I've seen so much info on here about using the float test with late eggs that I feel like I just have to share my recent experience.
I had a broody. Unproven. She had 12 eggs, and by the end she had 9. She was due this past Saturday or Sunday, and had been kinda "eh" about it. She had crushed eggs and pooped in the nest (so eggs were dirty). I wasn't expecting a hatch.
Saturday morning I checked and found 1 chick, mostly out of the shell, but crushed. Not pecked, just crushed. Like she wanted him to stay in the egg.
I was crushed.
Sunday, seeing no pips and smelling something pretty bad, I decided to float test the eggs and toss the bad. I had 2 that floated about 5-10% out of the water, everything else sank.
So did my heart.
Then I heard a peep peep... from the bottom of the pot!
One of the sinkers had internally pipped (really dark shell, couldn't candle). Well, then there should be an air sac and he should float... Unless he's out of air!
I did emergency egg surgery and over the course of the next 4 hours got him hatched.
2 days later, this is a picture of my little chick: Lucky 2B Alive.

I put the rest of the eggs back under the bad broody, just in case. Eggtopsies Monday revealed that nothing else made it past day 17ish. That broody will never brood again, and I was intending to give her chicks but that's not gonna happen now. I worked really hard to save Lucky. I'm not about to let her kill him.
Anyway, so the float test did NOT work for me, or Lucky wouldn't be so lucky...
I had a broody. Unproven. She had 12 eggs, and by the end she had 9. She was due this past Saturday or Sunday, and had been kinda "eh" about it. She had crushed eggs and pooped in the nest (so eggs were dirty). I wasn't expecting a hatch.
Saturday morning I checked and found 1 chick, mostly out of the shell, but crushed. Not pecked, just crushed. Like she wanted him to stay in the egg.
I was crushed.
Sunday, seeing no pips and smelling something pretty bad, I decided to float test the eggs and toss the bad. I had 2 that floated about 5-10% out of the water, everything else sank.
So did my heart.
Then I heard a peep peep... from the bottom of the pot!
One of the sinkers had internally pipped (really dark shell, couldn't candle). Well, then there should be an air sac and he should float... Unless he's out of air!
I did emergency egg surgery and over the course of the next 4 hours got him hatched.
2 days later, this is a picture of my little chick: Lucky 2B Alive.
I put the rest of the eggs back under the bad broody, just in case. Eggtopsies Monday revealed that nothing else made it past day 17ish. That broody will never brood again, and I was intending to give her chicks but that's not gonna happen now. I worked really hard to save Lucky. I'm not about to let her kill him.
Anyway, so the float test did NOT work for me, or Lucky wouldn't be so lucky...