How to incubate chicken eggs: temperature, humidity, 21 days

Most failed home hatches don't fail because the keeper picked the wrong temperature or humidity. They fail because nothing was actually held at the setpoint — the thermostat drifted, the hygrometer was off by 8 points, the door got cracked open at lockdown. The 21-day chicken-egg timeline is settled science. What you're really running is a 21-day discipline check on your instruments and your patience — calibrate hard, then leave the box alone.

The numbers below — 99.5°F forced-air, 55–60% humidity days 1–18, 70% at pip, 5–6 turns/day, stop turning day 18 — come from University of Maryland Extension FS-1114, “Hatching Eggs at Home”¹, cross-confirmed against Mississippi State²and Texas A&M AgriLife B-6092⁴ where they speak to the same setpoint. Where two publications differ, I take the more conservative figure.

Three calibration moves before any eggs go in

Most batch failures are baked in before the eggs even arrive. The three preflight moves below take an evening and prevent the failure modes that show up at day 7, day 14, and day 21:

• Don't trust the built-in thermometer. Incubator thermostats drift, and the stick-on hygrometers that ship with most home models read several points off out of the box. Run the incubator empty for 24 hours with a separate calibrated digital thermometer-hygrometer at egg level. If the readings disagree, trust the external instrument and adjust the setpoint until the egg-level number matches the target. Skip this and you're flying blind for 21 days.
• Use eggs under 7 days old, full stop.Hatchability falls off a cliff past 7–10 days of storage; by two weeks the rate is poor enough that a batch of older eggs is wasted incubator time. Cool storage (50–60°F) and daily turning before incubation buys you a few extra days but doesn't reset the clock. If a friend offers you eggs from a fridge — politely decline.
• Wide end up, every time. Lightly pencil-mark the wide end of each egg before loading. Chicks pip from the air cell, which sits at the wide end, and eggs incubated narrow-end-up have measurably lower hatch rates. The auto-turner cradle in most incubators only runs one direction — load it wrong once and the whole batch is upside down for 18 days.

Pick 99.5°F and don't chase the dial

UMD FS-1114 specifies 99.5°Ffor forced-air incubators — the type with a fan, which is what most modern home models are. Still-air incubators (radiant heat, no fan) need a higher reading at egg-surface level, around 101–102°F, because air stratifies vertically when nothing is mixing it. If you can't tell which kind you have, look for a fan; if there is one, you're running forced-air and 99.5°F is the number.

The tolerance is tighter than it sounds. Spikes above 102°F kill embryos within hours. Sustained operation below 98°F drags out development and produces a sloppy late hatch with elevated mortality. A drifting thermostat is what does most of the damage I've read about on backyard forums — the keeper sets 99.5 on day 1, the thermostat creeps to 100.8 by day 10, and the day-14 candling looks fine because the failure mode is delayed. Recheck the egg-level reading every few days; don't recheck the setpoint dial.

At lockdown some keepers drop the setpoint to 99°F to compensate for the chick's rising metabolic heat. FS-1114 doesn't mandate this and I'd skip it on a first run — one less variable to chase while the hatch is happening. Pick a number on day 1 and hold it. Consistency outranks precision here.

Run humidity dry-then-wet, not steady

The trap most beginner guides skate past: humidity failures in the two phases produce opposite problems, and they look like the same problem at the wrong time. Run too humid early and the air cell stays small — the chick drowns at hatch trying to absorb fluid it shouldn't be carrying. Run too dry late and the inner membrane shrink-wraps onto the chick and locks it in the shell. The fix isn't to split the difference; it's to deliberately run drier in phase one and wetter in phase two. UMD FS-1114's two phases:

• Days 1 through 18: 55–60% RH.The egg loses water through the shell at the right rate so the air cell grows on schedule. Hold it at the lower end of the range if your day-18 candling shows the air cell coming up short. I'd rather run a day-7 incubator on the dry side of 55% than the wet side of 60% — the air cell tells you at day 18 whether you got it right, and small-cell failure (drowning) is uglier than oversized-cell failure (a bit of dehydration that the chick generally survives).
• Day 18 onward (lockdown): 70% RH.The chick is breaking through the inner membrane into the air cell and then through the shell. The membrane has to stay flexible to let it rotate. Bump humidity at the first pip — some keepers preemptively bump on day 18, which is what I'd do; you don't lose anything by being early and you risk a shrink-wrap if you wait for a pip you missed.

The hatch-time killer is keeping humidity too low at lockdown. Top up the incubator's reservoir on day 17, add a damp sponge if your model runs dry, and don't open the incubator until the hatch is over. Every door-crack drops humidity within seconds; how much exactly depends on the room, but every extension publication agrees on the rule. The hygrometer reading in the first ten seconds after closing the door isn't the recovery — it's the bottom of the dip.

Buy the auto-turner — don't hand-turn 5×/day for 18 days

UMD FS-1114 specifies 5–6 turns dailyfor days 1–17; some other extension publications quote a minimum of three. The odd number matters because an even count lands the egg back in its starting overnight position every night, which defeats the point of turning. Most modern home incubators ship with auto-turner cradles that rock the eggs every 1–2 hours, which clears the bar comfortably. If yours doesn't come with one, buy it as an add-on; it's $30–60 and it eliminates the single highest-effort daily task in the 21-day cycle. I wouldn't hand-turn a batch unless I had to.

If you're hand-turning anyway, mark each egg with an X on one side and an O on the other so you can see at a glance which got turned this round. Set a phone alarm — you will forget at least once per cycle, and a missed turn is a survivable error; missing five in a row isn't.

