The ideal humidity for hatching chicken eggs is 50–55% relative humidity for the first 18 days, then 65–70% for the final three days before hatch. These two phases have different goals: the first phase allows the egg to lose moisture at the right rate, while the higher humidity at the end keeps the membrane soft so the chick can break free.
Days 1 Through 18: The Incubation Phase
For the bulk of incubation, keep your humidity between 45% and 55%. Many experienced hatchers target 50–55% as the sweet spot, though some sources recommend as low as 40%. The purpose of this range is to let moisture escape through the tiny pores in the eggshell at a steady, controlled rate. As water vapor leaves, the air cell at the blunt end of the egg grows larger. By day 18, a healthy egg should have lost roughly 10–12% of its starting weight. That air cell is what gives the chick its first breath of air before it pips through the shell.
If your eggs are losing too much weight (more than 18.5% by hatch time), your humidity is too low. If they’re losing less than about 9%, your humidity is too high. Both extremes raise embryo mortality. Low humidity during this phase can also reduce chick body weight at hatch. You can track moisture loss by weighing a few eggs every few days and calculating the percentage lost from their starting weight.
Days 19 Through 21: Lockdown
On day 18, you stop turning the eggs, boost the humidity to 65–70%, and leave the incubator closed. This final stretch is called “lockdown,” and the humidity increase serves a critical purpose: it keeps the inner membrane moist and pliable. A chick uses a small tooth on its beak to crack the shell from inside, and it needs to push through the membrane beneath it. When the membrane dries out and turns tough, the chick can get stuck, a problem hatchers call “shrink-wrapping.” It’s one of the most common causes of chicks dying in the shell, and it’s almost always a humidity problem.
Resist the urge to open the incubator during lockdown. Every time you lift the lid, humidity plummets and takes time to recover. If you see one chick hatch while others are still pipping, leave them alone. Chicks can survive 24 hours or more after hatching without food or water.
How to Measure Humidity Accurately
Cheap hygrometers (humidity sensors) are notoriously inaccurate, sometimes off by 10% or more. That kind of error is the difference between a good hatch and a failed one. Before your first set of eggs goes in, calibrate your hygrometer with the salt test.
- Set up: Place a teaspoon of table salt in a bottle cap and add a few drops of water so the salt is damp but not dissolved. Put the cap and your hygrometer inside a sealed ziplock bag.
- Wait: Leave the bag sealed and undisturbed for at least 6 hours. The damp salt creates a stable environment of exactly 75% humidity.
- Read the result: Without opening the bag, check the display. If it reads 75%, your sensor is accurate. If it reads 70%, you know it runs 5% low, and you can add that offset to every reading going forward.
Some digital hygrometers have a calibration button that lets you set the reading to 75% during the test. If yours doesn’t, just remember the offset and apply it mentally.
Wet Bulb vs. Relative Humidity
Older incubators and many university extension guides give humidity in wet bulb temperature rather than relative humidity percentage. A wet bulb thermometer has a damp wick over its sensor, and the evaporation from the wick cools it. The drier the air, the more cooling occurs, so the reading drops further below the actual (dry bulb) temperature. Mississippi State University Extension recommends a wet bulb reading of 85–87°F during incubation and about 90°F during the last three days, assuming a dry bulb temperature of 100°F. If your incubator displays relative humidity directly, you don’t need to worry about wet bulb readings. But if you’re using older equipment, a psychrometric conversion chart will translate between the two.
Adjusting for Your Climate and Altitude
If you live in a humid climate, you may struggle to keep humidity from climbing too high during incubation. Running fewer water trays, increasing ventilation, or using smaller water surface areas can help bring it down. In dry climates, adding sponges or extra water channels gives you more evaporative surface to raise humidity.
Altitude changes things more than most people expect. Water vapor moves through the eggshell pores faster at higher elevations because of lower air pressure. At around 4,900 feet (1,500 meters), you’d need to set your incubator roughly 8 percentage points higher than you would at sea level to achieve the same rate of egg weight loss. So if a sea-level hatcher runs 50% humidity, you’d aim for about 58% at that altitude. There’s a catch, though: if your breeding flock also lives at high altitude, their eggs may have adapted with less porous shells, meaning you might actually need to lower humidity slightly compared to the altitude-adjusted number. The best approach is to weigh eggs and track their moisture loss directly rather than relying on a single humidity number.
When Humidity Goes Wrong
Too much humidity during days 1–18 means the air cell stays too small. The chick may drown in excess fluid when it tries to pip into the air cell, or it may pip in the wrong position because the air cell didn’t develop where it should. You’ll often see eggs weeping fluid at hatch, and chicks that do emerge tend to be wet, swollen, and weak.
Too little humidity throughout incubation leads to oversized air cells and dehydrated embryos. Chicks that hatch from overly dry eggs are often small and may have dried membranes stuck to their down. During lockdown specifically, low humidity is the primary cause of shrink-wrapping, where the membrane tightens around the chick like plastic wrap, trapping it inside the shell even after it has pipped a hole.
If you’ve had poor hatches and suspect humidity is the issue, weighing eggs at day 7 and day 14 gives you an early signal. Plot the weight loss as a percentage of starting weight. If you’re on track for 10–12% total loss by day 18, your humidity is in the right range regardless of what the hygrometer says. The eggs themselves are the most reliable humidity gauge you have.