Where do I find the crystal load capacitance?

CL is on the crystal manufacturer's datasheet, typically listed as "Load Capacitance" in pF. Common values are 6 pF, 8 pF, 12 pF, and 18 pF.

What is stray capacitance?

Stray (parasitic) capacitance is the unavoidable capacitance from PCB traces, vias, and the microcontroller's I/O pin input capacitance. It is typically 2–7 pF; check your MCU datasheet for pin capacitance and add 1–3 pF for trace parasitics.

Why must C1 and C2 be equal?

The Pierce oscillator is symmetrical. Equal capacitors present the same load to each side of the crystal, ensuring stable oscillation and minimizing frequency error.

What if the calculated value is negative?

A negative result means your stray capacitance already exceeds the crystal's required load capacitance. This is uncommon — re-check your CL and Cs values. You may need a crystal with a higher CL or to reduce PCB parasitics.

Can I use the nearest E12 value instead of the exact result?

Yes. A small deviation (within ±20% of the ideal value) is acceptable for most applications. The tool highlights the closest E12 and E24 standard values for convenience.

Crystal Load Capacitance Calculator

Enter your crystal's load capacitance and stray capacitance to find the two external capacitors needed for a Pierce oscillator.

Crystal & Circuit Parameters

Crystal Load Capacitance, C_L (pF)

From the crystal datasheet. Common values: 6, 8, 12, 18 pF.

Stray / Parasitic Capacitance, C_s (pF)

PCB trace + MCU pin capacitance combined. Typically 2–7 pF.

Common C_L Presets

Results

Ideal C1 = C2 —

Nearest E12 standard —

Nearest E24 standard —

Formula (Pierce Oscillator)

C1 = C2 = 2 × (C_L − C_s)

C_L is the crystal's required load capacitance. C_s is the total stray capacitance (PCB traces + MCU pin). C1 and C2 are the two external capacitors placed between each crystal pin and ground.

Effective Load Verification

Effective C_L with E12 caps —

Effective C_L with E24 caps —

Target C_L —

Summary

This calculator computes the external capacitor values (C1 and C2) required for a Pierce crystal oscillator. It applies the standard formula C1 = C2 = 2 × (CL − Cs), where CL is the crystal's specified load capacitance and Cs is the total stray capacitance from PCB traces and MCU pins. Results include exact values and the nearest E12/E24 standard capacitor sizes.

How it works

Enter the crystal's specified load capacitance (CL) in picofarads — find this on the crystal datasheet.
Enter your PCB stray capacitance (Cs), typically 2–7 pF depending on layout and microcontroller pin capacitance.
The calculator solves the Pierce oscillator formula: C1 = C2 = 2 × (CL − Cs).
The result shows the required capacitor value and the nearest standard E12/E24 values.
Use identical capacitors for C1 and C2 to keep the oscillator balanced.

Use cases

Selecting load capacitors for a microcontroller crystal (STM32, PIC, AVR, ESP32).
Debugging an oscillator that fails to start or drifts due to incorrect loading.
Verifying a schematic's capacitor values against the crystal datasheet.
Adjusting for board-specific stray capacitance measured or estimated from layout.
Teaching Pierce oscillator theory with an interactive example.

Frequently Asked Questions

Last updated: 2026-06-11 · Reviewed by Nham Vu