On this page
- Why does incubator humidity matter?
- Humidity by species
- The 12–14% weight-loss method
- Reading the air cell
- What is dry hatching, and who should try it?
- How to raise humidity in an incubator
- How to lower humidity in an incubator
- Hygrometer salt test
- Wet-bulb vs humidity table
- Signs humidity was wrong
- Frequently asked questions
Keep incubator humidity at 40–50% for chicken eggs on days 1–17, then raise it to 65–70% at lockdown on day 18. Ducks need 55–60%, rising to 65–75% from day 25. Coturnix quail need 45–50%, rising to 65–70% from day 15. The number is only a guide: what matters is that eggs lose about 12–14% of their weight by lockdown.
Why does incubator humidity matter?
Humidity controls how fast water evaporates out through the eggshell. As the egg loses water, the air cell at the blunt end grows. The chick needs that air cell to be the right size: it breathes from it for the first time at internal pip, and it needs room to move into hatching position.
- Too humid: eggs lose too little water. The air cell stays small, the chick is large and wet, and it may drown in fluid when it pips.
- Too dry: eggs lose too much water. The air cell is huge, the chick is small, and the membrane can dry and shrink onto it, trapping it in the shell.
Humidity problems are the most common reason home hatches fail. Wrong temperature is a close second, which is why the step-by-step hatching guide starts with checking both.
Incubator humidity by species and phase
All species incubate at 99.5°F (37.5°C) in a forced-air incubator. Humidity is lower for most of incubation and higher for the final three days.
| Species | RH until lockdown | Lockdown RH | Lockdown day | Hatch day |
|---|---|---|---|---|
| Chicken | 40–50% | 65–70% | 18 | 21 |
| Bantam chicken | 40–50% | 65–70% | 17–18 | 20–21 |
| Duck (most breeds) | 55–60% | 65–75% | 25 | 28 |
| Muscovy duck | 55–60% | 65–75% | 32 | 35 |
| Goose | 50–55% | 70–75% | 3 days before hatch | 28–33 |
| Turkey | 50–55% | 65–70% | 25 | 28 |
| Coturnix quail | 45–50% | 65–70% | 15 | 17–18 |
| Bobwhite quail | 45–50% | 65–70% | 21 | 23–24 |
Many duck and goose keepers also cool and mist eggs once a day from about day 10 until lockdown, which helps in dry climates. Species details are in the duck egg and quail egg incubation guides.
The 12–14% weight-loss method: are your eggs drying at the right rate?
Weighing a few eggs is the most accurate way to check humidity, because it measures the result rather than a sensor reading. Chicken eggs should lose about 12–14% of their starting weight by day 18, or roughly 0.7% a day.
How to do it
- On day 0, number 4–6 eggs with a pencil and weigh them together on a kitchen scale that reads to 1 g (0.1 g is better).
- Write down the total. Divide your target loss by your lockdown day to get the expected loss per day.
- Reweigh the same eggs every 3–4 days, at about the same time of day.
- If they’re losing weight too slowly, lower the humidity a few percent. Too fast, raise it. Reweigh after 3–4 days to see the effect.
Worked example
Six chicken eggs weigh 348 g together on day 0. A 12–14% loss means they should weigh about 299–306 g on day 18.
| Day | Target loss so far | Target total weight |
|---|---|---|
| Day 0 | 0% | 348 g |
| Day 7 | 4.7–5.4% | 329–332 g |
| Day 14 | 9.3–10.9% | 310–316 g |
| Day 18 | 12–14% | 299–306 g |
Say the eggs weigh 335 g on day 7. That’s a 13 g loss, or 3.7%, behind the 4.7–5.4% target. Humidity is running a little high, so lower it by about 5% and reweigh on day 10 or 11. Small corrections in the first two weeks are easy; there is little you can do once lockdown starts.
For other species, use the same method and aim for the same 12–14% range by their own lockdown day. Treat it as a guide rather than an exact rule.
Reading air cell size when candling
If you don’t want to weigh eggs, candle them and look at the air cell. It should grow steadily: a shallow cap on day 7, about a fifth to a quarter of the egg by day 14, and roughly a third by day 18. These are approximate. Shell shape and egg size vary, so compare eggs in the same batch.
Pencil around the air cell edge each time you candle. If it’s barely moving between day 7 and day 14, humidity is too high. If it’s racing ahead, humidity is too low. The egg candling guide shows what else to look for at each stage.
What is dry hatching, and who should try it?
Dry hatching means adding no water to the incubator from day 1 to lockdown, then adding water to raise humidity to 65–70% for the hatch. The incubator simply runs at whatever humidity the room air gives it, often somewhere around 25–40%.
It works best for keepers in humid climates or with a humid room, where adding water pushes the incubator too high and the eggs lose too little weight. It also suits anyone whose last hatch showed signs of high humidity, such as small air cells and wet, sticky chicks.
- Run the incubator dry and watch the hygrometer for the first few days.
- Weigh eggs or check air cells. Dry hatching still needs checking, not guesswork.
- If humidity drops below about 25%, or eggs are losing weight too fast, add a small amount of water.
- At lockdown, fill the water channels to reach 65–70%.
Skip dry hatching in a very dry climate or a heated winter room, where the incubator may fall well below 25%.
How to raise humidity in an incubator
Humidity depends on how much water surface is exposed to warm air, not on how deep the water is. To raise humidity, add surface area.
- Fill more water channels or add a shallow dish of water.
