The model shown in the following figure can
be used to calculate correct load capacitor for a given crystal. This model includes
internal capacitors C_{Ln},
external parasitic capacitance C_{ELn} and external load capacitance C_{Pn}.

Figure 1. Crystal Circuit With Internal,
External and Parasitic Capacitance

Using this model the total capacitive load for the crystal can be calculated as shown in the equation below:

where C_{tot} is the total load capacitance seen by the crystal. This value should be equal to the load capacitance value found in the crystal manufacturer datasheet.

The parasitic capacitance C_{ELn} can in most applications be disregarded as these are usually very small. If accounted for, these values are dependent on the PCB material and PCB layout.

For some crystal the internal capacitive load provided by the device itself can be enough. To calculate the total load capacitance in this case. C_{ELn} and C_{Pn} are both zero, C_{L1} = C_{L2} = C_{L}, and the equation reduces to the following:

See the related links for equivalent internal pin capacitance values.