Stop turning at day 18, hard.The chick is positioning itself for hatch, head pointed into the air cell. Continued turning past lockdown disorients it and tanks hatch rates. On an auto-turner, pull the cradle out and lay the eggs directly on the wire floor so they don't roll into one corner.

Candle on day 7 — pull the clears, don't carry them

Candling means shining a bright LED through the shell in a dark room to see whether the embryo is developing. It's the only way to identify duds before they rot and stink up the whole incubator. Pull the clears every time — there's no payoff in carrying them “just in case.” A clear at day 7 is a clear at hatch.

• Day 7: clear eggs (no embryo development) show up as evenly translucent. Fertile, developing eggs show a small dark spot with thread-like blood vessels radiating outward. Pull the clears.
• Day 14: the embryo fills most of the egg. Fertile, developing eggs are mostly dark with the air cell visible at the wide end. Eggs that have stopped developing (“blood rings” or no movement at all) get pulled.
• Day 18: optional final candling before lockdown. Confirms air cell size is correct. If the air cell is much smaller than expected, humidity has been running high earlier in the cycle.

The MSU “Important incubation factors” page³and Texas A&M B-6092 both cover candling milestones with photos useful for identifying normal vs abnormal development.

Lockdown — close the box on day 18 and walk away

The day-18 transition is four moves, in this order:

1. Stop turning. Pull the auto-turner cradle.
2. Raise humidity to 70% — preemptively, not at first pip. The pip you don't see is the one that shrink-wraps.
3. Leave temperature at 99.5°F. Skip the optional 99°F drop on a first run; one less variable in flight.
4. Close the incubator and don't open it.

The urge to peek when the first chick pips is enormous. Ignore it. Every door-crack drops humidity instantly and can trigger shrink-wrapping in chicks that haven't pipped yet. A pipped chick that takes 24 hours to fully emerge is normal — slow is normal. If a chick pipped 36+ hours ago and has clearly stopped progressing, intervention may be warranted, but help should come after the hatch is done, not during. The cost of one assisted-hatch attempt mid-cycle can be the rest of the batch.

Hatch typically runs from day 20 morning through day 21 evening, with slower-developing breeds (cochins, brahmas) sometimes stretching to day 22. Anything still unhatched on day 24 isn't coming.

Leave chicks in the incubator until they're fully dry

Newly hatched chicks need 6–12 hours in the incubator to dry off before moving to the brooder. They're done absorbing the yolk sac and don't need food or water during that window — the absorbed yolk sustains them for up to 72 hours, which is why mail-order day-old chicks survive shipment. Resist the urge to move a wet chick to a fluffy brooder; a damp chick chills fast outside the incubator and a chilled chick won't eat.

Once chicks are dry and fluffy, move them to a brooder pre-heated per UMN Extension's “Raising Layer Chicks and Pullets” guidance⁶: 90–95°F at chick height for week 1, drop 5°F per week through week 6. The brooder requires its own setup — the brooder heat lamp wattage calculator covers the temperature schedule, advisory wattage range, and mandatory safety checklist.

Skip the auction eggs — buy from a known flock

USDA APHIS Defend the Flock's⁵ recommendations for backyard incubation come down to three calls. Get these wrong once and you can contaminate the incubator (and an existing flock) for years:

• Source from a known flock.Auction eggs and unknown-seller hatching eggs can carry Salmonella Pullorum, Mycoplasma, and other pathogens that survive the incubator and infect every future batch. The price difference vs eggs from an NPIP-certified breeder isn't worth the disease risk on a backyard scale.
• Disinfect with a poultry-safe sanitizer between batches. Hot soapy water alone doesn't reach the porous interior surfaces; pick something labeled for poultry equipment (Virkon S, Tek-Trol) and follow the dwell time on the bottle.
• Quarantine hatched chicks 30 days before mixing. Even chicks from a clean source carry environmental microorganisms an established flock hasn't seen. Thirty days is the standard; I'd hold to it even when chicks look healthy on day 14.

When the hatch fails — read it from the timing

Where in the cycle the failure shows up tells you which setpoint was wrong. Use this table at the post-mortem, not as a worry list during the run:

Common questions

Related calculators and pages

• Brooder heat lamp wattage calculator →
• Coop ventilation calculator →
• Coop size + run space calculator →
• Feed amount calculator →
• Coop ventilation explained →
• Methodology + sources →

1. 1. University of Maryland Extension FS-1114 — Hatching Eggs at Home — primary anchor for 99.5°F, 55-60% humidity days 1-18, 70% at pip, day-18 turning stop, 5-6 turns daily, 21-day timeline. ↩
2. 2. Mississippi State University Extension — Care and Incubation of Hatching Eggs — cross-confirmation. ↩
3. 3. MSU Extension — Important incubation factors — candling and failure-mode framing. ↩
4. 4. Texas A&M AgriLife Extension B-6092 — Incubating and Hatching Eggs — cross-confirmation. ↩
5. 5. USDA APHIS Defend the Flock — biosecurity framing (source flocks, incubator disinfection, chick quarantine). ↩
6. 6. UMN Extension — Raising Layer Chicks and Pullets — post-hatch brooder handoff (90-95°F at chick height for week 1). ↩

The 21-day timeline, 99.5°F forced-air setpoint, 55-60% / 70% humidity split, and day-18 turning stop are all in UMD FS-1114 specifically. Where the secondary publications use slightly different ranges, this guide uses UMD's numbers as the primary anchor. By Jimmy L Wu. Reviewed 2026-05-12. Not veterinary advice — for sick chicks or any animal-health emergency, consult an avian or livestock veterinarian, or your county Cooperative Extension office.