- Add a clean, damp sponge or cloth to the water tray. It wicks water and increases the evaporating surface.
- Use warm water so you don’t chill the incubator while you refill.
- Refill from outside through a fill port or a length of aquarium tubing so you don’t open the lid, especially during lockdown.
- Close vents partly, but only within the manufacturer’s limits. Eggs need fresh air, especially at hatch.
- Raise the room humidity with a humidifier if the house is very dry.
How to lower humidity in an incubator
To lower humidity, take away water surface and let more dry air in.
- Remove water or empty one of the channels. Many keepers in humid areas use no water at all until lockdown.
- Open the vents a little more.
- Dry the room with a dehumidifier or air conditioning if the room itself is humid.
- Wait and remeasure. Humidity takes an hour or two to settle after any change.
Don’t chase the number. Opening the lid to fiddle with water every few hours causes bigger swings than a slightly off reading. Make one change, wait, and check egg weight or air cells before changing again.
Is your hygrometer accurate? The 75% salt test
Cheap digital hygrometers, including many built into incubators, can be off by 5–10% or more. The salt test checks yours at home in a few hours, because a damp salt paste in a sealed space holds the air at very close to 75% relative humidity.
- Put a few tablespoons of table salt in a bottle cap or small cup.
- Add a few drops of water and stir until it looks like wet sand. The salt must not dissolve completely.
- Seal the salt and the hygrometer together in a zip-top bag or airtight container. Don’t let them touch.
- Leave it at stable room temperature for 8–12 hours.
- Read the hygrometer. It should say 75%. If it reads 69%, it’s 6% low, so add 6 to every reading from now on.
Calibrate thermometers too: a hygrometer that’s spot-on is no use if the temperature is wrong. If both are suspect, an inexpensive second sensor is one of the best upgrades you can make. See our incubator recommendations for what to look for.
Wet-bulb readings and relative humidity
A wet-bulb thermometer has its bulb wrapped in a wick dipped in water. Evaporation cools it, and the drier the air, the lower it reads. Some incubators and older guides give humidity as a wet-bulb temperature instead of a percentage.
| Relative humidity | Wet-bulb °F | Wet-bulb °C | Typical use |
|---|---|---|---|
| 40% | ≈79 | ≈26 | Chicken, low end |
| 45% | ≈81 | ≈27 | Chicken, quail |
| 50% | ≈83 | ≈28.2 | Chicken high end, quail, turkey |
| 55% | ≈85 | ≈29.5 | Duck, goose, turkey |
| 60% | ≈87 | ≈30.5 | Duck, high end |
| 65% | ≈88.7 | ≈31.5 | Lockdown, all species |
| 70% | ≈90 | ≈32.3 | Lockdown, all species |
So for chicken eggs, a wet-bulb of roughly 79–83°F during incubation and about 89–90°F at lockdown matches the percentages above. Keep the wick clean and wet with distilled water, or the reading drifts high.
Signs of wrong humidity at hatch
The chicks and leftover eggs tell you how humidity went. Use these signs to adjust next time.
| Humidity too high | Humidity too low |
|---|---|
| Small air cells on day 18 | Very large air cells on day 18 |
| Large, soft-bellied, wet or sticky chicks | Small, dry chicks |
| Fluid at the pip hole; chicks drown after pipping | Tough, dry, white membrane that shrinks onto the chick |
| Unhealed, wet navels; bad smell | Chicks stuck in the shell, sometimes glued by dried membrane |
| Late, drawn-out hatch | Early pipping, then a stall |
If several of these line up, go through why didn’t my eggs hatch? to rule out temperature and fertility too.
Don’t open the lid to raise humidity at pip. Each opening drops humidity in seconds. Add warm water through a fill port or tube instead, and keep the lid shut until the hatch is over.
Frequently asked questions
What humidity should an incubator be for chicken eggs?
40–50% relative humidity for days 1–17, then 65–70% from lockdown on day 18 until the hatch is done. Use these as a starting point and check that the eggs lose about 12–14% of their weight by day 18, adjusting a few percent if they’re drying too slowly or too fast.
What humidity do duck eggs need?
Most duck eggs do well at 55–60% humidity until lockdown on day 25, then 65–75% until they hatch on about day 28. Muscovy ducks take 35 days, with lockdown on day 32. Daily cooling and misting from about day 10 is optional but helps in dry climates.
Is dry hatching better?
Not always. Dry hatching, with no water added until lockdown, suits humid climates and rooms where a normal setup runs too high. In dry homes it can push humidity below 25% and dry the eggs out. Whichever you choose, weigh the eggs or watch the air cells to confirm they’re on track.
How do I raise humidity quickly during lockdown?
Add warm water through an external fill port or a tube pushed through a vent, so the lid stays closed. A damp sponge in the water tray adds surface area and raises humidity quickly. Avoid opening the lid, which drops humidity sharply and can dry out the membranes of pipped eggs.
How accurate are incubator hygrometers?
Often not very. Built-in and cheap digital hygrometers can be off by 5–10% or more. Test yours with the salt test: damp table salt sealed in a bag with the hygrometer for 8–12 hours should read 75%. Note the difference and correct every reading by that amount.
What wet-bulb temperature is 50% humidity?
At 99.5°F dry-bulb, 50% relative humidity is a wet-bulb reading of about 83°F (28°C). Around 81°F is 45%, 85°F is about 55%, and 88–90°F covers the 65–70% lockdown range. Keep the wick clean and wet for accurate readings.