Relative Humidity (RH) “humidity” – simplified, is the amount of moisture present, relative to the amount that would be present if the air were totally saturated. This is usually expressed as a percentage and is measured with a “hygrometer”. RH can also be measured with a wet-dry-bulb thermometer, expressed in degrees. RH can get quite technical, involving temperatures, dew points, vapor pressures, wet bulb, etc., but for basic egg incubation purposes, I will not go into those technicalities. We will stick to hygrometer and percentages.

Most store-bought incubators usually come with humidity guidelines, but the actual relative humidity needed for your own incubations can vary drastically from those instructions. There are several factors that dictate what setting(s) will work best for you.

First the basics
Why worry about it? The relative humidity inside the incubator controls the moisture loss inside the egg. Most every bird egg needs to lose a certain percentage of weight/moisture for the bird to develop properly and hatch. This is accomplished through pores in the egg shell. Humidity affects how quickly this moisture evaporates through those pores. Humidity too low can cause too much moisture loss, the air cells may get too large, the chick can get crowded, and can have a difficult time hatching. If humidity is too high, and not enough moisture is lost, then the air cells can’t grow enough, and chicks can drown; or chicks can grow too large to be able to maneuver and hatch; or there isn’t enough air in the cell for the chick to use to complete its cycle and emerge. It’s not really rocket science for incubation purposes, and there is some flexibility, but too much to one extreme will usually cause problems.

Factors to consider:
Local conditions
Those living in humid areas usually find that "dry incubation" works just fine. They can usually maintain enough natural humidity for proper air cell growth. Those living in drier areas may need to add some water.

Egg shell quality
All eggs are not created equally. Some are very thick, some have very heavy blooms, some can be waxy, some can be glossy, and some are very porous. Examining eggs by feeling them, scratching them, and candling can give you an idea as to how much moisture will be able to evaporate from within the egg. More porous eggs usually need a little higher humidity, because the moisture loss is naturally greater. Glossy shells, and very dark colored shells usually require lower humidity.

Time of year plays a big part for some of us. Summers can be quite frustrating for those of us in humid areas, and equally frustrating for those in dry areas. Fluctuations can cause incubators to change from day to day. Winter causes many folks to use artificial heat, and homes can be much drier as a result, even if it’s wet outside! Which brings us to….

Incubator location
Always try to locate your incubator in a stable space, where temperature and humidity is most controllable. Not hot, not cold, just stable. Be aware that changes to the room will also cause changes inside the incubator. The heater will work harder in a room that is too cool, but a room that is very warm can cause issues with keeping humidity as constant as possible.

Other things you may need to take into consideration:
Egg sizes/qualities/quantities being incubated together

If you plan to incubate very many eggs at one time, keep in mind that eggs of different sizes/qualities can lose moisture at different rates. Also, an incubator full of eggs may not maintain the same RH as an incubator with only a few eggs, so what worked at one time may not work with each and every set.
Age of eggs at set time
Air cells are naturally smaller in freshly laid eggs. If the eggs are older, some moisture has already been lost, so a slightly higher humidity setting may be necessary.
Incubating at higher elevations can require different settings, much like alternate baking instructions on a cake box.

So how do we choose?
Personally, I have found that monitoring air cells is the easiest way to ensure that the eggs are losing the proper amount of weight/moisture, and that the air cells are growing at an acceptable rate.

This chart shows an approximate guideline for chicken and duck eggs, as to how large and how quickly the air cells should grow.
Candling the eggs on the days indicated and comparing to this chart can guide you as to whether you need to increase or decrease humidity. Minor adjustments should be made over a few days, rather than large increases or decreases at one time.

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After considering all these factors, you still don’t know where to start, I suggest testing your incubator "dry" to determine what it will maintain with no water added. "Dry incubation" does not mean zero humidity. It refers to not adding water to the incubator in order to unnaturally increase humidity. A good starting point is in the 30-40% range for most eggs. Choose a number on the lower side of the range for very humid locations, on the higher side of the range for drier locations. Add a few percentage points if incubating very porous shells, subtract a few points for dark or denser shells. If your "dry" test maintains less than your goal number, add a little water to your incubator.
I recommend starting with lower RH, and increasing later, if needed. It is easier to slow down the air cell growth by increasing humidity later, than it is to do the reverse.

Keep in mind that RH is determined by the amount of surface area of water. The depth of the water has no real bearing, except that more shallow water will evaporate more quickly and additional water may need to be added more often to maintain the proper number. For example, 8 fluid ounces of water in a tall glass will produce less humidity than the same 8 ounces of water in a flat pan. The surface area in the pan is greater than the surface area in the tall glass. But the pan’s water will evaporate faster.

Final 3 days – Hatch time!
Three days before hatch date (day 18 for chickens, day 25 for ducks, etc.) humidity should be raised all at once. By this time, the chick is occupying much of the egg, and this dramatic increase in humidity helps to keep the membrane from drying out any further. It also triggers the chick’s instinct to move into position to break out. Hatching humidity can range from 60-75% most commonly, but this range can also vary for you. Candle 3 days prior to hatch date, and if your air cells are on the large side, it is safe to run a little higher humidity. If they are smaller, then running a little lower may be your preference.

** Final note – Hygrometers can and should be tested for accuracy. A scientific and simple “Salt Test” can determine if your hygrometer is correct. More detailed instructions for this test can be found online, but the gist of it is as follows:
Pour a little table salt into a small container (bottle cap, shot glass, small bowl, etc). Add enough water to make it wet, but not so much as to dissolve the salt, or be too wet. You want to make a salt slurry. Place your salt slurry and your hygrometer(s) in a Ziploc bag or airtight container and let it sit at room temperature until the hygrometer stabilizes. This could take up to 8 or 10 hours, depending on the amount of slurry used, but usually 2-4 hours will give you some indication. The RH of salt slurry at room temperature should be 75%, so if your hygrometer is reading higher or lower than 75%, you will need to adjust for any difference. I personally write the variance on a piece of masking tape and stick it to my hygrometer. Some hygrometers have an adjustment knob, so they can be set to a particular percentage. Keep in mind that this also doesn't mean that it will be perfectly accurate at 100 degrees and around 30% humidity for incubation, but it should be relatively close. Also a point to note is that some hygrometers have a lower limit, and may not read less than 20-30%, so check the packaging when you purchase one.​
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An example of how far off some meters can be, without being properly calibrated:
Image thanks to @casportpony

Happy Hatching!