Crystal Load Capacitance Calculator
A quartz crystal only oscillates at its rated frequency when the circuit presents the load capacitance stated on its datasheet. In the common Pierce oscillator, two external capacitors sit in series across the crystal, and a stray capacitance from the board and pins adds in parallel. This calculator solves the standard relation for the matched capacitor value you need on each side, given the target load capacitance and an estimate of stray capacitance, and also reports the resulting effective load to confirm the match.
Crystal load capacitance formula
CL = (C1 * C2) / (C1 + C2) + Cstray
For matched capacitors C1 = C2 = C:
C = 2 * (CL - Cstray)
Series part = C / 2; Effective load = C/2 + Cstray
The two external capacitors appear in series to the crystal, so their combined series capacitance is C divided by 2 when matched. Adding stray capacitance gives the effective load the crystal sees.
Crystal oscillator facts
- Common datasheet load capacitances are 12, 18, and 20 picofarads.
- Stray capacitance typically ranges from about 3 to 5 picofarads.
- Matched load capacitors keep the Pierce oscillator symmetrical.
- Excess load capacitance pulls the frequency slightly low; too little pulls it high.
- If the target CL is less than the stray capacitance, no positive capacitor value exists.
Crystal load capacitance: frequently asked questions
What is crystal load capacitance?
Load capacitance (CL) is the total capacitance the oscillator circuit must present to the crystal so it resonates at its specified frequency. The crystal datasheet states a target CL, commonly 12, 18, or 20 picofarads. Getting close to this value keeps the oscillation frequency on target.
How are the two load capacitors calculated?
In a Pierce oscillator the two external capacitors C1 and C2 appear in series across the crystal, plus a stray capacitance. The relation is CL = (C1 * C2) / (C1 + C2) + Cstray. When C1 equals C2, each capacitor equals 2 * (CL - Cstray). This calculator returns that matched value.
What stray capacitance should I assume?
Stray (parasitic) capacitance comes from the PCB traces, pin capacitance, and microcontroller oscillator inputs. A typical assumption is 3 to 5 picofarads, but it depends on layout. This calculator exposes stray capacitance as an editable input so you can use your measured or estimated value.
What happens if the load capacitance is wrong?
If the effective load capacitance is too high the crystal oscillates slightly below its nominal frequency; too low and it runs slightly high. Large errors can also affect startup margin. Matching the datasheet CL keeps frequency error to a few parts per million.
Do C1 and C2 have to be equal?
They do not have to be equal, but matched capacitors are common and simplify the math and the symmetry of the oscillator. If you choose unequal values, use the series formula CL = (C1 * C2)/(C1 + C2) + Cstray and adjust until the result matches the datasheet target.
Official sources
- NIST: Time and Frequency Division: quartz oscillators.
- IEEE: IEEE standards for piezoelectric crystal terminology.
Reviewed by the CalculatorHub team, edited by James Graham, 16 June 2026. See our methodology.