Enter your crystal's target load and stray capacitance to find equal C1 and C2 values, or check the effective load of an existing pair.
This is the calculated value for each equal external capacitor. Component tolerance, board parasitics, startup margin, drive level, and measured frequency still need separate checks.
Compare this simplified result with the crystal datasheet load capacitance. A matching value alone does not prove startup margin or final frequency accuracy.
Table of contents
How to use our Crystal Load Capacitance Calculator
- Choose "Find matched capacitors" if you need equal C1 and C2 values, or choose "Check an existing pair" if your schematic or BOM already lists both capacitors.
- For "Find matched capacitors", enter the crystal's target load capacitance, CL, from its datasheet and your total stray capacitance estimate, Cs, in pF.
- For "Check an existing pair", enter the existing C1 and C2 values plus total stray capacitance, all in pF.
- Click "Calculate". In "Find matched capacitors", the first result is the mathematical value for each equal capacitor. In "Check an existing pair", compare the effective load capacitance with the crystal datasheet CL value.
- If you select a nearby standard capacitor value, calculate again with that value or pair to see the resulting effective load.
Definitions
Load capacitance (CL): The capacitance the crystal is specified to see in its oscillator circuit.
Stray capacitance (Cs): Unintended capacitance from oscillator pins, PCB traces, pads, and nearby circuit effects, entered as one total estimate.
C1 and C2: The two external capacitors used with a typical Pierce crystal oscillator.
pF: Picofarad, a very small capacitance unit equal to one trillionth of a farad.
Series capacitance: The combined contribution of C1 and C2 in this equation. It is smaller than either capacitor alone.
Common mistakes and quick fixes
Mistake: Entering the crystal datasheet CL value as C1 or C2.
Fix: CL is the capacitance the crystal should see. C1 and C2 are the external capacitors used to create that load with Cs included.
Mistake: Leaving out stray capacitance or entering a negative value.
Fix: Enter zero only when you intentionally want to ignore stray capacitance. Otherwise use a nonnegative estimate based on the oscillator device and board.
Mistake: Assuming equal capacitors are required in every circuit.
Fix: Equal values are an assumption used by "Find matched capacitors". Use "Check an existing pair" to calculate a design with different C1 and C2 values.
Mistake: Treating a matching load result as proof that the oscillator will work.
Fix: Also check the device guidance, startup margin, crystal drive level, capacitor tolerance, and frequency on the finished circuit.
Limitations & Key Assumptions / Boundary Conditions
- The calculation uses a simplified Pierce-oscillator load model with one total stray-capacitance value.
- "Find matched capacitors" assumes C1 and C2 are equal. Use "Check an existing pair" when they differ.
- Stray capacitance depends on the oscillator device, package, PCB layout, and measurement method, so an estimate may not match the finished board.
- A matching calculated load does not verify oscillator startup margin, crystal drive level, capacitor tolerance, temperature behavior, or final frequency.
- Choose fitted capacitor values using the oscillator or crystal manufacturer's circuit guidance, then verify the actual circuit.
Methodology
Load-capacitance equation
The calculator uses the simplified relationship between the two external capacitors, total stray capacitance, and the load seen by the crystal.
CL = (C1 * C2) / (C1 + C2) + Cs
CL is target or effective load capacitance, C1 and C2 are external capacitors, and Cs is total stray capacitance. All values are in pF. C1 and C2 must be greater than zero; Cs may be zero but cannot be negative.
Finding equal capacitors
When C1 and C2 are equal, the equation simplifies so each capacitor is twice the difference between target load capacitance and stray capacitance.
C1 = C2 = 2 * (CL - Cs)
For CL = 12.5 pF and Cs = 2 pF, each matched capacitor is 2 * (12.5 - 2) = 21.00 pF. The target CL must be greater than Cs; otherwise the calculation would require zero or negative external capacitance.
Checking an existing pair
For installed capacitors that may differ, the calculator keeps C1 and C2 separate. With C1 = 18 pF, C2 = 22 pF, and Cs = 2 pF, the effective load is (18 * 22) / (18 + 22) + 2 = 11.90 pF.