GuideIncubation · check

How to candle chicken eggs

Candling means shining a bright light through an egg to see what's developing inside. Three checkpoints: day 7, day 14, day 18. Day 7 catches infertile eggs and early death (blood rings). Day 14 confirms continued development. Day 18 is the final viability check before lockdown. Skipping candling means dead eggs sit in the incubator until day 21+, sometimes rotting and bursting and contaminating the batch.

Dark-shelled breeds (Marans, olive eggers) are mostly opaque to candler light and are the exception — for those, wait for hatch and only investigate eggs that haven't pipped by day 22.

What you need

Bright candler. $15–25 LED candler from a feed store or online (Brinsea OvaScope, Magicfly, generic poultry-supplier units). Or a 250+ lumen LED flashlight.
Dark room. Ambient light washes out the view. A closet, bathroom with the lights off, or any interior room without windows works.
Warm clean hands. Cold hands shock the egg. Cold incubator-to-cold-room transfer drops embryo temperature; minimize the time outside the incubator.
Egg cup or stable surface.Don't roll the egg or shake it. Hold blunt end up against the candler, gently rotate to see different angles.

Each candling session should take less than 60 seconds per egg. Long sessions (multiple minutes) cool the egg and risk development.

Stage-by-stage checklist

Day	What to look for	Note
Day 0–6	Don't candle.	Embryo too small; risk of disturbance > information gained
Day 7 (first candling)	Pea-sized dark embryo with radiating blood vessels (the 'spider')	Most common discard point — clears + blood rings removed here
Day 10–12	Embryo growing visibly; vessels denser; air cell expanding	Optional intermediate check
Day 14	Embryo fills ~half the egg; visible movement; clear air cell line	Second discard point — stopped developments removed
Day 18 (lockdown)	Egg is mostly dark with chick visible against air cell; large air cell at blunt end	Final go/no-go before lockdown begins
Day 19+	Don't candle — lockdown rules apply	Wait for hatch; humidity disturbance > info

Day 7 — the first candling

The most informative checkpoint. Look for one of three patterns:

Developing (keep). A small dark embryo (about pea-sized) at the top of the yolk, with blood vessels radiating out in a spider-web pattern. The vessels are red and clearly defined. You may see a brief twitch or movement from the embryo. Air cell at the blunt end is small but visible. This is normal day-7 development.
Clear / infertile (discard). No embryo, no vessels, just a pale yellow yolk shadow. The egg looks like it would have looked if you cracked it on day 0 — no development happened. Either the egg was infertile or development never started. Discard.
Blood ring (discard). A thin red ring of blood around the inside of the shell, often with no visible embryo. The embryo started but died early; the blood settled into a ring as circulation stopped. Genetic issue or temperature error. Discard.

Mark questionable eggs with a pencil and recheck them in 2–3 days. Some early embryos are harder to see than others; a recheck after another day or two often clarifies. Don't discard aggressively at day 7 — give borderline cases another chance.

Day 14 — mid-development check

The embryo now fills roughly half the egg. The vessels are dense and the air cell at the blunt end is visible as a clearly-defined pocket. Movement is often more obvious than at day 7 — kicks, body rotations, sometimes a flutter you can watch for several seconds.

What to discard at day 14:

No movement, no visible vessel network. Likely stopped at some point between day 7 and 14. Discard.
Brown or murky shell appearance. Indicates embryo death and bacterial breakdown. The egg is rotting and will burst if left in the incubator. Discard immediately.
Strong rotten smell.Don't even candle — discard. Burst rot eggs contaminate viable eggs in the same incubator.

Eggs with reduced movement but still showing a developing air cell get a benefit-of-the-doubt pass. Recheck at day 17 before lockdown.

Day 18 — lockdown candling

The final check before lockdown begins. By this point, viable eggs are mostly dark (the chick fills most of the shell), with a clear air cell at the blunt end. You may see the chick's beak silhouette against the air cell — sometimes the chick is already pipping internally.

Quick check: the air cell should be roughly 1/3 the way down the egg, slightly tilted toward one side. A small or absent air cell suggests humidity was too high during incubation (the embryo didn't lose enough moisture). An oversized air cell suggests humidity was too low.

Don't open the incubator after day 18 except for this single brief candling. Lockdown humidity disturbance is usually fatal at the wrong moment.

