Crystal Load Capacitor Calculator

A crystal needs its two load caps to present the rated CL: CL = (C1·C2)/(C1+C2) + Cstray. With C1 = C2: C = 2·(CL − Cstray).

Usage: Enter the crystal's datasheet load capacitance and your board's stray; use NP0/C0G capacitors of the resulting value.

When you need it: Choosing the two load capacitors for a crystal so the oscillator starts reliably and runs on frequency, accounting for the board's stray capacitance.

Worked example: For a crystal specified at CL = 12 pF with ~5 pF of stray, each load cap is Cx = 2 × (CL − Cstray) = 2 × (12 − 5) = 14 pF, one from each crystal pin to ground.

Tips & gotchas:

• Wrong load capacitance pulls the frequency off by ppm — a common cause of clocks that are "almost" right.
• Include the ~3–5 pF of pin and PCB stray capacitance in the sum, not just the caps you place.
• Too much load can stop start-up or push the crystal's ESR margin past the drive level.
• Keep the oscillator traces short and guarded with ground; it's a high-impedance, noise-sensitive node.