Incubator Humidity for Chicken Eggs: The Numbers and How to Check Them

Most chicken eggs hatch well when incubator humidity sits somewhere between 55% and 65% for the first 18 days and goes up to 65% to 70% for the last three. That range is wider than most guides admit, because extension services don't agree on a single number. The good news: you don't have to trust any of them blindly. Your eggs tell you whether the humidity is right, and a kitchen scale is enough to read them.

Why humidity matters at all

An egg is not sealed. Water evaporates through the shell the whole time it sits in the incubator, and that is supposed to happen. As moisture leaves, the air cell at the blunt end grows, and the chick needs that space at the end. Mississippi State University's hatching guide puts a number on it: the egg's weight must drop by about 12% during incubation for a good hatch. University of Missouri Extension gives a target window of 11% to 14% weight loss from day 1 to day 18.

Humidity is simply the dial that controls how fast that water leaves. Low humidity, fast loss. High humidity, slow loss. Get it wrong for long enough and the chicks pay for it at hatch:

• Too dry: Missouri Extension notes that low humidity can cause the chick to stick to the shell and membranes at hatching. Mississippi State adds short down and small, weak or malpositioned chicks to the list.
• Too wet: Excessive humidity can weaken a chick so much that it can't get out of the shell, according to Missouri Extension. Mississippi State describes large, soft, "mushy" chicks as a sign of high humidity.

The numbers, and why they don't match

Here is what three extension services recommend for chicken eggs:

Nobody here is wrong. The right number depends on your incubator, your eggs and how dry your house is, which is why the recommendations spread out. Pick a starting point in the middle, around 58% to 60%, and then let the eggs correct you.

One thing to keep in mind: some guides give humidity as a wet-bulb temperature instead of a percentage. Mississippi State Extension points out that the two only convert with the dry-bulb temperature known, using a psychrometric table. A wet-bulb reading of 84 to 86 °F is not a percentage, so don't mix the two up.

Temperature matters for the same reason. Missouri and Maryland Extension both give 99.5 °F for forced-air incubators. Mississippi State says still-air units run warmer, at about 102 °F. Your humidity reading only means something when the temperature is right.

Weigh your eggs: the check that beats the hygrometer

A hygrometer tells you about the air. A scale tells you about the eggs, and the eggs are what you're actually trying to get right. All you need is a kitchen scale that reads in grams.

1. Weigh the empty tray or carton you'll use, and write it down.
2. On setting day, weigh all the eggs together in that tray. Subtract the tray. That's your starting weight.
3. Work out your day 18 target. Multiply the starting weight by 0.88 for a 12% loss. Missouri's 11% to 14% window gives you a range around it.
4. Weigh again every few days, at the same time of day, and compare.

An example. Twelve eggs weigh 684 g on day 1. A 12% loss means about 82 g, so the day 18 target is roughly 602 g. The 11% to 14% window puts it between about 588 g and 609 g. If you spread the loss evenly over 18 days (a simplification, but a useful check), you'd expect roughly 652 g on day 7 and 620 g on day 14.

If the eggs are heavier than the line, they're losing water too slowly: lower the humidity. If they're lighter, they're losing too fast: raise it. You're correcting a trend, not reacting to a single reading.

💡 Pro tip: Weigh the whole batch, not single eggs. Egg sizes vary, and one odd egg throws off the math. A dozen together averages it out.

Read the air cell when you candle

If you'd rather not weigh, the air cell gives you the same information visually. Mississippi State's guide compares the normal air cell size at 7, 14 and 18 days of incubation and recommends adjusting humidity based on what you see. A cell that looks small for the day means the eggs are holding too much water. One that looks large means they're drying out.

Candling also lets you spot eggs that aren't developing. University of Maryland Extension suggests candling after about 10 days for infertile eggs and dead embryos.

How to raise humidity (without hurting the eggs)

Mississippi State's guide is clear: humidity goes up when you increase the exposed water surface area, and the water pan area should be about half the incubator's floor or more.

• Fill every water channel your incubator has.
• Add a second shallow dish or a clean sponge to increase the surface.
• Expect to add more water in a dry house. Maryland Extension notes that dryer areas need more water than humid ones.

What not to do: tape the vents shut. Mississippi State Extension warns that closing vents may push the humidity reading up, but the embryos suffer from poor ventilation. Higher humidity is never worth less oxygen.

How to bring humidity down

Too-high humidity is usually the easier fix. Take water out: empty a channel, remove the sponge, and give the reading a day to settle before you change anything else. If your incubator has an automatic humidifier, check the set point. Mississippi State's hatchery guide notes that in machines relying on evaporation from pans, humidity rarely runs too high; forced spray systems are the more common cause.

💡 Pro tip: Keep a short log: date, temperature, humidity and egg weight. When a hatch goes well, you'll know exactly what to repeat. When it doesn't, you'll know where to look.

Lockdown: the last three days

Around day 18, three days before the expected hatch on day 21, you change three things at once:

• Stop turning. Maryland Extension says to stop at day 18. Mississippi State says not to turn during the last three days. Missouri Extension notes turning runs through day 17.
• Raise the humidity to 65% or more (Mississippi State) or 70% (Missouri and Maryland).
• Keep the lid closed. Mississippi State's guide says to keep the incubator closed during hatching to hold temperature and humidity, with the air vents almost fully open in the latter stages.

That last one is the hardest. Every time the lid comes up, humidity drops, and chicks that have started to pip can dry out against the membrane. Watch through the window.

The short version

• Days 1 to 18: start around 58% to 60%. Extension advice ranges from 55% to 65%.
• Last three days: 65% to 70%, no turning, lid closed.
• Check with the eggs, not just the hygrometer: aim for about 12% weight loss by day 18.
• Raise humidity with more water surface, never by closing vents.

Once your chicks are out and growing, the next questions are housing ones. We've covered how much roost space they'll need and how to add new birds to an existing flock.

Looking for an incubator that takes some of the routine off your hands? Our E24 holds 24 eggs and has automatic turning, a humidity pump and a built-in candling light. You'll still want to weigh and candle; the equipment takes over the routine, not the checking. See it with our brooder plate in Hatching & Brooding.

Sources

• Important Incubation Factors, Mississippi State University Extension
• Care and Incubation of Hatching Eggs, Tom W. Smith, Mississippi State University
• Hatchery Management Guide for Game Bird and Small Poultry Flock Owners, Mississippi State University Extension
• Small Flock Series: Incubation of Poultry (G8353), University of Missouri Extension
• Hatching Eggs at Home (FS-1114), University of Maryland Extension