PreBuild Engineering
PCB calculators / Circuit design
Free tool · PreBuild Engineering

Crystal load capacitor calculator

Get the two capacitors that make your crystal run on frequency.

Inputs

pF
From the crystal datasheet
pF
Typically 2–5 pF; check the MCU datasheet

Result

—

Why it matters

Wrong load capacitors pull the crystal off frequency (bad for UART, USB and RF timing) or stop it from starting reliably at temperature extremes.

Formula

CL = (C1 · C2)/(C1 + C2) + C_stray With C1 = C2 = C: C = 2 · (CL − C_stray)

Design tips

  • Keep the crystal and its capacitors close to the MCU, with a ground guard and no signals routed underneath.
  • A common mistake is using CL itself as C1 and C2.

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Results are engineering estimates from standard formulas and published approximations. Always confirm against component datasheets, your fabricator's capabilities and, for critical designs, simulation or measurement.