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Quartz crystal parameters calculator

The number that is not in the datasheet

The motional capacitance Cm is needed both for a filter and for a VXO, and manufacturers usually do not quote it. What is more, the spread between specimens from one batch exceeds the difference between types, so taking it from somebody else's table is pointless — you have to measure it yourself. There are two ways, and both can be done at home.

LmCmRmC0fsfp
The series branch sets fs; the parallel holder capacitance pushes fp higher
Where Cm comes from
Series resonance fs (MHz)
Parallel resonance fp (MHz)
Holder capacitance C0 (pF)
Motional resistance Rm (Ω)
Resonator parameters
Motional capacitance Cm
Motional inductance Lm
Parallel resonance fp
Separation fp − fs
Q factor
C0/Cm ratio
Frequency pulling
Load capacitance from (pF)
to (pF)
Offset above fs
Tuning range

📐 The formulas​

From measured fs and fp (easiest with a NanoVNA):

Cm = C0 × ( (fp/fs)² − 1 )

The G3UUR method (an oscillator with a switched series capacitor):

Cm = 2 × (Cs + C0) × Δf / fs

Derived quantities:

Lm = 1 / ( (2π·fs)² · Cm )
Q  = 1 / (2π·fs·Cm·Rm)
fp = fs · √( 1 + Cm/C0 )

Pulling the frequency with load capacitance:

Δf = fs · Cm / ( 2 · (C0 + CL) )

A check point — a typical 10 MHz HC-49:​

Cm = 20 fF, C0 = 4 pF, Rm = 10 Ω → Lm = 12.67 mH, Q ≈ 79,600, fp = 10.024969 MHz (24.97 kHz higher).

Where this calculator stops:
  • The G3UUR method needs a warmed-up, stable oscillator: a hundred hertz of drift ruins the result, because Δf is itself only hundreds of hertz
  • C0 has to be measured separately, with an ordinary LC meter at a frequency well away from resonance
  • Rm is entered, not calculated: it is measured from the loss at resonance
  • You cannot pull the frequency below fs with capacitance — that needs a series inductor

🎯 In practice​

  • Before designing a crystal filter — without Cm it cannot be computed.
  • Designing a VXO: how far the crystal can realistically be stretched.
  • Selecting crystals from a batch — and seeing how alike they are.
Practical advice:
  1. A NanoVNA gives fs and fp straight off the trace — quicker and more accurate than the G3UUR method.
  2. Measure the whole batch in one sitting: you will see both the spread and which specimens suit a filter.
  3. An Lm in millihenries is not a mistake: a crystal is equivalent to a tuned circuit with an enormous inductance.
  4. C0/Cm for an ordinary crystal is about 200. A wildly different figure means a measurement error.

© 2026 UR3PKI · CyberDev.Space · Content licensed under CC BY-NC-SA 4.0.

How to cite this calculator
UR3PKI. «Quartz crystal parameters calculator». CyberDev.Space. https://cyberdev.space/en/radio/calculators/others/crystal_params (licence CC BY-NC-SA 4.0).

The licence lets you use this material freely, including in teaching materials, but only with attribution to the author and a link to the source.

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