Chicken eggs should be incubated at 40 to 50 percent relative humidity for the first 18 days, then raised to 60 to 65 percent for the final 3 days of hatching. Those two distinct phases are the core of successful incubation, and getting humidity right in each one is just as important as holding a steady temperature.
Humidity for Days 1 Through 18
The first 18 days are called the setting stage. During this period, your incubator should hold a relative humidity between 40 and 50 percent. If you’re using a wet-bulb thermometer instead of a digital hygrometer, that translates to a wet-bulb reading of roughly 85 to 87°F at a dry-bulb temperature of 99.5°F.
The goal during this phase is controlled moisture loss. A developing embryo needs the egg’s contents to slowly dehydrate so that an air cell forms at the blunt end of the egg. That air cell is what the chick will eventually use to take its first breath before it starts pipping through the shell. If humidity stays too high, the air cell never grows large enough. If it drops too low, the egg dries out faster than the embryo can tolerate.
Humidity for Lockdown and Hatch
Around day 18, you enter what’s commonly called lockdown. You stop turning the eggs, avoid opening the incubator, and raise humidity to 60 to 65 percent. This higher moisture level keeps the inner shell membrane soft and pliable so the chick can rotate inside the egg and break free. A membrane that dries out and turns leathery can trap a fully developed chick, which is one of the most common causes of failed hatches in otherwise healthy embryos.
Lockdown humidity stays at this level until all viable chicks have hatched, typically by day 21. Resist the urge to open the incubator during this window. Every time you lift the lid, humidity plummets and can take 30 minutes or more to recover, which puts pipping chicks at risk.
Why 12 Percent Weight Loss Matters
The simplest way to confirm your humidity is dialed in correctly is to track egg weight loss. Healthy chicken eggs should lose about 12 percent of their fresh weight between the day they’re set and the day the embryo begins to pip. Research published in Veterinary World found that hatchability drops significantly when eggs lose less than 10 percent or more than 15 percent of their starting weight.
To monitor this, weigh a sample of your eggs on day 1 and again on day 18. If a 60-gram egg weighs about 53 grams at lockdown, it has lost roughly 12 percent, and your humidity has been in the right range. If the eggs haven’t lost enough weight, your humidity has been too high, and you should reduce the water surface area in the incubator. If they’ve lost too much, add more water or a larger water tray.
What Happens When Humidity Is Wrong
Humidity that’s consistently too low causes dehydration. Embryos can develop head and skull malformations, abdominal wall defects, and other structural problems. Even when embryos survive to hatch day, they may be too weak or too dried out to pip successfully. Chicks that do emerge from overly dry incubations tend to be smaller and less vigorous.
Humidity that’s consistently too high creates the opposite problem. The air cell stays too small, leaving the chick without enough air space to transition to breathing with its lungs. Research in BMC Veterinary Research documented a spike in late-stage mortality in embryos incubated at high relative humidity, most likely caused by overhydration. High humidity during lockdown specifically can result in what hatchers call “mushy chick” syndrome: weak, wet chicks that struggle to absorb their yolk sac and are prone to infection.
The Dry Incubation Method
Some experienced hatchers use a technique called dry incubation, which skips adding water to the incubator entirely during the first 18 days. The idea is to let the incubator’s natural ambient humidity (from the eggs themselves and the surrounding environment) provide all the moisture the eggs need, as long as it doesn’t drop below about 15 percent. In dry climates or air-conditioned rooms, natural humidity can sit well below that floor, making this method impractical without monitoring.
Even with the dry method, lockdown humidity still needs to reach 60 to 65 percent. That means adding water trays or wet sponges on day 18, just as you would with traditional incubation. The dry method simply changes the approach for the setting stage, not the hatch stage. It works best for hatchers who have found that their local climate pushes standard incubators above 50 percent even without adding water, causing eggs to lose too little weight.
Calibrating Your Hygrometer
Cheap hygrometers can be off by 10 to 15 percentage points out of the box, which is enough to ruin a hatch. Before you trust any reading, calibrate with a salt test. Mix about half a cup of table salt with just enough water to make a damp slurry (not a puddle), place it in a small container, and seal it inside a zip-lock bag with your hygrometer. Leave it at room temperature for 12 to 24 hours. The reading should settle at exactly 75 percent. If your hygrometer reads 70 percent, you know it reads 5 points low and can adjust accordingly.
Digital hygrometers with a probe tend to be more accurate than the small analog dial types, but both benefit from calibration. If you’re running a still-air incubator without a fan, humidity can also vary significantly between the top and bottom of the unit, so place the sensor at egg level.
Practical Tips for Controlling Humidity
Most small incubators control humidity through water channels or trays built into the floor. Filling more channels raises humidity; leaving some dry lowers it. If you need finer control, try these adjustments:
- To raise humidity: Add a small sponge to increase the evaporation surface area without adding a large volume of water. Warm water evaporates faster than cold, so use lukewarm water when topping off.
- To lower humidity: Reduce the number of filled water channels, or increase ventilation slightly by opening an additional air vent on the incubator.
- To stabilize swings: Place the incubator in a room with relatively stable temperature and humidity. Garages, porches, and rooms near exterior doors tend to cause wider fluctuations than interior rooms.
Humidity will naturally rise on its own during lockdown as chicks begin to pip and release moisture from inside the egg. This is normal and part of why the target range has some flexibility built in. Aim for 60 percent at the start of lockdown, and expect it to climb toward 65 or slightly higher as hatching progresses.