Crystal Load Capacitance Calculator
A quartz crystal hits its marked frequency only when the surrounding circuit presents the load capacitance it was cut for. In the common Pierce oscillator, two loading capacitors sit in series across the crystal, and board and pin stray capacitance adds to them. Enter the two loading capacitors and the stray capacitance to find the load capacitance the crystal sees. The calculator also reports the equal-capacitor value needed to match a target load.
Load capacitance formula
CL = (C1 * C2) / (C1 + C2) + Cstray
For equal caps C: CL = C/2 + Cstray
Matching cap for a target: C = 2 * (CL_target - Cstray)
The two loading capacitors are effectively in series as seen by the crystal, and stray capacitance from the board and pins adds on top.
Worked example
With C1 = C2 = 18 pF and stray = 4 pF: series part = (18 * 18) / (18 + 18) = 324 / 36 = 9 pF. CL = 9 + 4 = 13.00 pF. To instead hit a 20 pF target load with equal caps, C = 2 * (20 - 4) = 32.00 pF each.
Crystal load capacitance: frequently asked questions
What is crystal load capacitance?
Load capacitance (CL) is the total capacitance the oscillator circuit presents across the crystal. For the common Pierce circuit it is the series combination of the two loading capacitors plus stray capacitance: CL = (C1 C2) / (C1 + C2) + Cstray.
Why does load capacitance matter?
A crystal is cut to oscillate at its marked frequency only when driven with its specified load capacitance. If the circuit load differs from the crystal's rating, the actual frequency drifts slightly above or below the nominal value.
What is stray capacitance?
Stray capacitance is the unavoidable capacitance from PCB traces, pads, and the IC pins, typically around 3 to 5 pF. It adds to the load the crystal sees, so it must be included. The field is editable.
How do I pick the loading capacitors for a target load?
If the two loading capacitors are equal (C1 = C2 = C), then CL = C/2 + Cstray, so the required value is C = 2 (CL - Cstray). This calculator reports that matching capacitor value alongside the load.
Sources
- NIST: SI units (farad).
- The Pierce-oscillator load-capacitance relation is a standard result of crystal oscillator design. Stray capacitance is a user-editable estimate.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.