Crystal Load Capacitance Calculator

How it is calculated

For a Pierce oscillator the crystal sees the two capacitors C1 and C2 in series, plus the stray capacitance of the board and the chip pins in parallel: CL = (C1 × C2) ÷ (C1 + C2) + Cstray. All three inputs are in picofarads (pF) and the result is shown in pF with two decimals. C1 and C2 must be greater than zero. Stray capacitance is optional: if you leave it empty it counts as 0, and a negative value gives no result. The tool does not know your actual stray value, so it must come from your layout or from the microcontroller datasheet. To design backwards from a crystal's rated load, with C1 = C2 use C1 = 2 × (CL − Cstray).

Common mistakes

• Setting C1 and C2 equal to the crystal's rated load capacitance. The two capacitors are in series, so they must be roughly twice the rated value minus the stray part.
• Leaving stray capacitance at zero. Traces and pins add a few pF, so the real load is higher than the capacitors alone suggest.
• Mixing units, such as entering 0.022 for a 22 nF part or values in farads. All fields are in pF.
• Using the sum C1 + C2 as if the capacitors were in parallel, which gives a load that is several times too large.
• Forgetting that the printed capacitor tolerance and PCB variation shift the load, so a few kHz of frequency error does not always mean the formula is wrong.