Crystal load capacitance calculator
Pick the load capacitors for a crystal oscillator: enter the crystal’s specified load capacitance (CL) and stray capacitance, and read the two matching load capacitors.
Crystal network
Load capacitors
Required load capacitor
C1 = C2 = 2·(CL − Cs − Ct)
Nearest standard value
E12, closest to 20 pF
Actual load with standard pair
(C·C)/(C+C) + Cs = 11 pF
Deviation from target CL
How far the standard pair sits from the datasheet CL
About the model
The crystal manufacturer specifies CL for the load network C_load = (C1·C2)/(C1+C2) + Cs, where C1 and C2 are the two pin-to-ground load capacitors and Cs is the stray/PCB capacitance seen at each pin. Two equal caps are standard because each pin sees ground symmetrically. Using 2·(CL − Cs) per pin cancels the strays so the network lands on CL; a wrong load shifts the oscillation frequency by up to tens of ppm.
The nearest E12 capacitor may land slightly off target — the deviation row above shows how far. For tight ppm budgets pick the next standard value and fine-tune with a trimmer.
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Frequently asked questions
How do I pick the load capacitors for a crystal?
The tool sizes the two pin-to-ground capacitors as C1 = C2 = 2 × (CL − Cs − Ct). For a crystal with a 12 pF load spec and 2 pF stray that is 2 × (12 − 2) = 20 pF per pin, and the nearest standard E12 value is reported with the load it actually presents.
What is load capacitance CL and where does it come from?
CL is the capacitance the oscillator network must present to the crystal, specified on its datasheet and typically 8 to 30 pF. The network load is C_load = (C1 × C2) / (C1 + C2) + Cs, so with two equal caps the actual load is C / 2 + Cs.
Why does a wrong load capacitance shift the crystal frequency?
The crystal oscillates slightly off its nominal frequency when the presented load does not match the datasheet CL — a wrong load can shift the frequency by up to tens of ppm. The deviation row shows how far the nearest standard E12 pair sits from your target CL.
First-order two-capacitor model; real oscillator layouts add stray across the pins and loading from the driver. ICBOMS provides this tool for reference only.