Part of it depends on how you calculate hatch rate. One part is hatchability. Some eggs are just never going to hatch. Some people think of these as not fertile but it’s a lot more than that. The overall health and nutrition of the parent flock plays a part. How the eggs are stored and handled play a part. If the eggs are stored too hot or too cold or even worse, warmed up then cooled off through a few cycles, the hatchability can go down, sometimes way down. Shipped eggs are notorious for poor hatchability because they often get shaken up no matter how well they are packed.
Then you have the part that depends on your incubator and incubation skills. That includes temperature, humidity, turning, and sanitation, air flow, and some others I’m sure I don’t know about. Sanitation is a cross-over. If the eggs are dirty to start with or if the incubator is not clean, bacteria can get inside the egg and multiply. That’s horrible. You don’t want an egg stinking or exploding.
The egg shell is porous so the developing chick inside can breathe, so it needs fresh air in the incubator to get good air exchange. That’s not real important in the first week or so, but the older the chick gets, the more it needs fresh air. In your forced air, that should not be a problem.
Then you have to remember that not all eggs that go into the incubator are exactly the same, even if laid by the same hen. You get differences in how and how long they are stored, and some have different porosities or shell thickness. The consistency of the egg white can vary, some are more runny than others. What may be perfect conditions inside the incubator for one egg may not be very good for another.
There are a whole lot more things involved in hatch rate than just humidity. You can have perfect humidity and still have a horrible hatch rate if something else is wrong. That’s why I try to analyze my eggs that don’t hatch, to see what went wrong and if I can correct something. It’s not always easy to figure out what actually went wrong. In most cases if the egg did not start to develop or stopped developing in the first week of incubation, it is because of something that happened before the egg went in the incubator. If it dies during the last week it is usually because of something to do with your incubation.
Averages don’t really mean anything unless you have enough data points to get a statistically significant number. The commercial hatcheries, the ones hatching maybe 1,000,000 chicks a week for the commercial broiler industry, using incubators that hold maybe 60,000 or even 120,000 eggs each usually hatch 90% of their eggs. About half of the failures have to do with the hatchability of the eggs, something that happened before the eggs went in the incubator. The other 5% has something to do with the actual incubation, though some of that is just due to the differences in the eggs that went in, the porosity or something like that. These huge incubators are generally housed in the same room. Each individual incubator has to be tuned to get the best hatch rate. They don’t set the humidity the exact same for each one, although they are in the same room. They tweak each one based on analysis of the unhatched eggs to get the best setting for that individual incubator, even if they are the same make and model.
I shoot for an average humidity of between 40% and 45% during incubation and 65% to 70% during lockdown, though when the chicks start to hatch, the humidity can get up as high as 90%. I don’t worry about it being too high during lockdown, I just don’t want it too low.
If you consider hatch rate to be eggs going into the incubator to chicks coming out, I’ve had between 33% to 100% on shipped eggs, which is the worst case. That 100% was 5 out of 5 eggs. With my own eggs I have between 70% to 100% but 100% is really rare and come from only doing a few at a time. I usually incubate 30 to 35 eggs at a time, though that can vary too. I put 14 of my own eggs in the incubator about 12 days ago. 3 of those were clear. We’ll see how many actually hatch. Those chicks will go under a broody when they hatch.
If you only look at the ones that make it past the first week (discount the clears) I usually have between 80% to 100%. With as few eggs as I incubate at a time, I get some wild percentage shifts.
I don’t know if you are going to try to understand this or not. Humidity is an important factor but it is not the only one. That’s why I suggest you pick something and try it, then adjust as you need too. There is science involved, but also some art.
