Candling Chicken Eggs: What You See on Days 1 to 21
What a candled egg looks like on each candling day, how to tell fertile from infertile, what a blood ring means, and how the air cell tells you your humidity is wrong.
Candle chicken eggs on days 7, 14 and 18. On day 7 a developing egg shows a dark spot with fine blood vessels radiating outward; a clear egg was never fertile, and a ring of blood means the embryo died in the first few days. By day 14 most of the egg is dark and the air cell at the blunt end is visibly larger. By day 18 the air cell takes up about a third of the egg — if it is much smaller than that, your humidity has been too high. Embryos are easiest to see from about day 8 onward, and a bright LED in a dark room works as well as a purpose-made candler.
Get candling dates for your own set date, or for ducks, quail and turkeys, from the incubation calendar calculator.
What you are looking at
Candling means shining a bright light through the shell so you can see inside without opening the egg. Three things are visible, and each tells you something different:
- The embryo — a dark mass that grows until it fills the egg.
- The blood vessels — fine lines radiating out from the embryo. Their presence is the clearest sign of a live embryo, and their absence is the clearest sign of a dead one.
- The air cell — a pocket at the blunt end that grows as moisture leaves the egg. Its size is a direct read on whether your humidity is right.
Day 7: is it fertile?
The first candling is the one that answers the question everybody wants answered.
A developing egg shows a small dark spot with fine blood vessels branching outward from it — Texas A&M describes it as a dark spot in the large end surrounded by a faint outline of blood vessels. The spot may drift away from the light as you turn the egg.
A clear egg is one of two things: infertile, or an embryo that died so early nothing formed. You cannot tell which by looking, and for practical purposes it does not matter. Remove it.
A blood ring is a red circle or smear with no embryo attached, and it is unambiguous: the embryo started and died. Texas A&M places that death in the first four days of incubation. A blood ring usually points at eggs that were too old at set, or a temperature problem in the first few days.
Expect some of each. Only 80 to 90 percent of incubated eggs are fertile to begin with, so a clear egg or two in a dozen is normal, not a failure of your incubator.
Remove clear eggs and blood rings when you find them. A dead egg left in a warm incubator can rupture and contaminate the rest of the clutch.
Day 14: is it still going?
By day 14 most of the egg is dark, because the chick now fills most of the space. You are looking for two things:
- The air cell is clearly bigger than it was on day 7.
- Movement. Hold still and watch for thirty seconds. A live embryo at this stage often shifts visibly.
What you will not see much of any more is the vessel detail — the embryo has grown enough to block most of the light. That is expected, and an egg that looks uniformly dark on day 14 is a good sign rather than a worrying one.
An egg that has gone from developing on day 7 to a loose dark mass with no vessels and no movement has quit somewhere in between. Texas A&M lists mid-incubation deaths under temperature problems, power failures, and eggs not being turned.
Day 18: the humidity check
Day 18 is the last candling before lockdown, and its real job is not counting live embryos. It is checking your humidity.
The air cell should take up roughly a third of the egg. That is the visible result of the egg losing moisture steadily across incubation — Mississippi State puts the target at about 12% of the egg’s starting weight lost by hatch, and the air cell is how you read that without a scale.
| What you see on day 18 | What it means | What to change next time |
|---|---|---|
| Air cell about a third of the egg | On track | Nothing |
| Air cell noticeably small | Humidity too high days 1–18 | Run drier early; verify with egg weights |
| Air cell unusually large, egg feels light | Humidity too low days 1–18 | Add water earlier |
An air cell that is too small is the more dangerous error. The chick needs that space to pip into and to take its first breath of air, and without it the odds of a chick that pips and then fails go up sharply.
After this candling, stop. Days 19 to 21 are lockdown: no turning, no opening, no candling. Full detail on the settings is in incubator temperature and humidity.
How to actually do it
A bright LED torch in a dark room. That is genuinely all you need — a purpose-built candler is more comfortable to hold and no more effective. Phone torches work but are on the dim side.
Cup your hand around the blunt end and press the light against the shell so light goes through the egg rather than around it. Tilt the egg rather than shaking it.
Be quick, and be gentle. Every minute out of the incubator is a minute of cooling, and Texas A&M warns that developing embryos have delicate blood vessels that rupture easily if eggs are jarred or shaken. Candle a few at a time rather than emptying the tray.
White eggs are far easier than brown. Texas A&M notes white eggs are easier to candle than dark or speckled ones. Very dark shells — Marans, Welsummer — can be close to impossible to read reliably.
When in doubt, leave it in. A doubtful egg costs you nothing but tray space. A wrongly discarded one costs you a chick. Only pull eggs you are confident about: clear, or an obvious blood ring.
What each candling day is actually for
| Day | Question it answers | Act on it by |
|---|---|---|
| 7 | Which eggs are fertile and developing? | Removing clear eggs and blood rings |
| 14 | Are they still alive? | Removing eggs that have quit |
| 18 | Is my humidity right? | Adjusting lockdown humidity, and next hatch’s early humidity |
Other species run to a different calendar — quail are candled around days 5, 10 and 14, ducks at 7, 14 and 25. The incubation calendar generates the dates for each. For what is developing on every day of the 21, see the day-by-day incubation chart.
Frequently asked questions
When should I candle chicken eggs?
Days 7, 14 and 18. Day 7 tells you which eggs are fertile, day 14 confirms they are still developing, and day 18 checks the air cell before lockdown. Do not candle after day 18.
What does a fertile egg look like when candled?
A dark spot with fine blood vessels radiating outward from it, sitting toward the large end. From about day 8 the embryo is easy to see; earlier than day 7 it is difficult even for experienced keepers.
What is a blood ring?
A red ring or smear with no embryo attached. It means the embryo started developing and died, within the first four days of incubation. Remove the egg — it can rupture and contaminate the incubator.
Can I candle brown eggs?
Yes, but it is harder, and very dark shells such as Marans or Welsummer may not be readable at all. A brighter light and a fully dark room help. If you cannot tell, leave the egg in.
How big should the air cell be at day 18?
About a third of the egg. Much smaller means humidity has been too high, which leaves the chick too little room to pip into. Much larger means it has been too dry.
Does candling hurt the embryo?
Not if you are quick and gentle. The risks are cooling and physical jarring rather than the light itself — developing blood vessels rupture easily if the egg is shaken.
Why are some of my eggs clear at day 7?
Most likely they were never fertile. Only 80 to 90 percent of incubated eggs are fertile, so one or two clear eggs in a dozen is a normal result rather than an incubator problem.
Sources
- Texas A&M AgriLife Extension — E-635, Hatching Eggs in the Classroom (PDF): embryos easily visible after 8 to 12 days; a live embryo as a dark spot in the large end surrounded by a faint outline of blood vessels; blood ring as a death within the first four days; 80 to 90 percent fertility; white eggs easier to candle than dark or speckled; removing infertile and dead eggs before they rupture; and the warning about rupturing blood vessels in jarred eggs.
- Mississippi State University Extension — poultry incubation guide (PDF): candling at 7, 14 and 18 days as the method for checking humidity by air-cell size, and the 12% egg weight loss target across incubation.
- Mississippi State University Extension — stages in chick embryo development: the developmental sequence behind what is visible on each candling day.
The illustration is our own, drawn to the proportions described in the sources above rather than traced from a photograph. Candling dates for chickens (7, 14, 18) match the incubation calendar; the schedules it uses for non-chicken species are our own spacing rather than published dates, and are labelled as such there.
Figures are planning guidance for backyard and classroom incubation, not veterinary advice. Last reviewed August 2026.