator, about half way through incubation, and at lockdown. Prepare a secure place to set the eggs so that they don’t roll around while you weigh each one. Print off an egg weight loss chart from this Raising Chicks Record Journal or make your own chart.

How to Weigh Hatching Eggs

1. Number each egg and put the number in the first column of your chart. This will make it easy for keeping track of the weight loss for each egg.
2. Weigh each egg and write the beginning weight (BW) in the column after the egg number.
3. Determine the total weight loss (TWL) for the incubation period (IP = 18 days for chicken eggs) for each egg based off of each egg’s beginning weight: TWL = BW – (BW x .13)
4. You can write the TWL next to the egg’s number on the right hand side of the chart, however, you won’t be needing that number for any other calculations.
5. Note: Use .13 in your calculations since the average percentage of egg weight loss for most poultry species is 13%.
6. You will want to find the daily weight loss (DWL) of each egg so that you can calculate the appropiate weight of each egg on any given day of incubation. Write each egg’s corresponding DWL in another column of your egg weight loss chart: DWL = (BW x 0.13)/IP
7. To determine an egg’s appropiate weight for any given day of incubation (totally daily weight loss, TDWL), use the following formula: TWDL = BW – (DWL x DI)
8. Fill in each egg’s TWDL in your egg weight loss chart for each day you plan to weigh the eggs.
9. Each day you weigh the eggs throughout incubation, write each egg’s weight in the chart row that corresponds with the egg’s number and the candling day. Compare the egg’s actual weight to the TDWL for that day.

Abreviation Guide:

• BW– beginning weight
• IP– incubation period
• DI– any given day of incubation (e.g. day 10 of incubation)
• TWL– total weight loss
• DWL– daily weight loss
• TDWL– total daily weight loss

Adjusting Humidity Based on Egg Weight Loss

If a majority of the eggs are below their TWDL weight for a given day of incubation, then the humidity levels are too low in the incubator. The eggs are losing weight too fast for proper development. If a majority of the eggs are above their TWDL weight for a given day of incubation, then the humidity levels are too high. The eggs are not losing enough weight for a successful hatch.

If you are incubating large volumes of eggs at one time, it may be a good idea to only weigh a select handful of eggs instead of spending the time weighing every egg. Choose the eggs from various spots throughout the incubator and make sure you weigh the same eggs each time.

#3. Wet Bulb Readings

Another way to monitor the humidity levels in the an incubator is by taking wet bulb readings. Wet bulb readings are different than relative humidity percentages. A wet bulb reading is achieved by using a standard stem thermometer and enclosing the tip, or bulb, with a moist wick or cloth. The moisture will constantly evaporate from the moist wick or cloth and will cool the bulb. The thermometer will then read a cooler temperature that can be converted to a relative humidity percentage.

You will need to place the wet bulb thermometer in the incubator and allow it to stabilize at a wet bulb reading. Make sure you place the thermometer in a spot where you can view it while the incubator is closed. Ideally, you should be able to stick the thermometer in one of the incubator’s ventilation holes. Then the wick should be fastened to the thermometer bulb inside the incubator. To have an on going wet bulb reading, the wick should be long enough that it can dangle in a water source within the incubator, such as the incubator’s water pan or pot that helps supply humidity.

Wicks will loose absorbency over time, so make sure you have replacements on hand. You can buy wet bulb wicks from some poultry suppliers. Or you can make homemade wicks using cheesecloth or gauze pads. Using distilled water in your incubator’s water pan will also prevent the wick from becoming crusted with minerals from hard water.

Wet bulb readings will be more accurate in forced-draft incubators. Converting the wet bulb reading to a relative humidity reading will depend on the temperature within the incubator and the type of incubator you are using.

Here are some common wet bulb readings converted to relative humidity percentages.

Forced-Draft Incubator at 99.5°F

• 80.8°F wet bulb = 45% humidity
• 82.8°F wet bulb = 50% humidity
• 84.7°F wet bulb= 55% humidity
• 86.7°F wet bulb = 60% humidity
• 88.5°F wet bulb= 65% humidity
• 90.3°F wet bulb= 70% humidity
• 91.9°F wet bulb = 75% humidity

Still-Air Incubator at 101°F

• 82.2°F wet bulb = 45% humidity
• 84.2°F wet bulb = 50% humidity
• 86.2°F wet bulb= 55% humidity
• 88.2°F wet bulb = 60% humidity
• 90.0°F wet bulb= 65% humidity
• 91.7°F wet bulb= 70% humidity
• 93.6°F wet bulb = 75% humidity

#4. Air Cell Size

Lastly, another way to monitor humidity levels throughout incubation is by keeping track of the air cell size of each egg. The air cell is located at the blunt end of the egg. As moisture evaporates from the egg, the contents within the egg shrink and the air cell size gets bigger. You can note the air cell size of each egg when you candle the eggs throughout incubation.

The air cell will increase at a steady rate throughout incubation. It is helpful to have an air cell chart sized to match the size of the eggs you are incubating to reference when you candle the eggs. You can get a free egg air cell chart in our Trading Post!

After the chicks hatch, it can also be helpful to note the proportions of the eggshells. A chick will occupy about two thirds of the bottom half of the egg. The remaining third will be the air cell. When a chick hatches, the bottom half of the egg should be about twice as big as the top half (cap). If the cap of the eggshell is very small, than the humidity was too high during incubation and not enough moisture evaporated from the egg. If the cap of the eggshell is large, than the humidity was too low during incubation and too much moisture evaporated from the egg.

Humidity Tips!

Knowing how to monitor humidity levels throughout incubation is extremely important for maintaining the proper humidity levels in the incubator. Even if you have a fully automatic incubator that controls the humidity digitally, you still need to know what humidity level you should program into the incubator for a successful incubation and hatch. Correct incubation humidity levels will vary depending on ambient humidity, egg size, incubator capacity, and the type of incubator you are using. However, the methods for monitoring humidity levels can be used no matter what incubator you use or what type of eggs you are hatching!

Use a digital hydrometer and/or a wet bulb for monitoring the relative humidity in the incubator at any point during incubation. Keep track of air cell growth every time you candle the eggs. The air cells should steadily grow bigger over the course of incubation. Lastly, keeping track of egg weight loss is an accurate way to monitor proper humidity levels throughout incubation as well.

Need more tips for incubating eggs and hatching chicks on the homestead? Feel free to reach out with any questions you have! And make sure you join our modern pioneer newsletter for helpful resources and exclusive information on raising your homestead flock naturally and sustainably!

by Alexa