The fertility rate is the number of eggs that are fertile out of the number of eggs set.

For example:

If you set 120 eggs and upon candling (or use another method of your choice) you find that 30 of those eggs are infertile then your fertility rate is 75%. How do we arrive at that number? You take 120 and subtract 30 which leaves you 90. You would then divide 90 by 120 which gives you .75 then multiply .75 by 100 to get the percentage of 75%

120 - 30 = 90

90 / 120 = 0.75

0.75 x 100 = 75

Which translates to 75%

The hatch rate is the number of eggs that actually hatch out of the number of eggs that make it into the Hatcher.

For Example:

If out of th 90 eggs that make it to the hatcher 12 do not hatch then your hatch rate would be 87%. We arrive at that number by doing the same math with this new set of numbers. You would take 90 and subtract 12 which leaves you 78. You would then divide 78 by 90 which gives you 0.8666 (round up to 0.87) then multiply 0.87 by 100 to get a percentage of 87%

90 - 12 = 78

78 / 90 = 0.8666 (0.87)

0.87 x 100 = 87

which translates to 87%