How do I know if my incubator humidity is too high or too low?

Candle the egg and compare the **air cell** to a day-by-day growth chart. If the air cell is **too small** for the day, the egg isn't losing enough moisture and your **humidity is too high** — lower it. If the air cell is **too large**, the egg is drying out and your **humidity is too low** — raise it. The most objective version is weight: eggs should lose about **11–14% of their set weight by day 18**. Because every hygrometer and room differs, the air cell (or weight loss) is a more reliable guide than the percentage on your display — calibrate the hygrometer, then adjust toward the air cell rather than a fixed number.

What humidity should I use for hatching chicken eggs?

Extension sources put incubation at roughly **45–55%** for days 1–18 (Mississippi State cites about 58–60%; Missouri 60–65%), then **65–75%** at lockdown (Missouri specifies 70% for the final three days). Treat these as a starting band and fine-tune to an air cell that reaches about a third of the egg by day 18.

What does a too-small air cell mean?

The egg is holding too much moisture, which means your **humidity is too high**. Lower it — fewer or smaller water channels and more ventilation — and re-candle in a day or two to confirm the air cell starts growing.

What does a too-large air cell mean?

The egg is losing water too fast, which means your **humidity is too low**. Raise it by adding water surface area (a wet sponge works quickly) and partially closing vents. A too-large air cell leaves the chick short of fluid and prone to shrink-wrapping at hatch.

Why did my chicks drown in the shell?

Fully formed, wet chicks that die around pip usually point to **humidity that was too high** during incubation — the egg never lost enough moisture, so there wasn't room for the chick to position and pip. Missouri Extension notes that excessive humidity 'will weaken the chick to the point that it can't emerge.' Verify temperature too, since it can cause similar late deaths.

What is shrink-wrapping and how do I prevent it?

Shrink-wrapping is when the shell membrane dries and shrinks tight around a pipped chick, gluing it in place so it can't zip. It's caused by **humidity too low** at hatch — often from opening the incubator during lockdown. Prevent it by raising humidity to 65–75% at lockdown, keeping the incubator closed, and confirming the air cell wasn't oversized going in.

Incubation Humidity Too High or Too Low: How to Read the Air Cell

Verified September 2026

The short version

The fastest way to tell is the air cell, not the number on your hygrometer. Candle and compare the air cell to a growth chart: a too-small air cell means humidity is too high (the egg isn't losing enough moisture), and a too-large air cell means humidity is too low (it's losing too much). Chicken eggs run about 45–55% relative humidity for days 1–18, then 65–75% at lockdown, but the target is really an air cell that reaches roughly one-third of the egg by day 18 — around 11–14% weight loss. Correct by adjusting humidity toward that air cell, not toward a fixed percentage.

Read the air cell, not a fixed percentage

Every incubator, room, and hygrometer is slightly different, so a humidity number that works for one hatcher can be wrong for another. The reliable signal is how much moisture the egg is actually losing, and you read that from the air cell — the pocket of air at the large end that grows as water evaporates through the shell.

Candle each egg against a growth chart around day 7, day 14, and again before lockdown, and mark the air-cell boundary in pencil. If the air cell is too small for the day, the egg is holding too much water and your humidity is too high — lower it. If the air cell is too large, the egg is drying out too fast and your humidity is too low — raise it. University of Missouri Extension states it plainly: 'If the air cell is too large, the humidity must be increased… If the air cell is too small, the humidity must be decreased.'

The quantified version of the same idea is weight loss: eggs should lose about 11–14% of their set weight by day 18 (Missouri Extension). Weighing a sample of eggs on a kitchen scale is the most objective check of all — but for most hatchers, candling the air cell against a chart is enough.

Symptoms of humidity that is too high

When humidity is too high through incubation, the egg can't evaporate enough water. The air cell stays too small, the chick develops in an egg that is too 'full,' and there isn't enough room to draw the last of the yolk in or to turn and pip cleanly.

The classic late-stage picture is drowned chicks and wet dead-in-shell: fully formed chicks that die around internal or external pip, often with a lot of sticky fluid in the shell, or chicks that pip but are too swollen and weak to zip. Missouri Extension notes that 'excessive humidity will weaken the chick to the point that it can't emerge from the shell at hatching.' If you're finding big, wet, fully developed chicks that never made it out, suspect humidity that ran too high (and confirm your temperature was correct).

Symptoms of humidity that is too low

When humidity is too low, the egg loses water too fast. The air cell grows too large, leaving the chick short of fluid at hatch. The membrane inside the shell dries out and turns tough and leathery instead of staying moist and slippery.

