# Load capacitance for a microcontroller crystal?

I know there are lots of similar questions on here, but I've read many of them and am still a little confused.

I'm a student making a board similar to the Arduino Pro Micro, using a Atmega32U4 microcontroller. SparkFun's version shows the crystal circuit as this: The above iamge shows 22pF capacitors being used, and I'm trying to determine what the load capacitance of the crystal should be.

I found the formula for the capacitors to be: CL = (C1 * C2) / (C1 + C2) + Cs

If I assume the stray capacitance is 5pF (seems to be a somewhat standard assumption) and use 22pF capacitors, then the above equation gives: CL = (22*22)/(22+22) + 5 = 16pF

So if I want to use 22pF capacitors in the circuit, I need a crystal with a load capacitance of 16pF, correct? Looking on Digi-Key, the closest I see is 18pF.

If using an approximate value for the crystal load capacitance (such as 18pF when 16pF is calculated) is unacceptable, then I could go in the other direction, first choosing the crystal and based on it's load capacitance calculate the required capacitors values.

For example, I see a crystal with 8pF load capacitance, so using the above equation I can calculate CL = 6pF. However, every single schematic I see for an Arduino or Atmega circuit shows 18pF, 20pF, or 22pF capacitors being used, which makes me question whether lower capacitor values such as 6pF are acceptable?