Chicken eggs need a steady temperature of 99.5 to 100°F (37.5 to 37.8°C) in a forced-air incubator for the full 21-day incubation period. If you’re using a still-air incubator, set it higher: 101 to 102°F (38.3 to 38.9°C). That difference matters more than most beginners realize, and getting it wrong by even a few degrees can kill embryos or produce weak chicks.
Forced-Air vs. Still-Air Settings
The type of incubator you’re using determines your target temperature. A forced-air incubator has a built-in fan that circulates air evenly, so the temperature is roughly the same everywhere inside the unit. For these, aim for 99.5 to 100°F. Most experienced hatchers split the difference and hold at 99.5°F.
A still-air incubator has no fan, which means heat rises and layers inside the box. The top of the incubator can be several degrees warmer than the bottom. To compensate for this layering, you need to set the temperature 2 to 3°F higher, landing at 101 to 102°F. The goal is for the eggs themselves, sitting on the tray or turner, to experience roughly 99.5 to 100°F at their level even though the air above them is warmer.
Where to Place Your Thermometer
Your readings are only useful if your thermometer is in the right spot. In a still-air incubator, place the thermometer on the wire floor where you can read it through the viewing window. Because temperature increases the closer you get to the top of a still-air unit, a thermometer mounted high on the wall will give you a misleadingly warm reading. Keep it at egg height or slightly above the eggs.
In a forced-air incubator, placement is less critical because the fan distributes heat more evenly, but centering the thermometer near the eggs is still good practice. Regardless of incubator type, use a separate thermometer to verify your built-in gauge. Cheap incubators are notorious for inaccurate factory thermometers, and a reading that’s off by even 1 to 2°F can affect your hatch rate.
The Lockdown Period: Days 18 Through 21
On day 18, you stop turning eggs and enter what’s called “lockdown.” At this stage, the developing chick is positioning itself to pip through the shell, and the incubator should stay closed as much as possible. Many hatchers reduce the temperature slightly during these final three days, dropping to around 98.5 to 100°F (36.9 to 38.1°C). The reasoning is straightforward: by day 18, the embryo is large enough to generate its own metabolic heat. A cluster of near-term eggs can actually warm the incubator from the inside, so dialing back the thermostat a fraction helps prevent overheating.
Humidity also increases during lockdown, typically from around 45 to 50% up to 65 to 70%. Higher humidity keeps the membrane inside the shell from drying out and trapping the chick as it tries to hatch. While this article focuses on temperature, humidity and temperature work together. A spike in humidity can slightly affect how the thermostat reads, so monitor both closely during these final days.
What Happens When Temperature Goes Wrong
Chicken embryos tolerate brief, small fluctuations better than sustained ones. A momentary dip when you open the lid to turn eggs won’t cause problems. But consistent deviations tell a different story.
Running too hot is more dangerous than running too cool. Research on broiler embryos found that temperatures continuously elevated by about 5°F above standard during late incubation caused significantly higher embryo mortality. The embryos that did survive were smaller, hatched later, and showed signs of metabolic stress, essentially being forced into a state of malnutrition because the excess heat disrupted how they used their yolk reserves. Hatchability dropped notably.
Interestingly, embryos exposed to temperatures a few degrees below standard during the same late-stage window fared much better. Their hatching was delayed, but their overall development and growth were strikingly similar to embryos kept at normal temperatures. The takeaway: if you’re going to err, err slightly cool rather than slightly warm.
Prolonged exposure to temperatures below about 95°F will slow development to the point where embryos may not survive, and temperatures above 104 to 105°F for more than a short period are typically lethal. Power outages are the most common cause of dangerous temperature drops. If your power goes out, keep the incubator closed. The insulated box and the eggs’ own thermal mass will hold warmth longer than you’d expect.
Stabilize Before You Set Eggs
One of the most common beginner mistakes is placing eggs into an incubator that hasn’t been calibrated. Run your incubator empty for at least 24 hours before adding eggs. This gives you time to adjust the thermostat, verify your thermometer, and confirm the temperature holds steady. Incubators, especially tabletop models, can be sensitive to the room they’re in. A unit sitting near a sunny window or in an unheated garage will struggle to maintain a consistent internal temperature.
Choose a location with a stable ambient temperature, ideally between 70 and 80°F. The incubator’s heating element works less hard in that range, which means fewer temperature swings. Avoid placing it in direct sunlight, near heating vents, or in drafty areas. Once your eggs go in, the thermal mass of the eggs themselves will actually help stabilize the environment, but the first few hours after setting can see some fluctuation as the eggs absorb heat and the thermostat compensates.
Quick Reference by Incubator Type
- Forced-air incubator: 99.5 to 100°F (37.5 to 37.8°C) for days 1 through 17. Drop to 98.5 to 100°F during lockdown (days 18 to 21).
- Still-air incubator: 101 to 102°F (38.3 to 38.9°C) measured at egg level for days 1 through 17. Reduce slightly for lockdown, keeping thermometer readings around 100 to 101°F at egg height.
- Humidity, days 1 to 17: 40 to 50% relative humidity (wet bulb reading of 85 to 87°F in a forced-air unit).
- Humidity, days 18 to 21: 65 to 70% relative humidity (wet bulb reading around 90°F).
- Turning: Turn eggs at least 3 times daily through day 17. Stop on day 18.