Common candling mistakes

Candling before day 7. Embryos are too small to see; you risk disturbance for nothing.
Candling under bright room light. Ambient light obscures the egg interior. Always use a dark room.
Sessions over 60 seconds per egg. Cooling risk increases as time outside the incubator stretches. Keep it brief.
Discarding aggressively at day 7. Some embryos develop slower; a second-chance recheck at day 9–10 often confirms development.
Trying to candle dark Marans / olive eggs. Mostly opaque to candler light. Wait for hatch.
Rolling or shaking the egg. Hold steady, rotate slowly. Vibration disturbs the embryo, especially during sensitive development phases.
Skipping the day 14 check. Stop-developments between day 7 and day 14 sit in the incubator until hatch otherwise — and they sometimes rot.

Frequently asked

When should I candle chicken eggs?

Three checkpoints: day 7 (first candling — confirm fertile and developing), day 14 (mid-development check — confirm still alive), and day 18 (lockdown candling — final viability check before stopping turns). Don't candle before day 7; the embryo is too small to see reliably and unnecessary handling adds risk. Don't candle during lockdown (day 18+) except briefly at start; opening the incubator drops humidity and stresses developing chicks.

What do I need to candle eggs?

A bright LED candler ($15–25 from any feed store or online), or substitute with a small LED flashlight (250+ lumens) held against the blunt end of the egg in a dark room. Smartphone flashlights work for white or light-tan eggs but struggle with dark Marans or olive-egg shells. Specialized candlers do the job better but aren't strictly necessary for backyard hatching. The room should be dark — ambient light overpowers the candler and obscures the contents of the egg.

What does a fertile, developing egg look like at day 7?

You should see visible blood vessels radiating out from a small dark embryo (about the size of a pea), forming a 'spider' pattern across the inside of the shell. The embryo is at the upper center; veins branch out from it in a network. Movement may be visible — a small twitch or jerk. The air cell at the blunt end is small but present. Anything matching this pattern is developing normally.

What does an infertile or stopped-development egg look like?

Infertile (a 'clear'): no embryo, no blood vessels — just a pale yellow yolk floating in the egg, looks like an unfertilized egg from the carton. Discard. Early-death blood ring: a thin red ring of blood around the inside of the shell with no embryo or vessels — embryo started developing but died, usually due to temperature or genetic issue. Discard. Stopped at later stage: dark embryo with no movement, often a clear line where development halted, no growing veins. Discard. Don't keep guessing on questionable eggs past day 14 — they often rot in the incubator and can burst.

Should I candle dark-shelled eggs (Marans, olive)?

It's much harder. Dark Marans shells block 80%+ of candler light; olive eggs are similarly opaque. You can sometimes see the air cell and a hint of the embryo silhouette but rarely see vessels clearly. Two workarounds: (1) accept that dark eggs are mostly opaque to candling — wait for hatch and only candle what's clearly not pipped by day 22; (2) use a more powerful candler (commercial-grade 500+ lumens) that can punch through the pigment. For most backyard keepers with dark-egg breeds, fewer candling checkpoints and waiting for hatch is the practical answer.

Should I candle every egg or just questionable ones?

Candle every egg at days 7 and 14 — discarding clear or stopped eggs early prevents rot in the incubator and frees up space and humidity-stability. By day 18 lockdown, you should have only viable eggs in the incubator. Skipping candling and letting all eggs go through to day 21 means a higher chance of one rotting and bursting (which contaminates the entire batch), plus wasted time and humidity managing dead eggs. The 5-minute candling pass at days 7 and 14 saves the hatch.

By Jimmy L Wu. Reviewed 2026-05-01. Day-7 / day-14 / day-18 candling checkpoints and the embryo / blood-ring / clear visual patterns are settled across Cooperative Extension hatchery publications and university poultry-program references (notably University of Maryland Extension FS-1114 and Mississippi State Extension incubation guides). Air-cell-size humidity diagnostic (small = high humidity, large = low humidity) reflects extension-published incubation troubleshooting. Dark-shell breed candling difficulty is practitioner consensus rather than a published research finding — labeled HatchMath methodology. Not veterinary advice — for unusual or recurring hatch failures, consult an avian or livestock veterinarian, or your county Cooperative Extension office.