Electronics · Free Calculator
Crystal Load Capacitor Calculator
Choose the two capacitors for a Pierce crystal oscillator so the crystal sees its specified load capacitance.
C1 = C2Load capacitanceStray capacitanceMCU crystal
pF
pF
C1 = C226 pF
Nearest E1227 pF
Resulting CL18.5 pF
How this result was calculated
- C1 = C2 = 2 × (CL − Cs) = 2 × (18 − 5) = 26 pF
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Crystal Load Formula
CL = (C1 × C2) ÷ (C1 + C2) + Cstray
With C1 = C2: C1 = C2 = 2 × (CL − Cstray)
With C1 = C2: C1 = C2 = 2 × (CL − Cstray)
CL is on the crystal data sheet. Stray capacitance (PCB traces plus the MCU pins) is typically 2 to 5 pF.
Worked example
An 18 pF crystal with 5 pF stray needs C1 = C2 = 2 × (18 − 5) = 26 pF; fit 27 pF.
Key insight: wrong load capacitors pull the frequency by tens of ppm, enough to upset UART baud rates and real time clocks, so do not simply copy 22 pF from another design.
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Frequently Asked Questions
How do I calculate crystal load capacitors?
C1 = C2 = 2 × (CL − Cstray), using CL from the crystal data sheet.
What happens if the capacitors are too large?
The crystal runs slightly slow and may be slower to start.
What stray capacitance should I assume?
3 to 5 pF is a common estimate for a compact layout.
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