Crystal Load Capacitance and PPM Calculator
Calculate equal crystal load capacitors, evaluate selected capacitor values, and convert oscillator tolerance between PPM and absolute frequency error.
Load capacitor design and evaluation
Calculate crystal load capacitors, evaluate selected C1 and C2 values, and convert oscillator tolerance between PPM and absolute frequency error.
PPM and frequency tolerance
Crystal capacitor and frequency results
The equal-capacitor result assumes the common two-capacitor Pierce-style approximation where C1 and C2 are in series from the crystal pins and stray capacitance is added to the effective load. PPM is a fractional frequency error. Absolute error in hertz scales with the oscillator centre frequency. MCU pin capacitance, PCB stray capacitance, crystal drive level, ESR, motional parameters, package parasitics, temperature curve, ageing, and oscillator topology affect final tuning.
Choose load capacitors and convert frequency tolerance
Crystal oscillator load capacitors are usually chosen from the crystal load capacitance and a stray capacitance estimate. Final tuning still depends on the oscillator circuit, PCB layout, and the actual crystal.
Equal capacitors
Solve the common C1 = C2 starting point from the crystal load capacitance.
Selected value check
Evaluate existing C1 and C2 values against the target load capacitance.
PPM context
Convert oscillator PPM tolerance into hertz and measured error back into PPM.
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Engineering reference
Equations, assumptions, and design guidance
Calculates crystal load capacitors and oscillator frequency tolerance with a first-pass parasitic capacitance estimate.
Equations and variables
CL = (C1*C2)/(C1+C2) + CstrayC1 = C2 = 2*(CL - Cstray)df = f0*PPM/1000000- CL
- Specified crystal load capacitance (F)
- Cstray
- Pin, package, and PCB stray capacitance estimate (F)
- f0
- Centre frequency (Hz)
Assumptions and limitations
Assumptions
- The equal-capacitor result uses the common two-capacitor Pierce oscillator approximation.
- The stray capacitance estimate is added to the series combination of C1 and C2.
Limitations
- MCU pin capacitance, PCB stray capacitance, crystal drive level, ESR, motional parameters, package parasitics, temperature curve, ageing, and oscillator topology affect final tuning.
Worked example and design use
16 MHz crystal with 18 pF load
Inputs: CL = 18 pF, Cstray = 3 pF
Outputs: C1 = C2 = 30 pF
Design guidance
- Use the result as a starting point, then check the crystal and MCU oscillator notes and tune with measured frequency where accuracy matters.