The signature failure is shrink-wrapping: the dried membrane shrinks tight around a pipped chick and glues it in place so it can't rotate to zip. You'll see chicks that pipped, then stalled and died partway out, often with the membrane stuck to a sticky, tacky down. Missouri Extension: 'Low humidity can cause the chick to stick to the shell and membranes at hatching.' A too-low finish is also why opening the incubator during lockdown is dangerous — the sudden humidity drop can shrink-wrap a chick that had already pipped.

How to correct humidity mid-incubation

First, confirm your instrument before you chase a reading. Calibrate the hygrometer (a salt test is the common method) and cross-check against a second one — a lot of 'humidity problems' are really a hygrometer reading 10–15 points off. Then adjust toward the air cell, not toward a percentage.

To lower humidity (air cell too small): reduce the open water surface — use fewer or smaller water channels, or none — and improve ventilation by opening vent plugs so moist air can escape. Some hatchers in humid climates run a 'dry hatch,' keeping days 1–18 quite low and relying entirely on the air cell to tell them it's right. To raise humidity (air cell too large): add water surface area (a wet sponge increases evaporation faster than a small cup), top up channels, and partially close vents.

Make one change at a time and re-candle in a day or two to see which way the air cell is trending — humidity corrections show up slowly. As a starting point for chicken eggs, extension sources put incubation humidity in the 45–55% band (Mississippi State cites roughly 58–60%; Missouri 60–65%), rising to 65–75% at lockdown (Missouri specifies 70% for the last three days). Set within that range, then fine-tune to the air cell. This entry is about environment only — for a chick's health after a hard hatch, see a poultry health resource such as PoultryDVM (https://www.poultrydvm.com); this guide does not offer medical or dosing advice.

Common pitfalls

Trusting the hygrometer number instead of the air cell — calibrate it and candle to a growth chart, because instruments drift.
Confusing humidity failures with temperature failures; both cause dead-in-shell, so verify your thermometer before blaming humidity.
Opening the incubator during lockdown, which drops humidity fast and can shrink-wrap a pipped chick.
Making several humidity changes at once — adjust one thing and re-candle, since air-cell trends take a day or two to appear.
Chasing a single 'perfect' percentage copied from someone else's setup rather than your own air-cell readings.

Frequently asked questions

How do I know if my incubator humidity is too high or too low?: Candle the egg and compare the air cell to a day-by-day growth chart. If the air cell is too small for the day, the egg isn't losing enough moisture and your humidity is too high — lower it. If the air cell is too large, the egg is drying out and your humidity is too low — raise it. The most objective version is weight: eggs should lose about 11–14% of their set weight by day 18. Because every hygrometer and room differs, the air cell (or weight loss) is a more reliable guide than the percentage on your display — calibrate the hygrometer, then adjust toward the air cell rather than a fixed number.
What humidity should I use for hatching chicken eggs?: Extension sources put incubation at roughly 45–55% for days 1–18 (Mississippi State cites about 58–60%; Missouri 60–65%), then 65–75% at lockdown (Missouri specifies 70% for the final three days). Treat these as a starting band and fine-tune to an air cell that reaches about a third of the egg by day 18.
What does a too-small air cell mean?: The egg is holding too much moisture, which means your humidity is too high. Lower it — fewer or smaller water channels and more ventilation — and re-candle in a day or two to confirm the air cell starts growing.
What does a too-large air cell mean?: The egg is losing water too fast, which means your humidity is too low. Raise it by adding water surface area (a wet sponge works quickly) and partially closing vents. A too-large air cell leaves the chick short of fluid and prone to shrink-wrapping at hatch.
Why did my chicks drown in the shell?: Fully formed, wet chicks that die around pip usually point to humidity that was too high during incubation — the egg never lost enough moisture, so there wasn't room for the chick to position and pip. Missouri Extension notes that excessive humidity 'will weaken the chick to the point that it can't emerge.' Verify temperature too, since it can cause similar late deaths.
What is shrink-wrapping and how do I prevent it?: Shrink-wrapping is when the shell membrane dries and shrinks tight around a pipped chick, gluing it in place so it can't zip. It's caused by humidity too low at hatch — often from opening the incubator during lockdown. Prevent it by raising humidity to 65–75% at lockdown, keeping the incubator closed, and confirming the air cell wasn't oversized going in.

Sources

Small Flock Series: Incubation of Poultry (G8353) — humidity, air cell & weight loss — University of Missouri Extension (University extension)
Important incubation factors (incubation vs hatch humidity) — Mississippi State University Extension Service (University extension)
Relationship of Air Cell Size to Shell Temperature and Weight Loss in Broiler Hatching Eggs — Mississippi State University Extension Service (University extension)