About the Crystal Load Capacitor Calculator
This crystal load capacitor calculator works out the pair of capacitors that go from each crystal pin to ground in a Pierce oscillator — the circuit used by almost every microcontroller, RTC and USB chip. Enter the load capacitance (CL) from the crystal datasheet and an estimate of the stray capacitance of the PCB and chip pins, and it returns the exact capacitor value, the nearest standard E12 value, and the load the crystal will actually see with that part fitted.
Getting CL wrong does not stop the oscillator, but it pulls the frequency off by several parts per million — enough to make a real-time clock drift by seconds a day or a UART or CAN bus run out of tolerance. If you enter the crystal’s shunt capacitance C0 and motional capacitance C1 (both on good datasheets), the calculator estimates that frequency error in ppm. A second mode checks an existing design with unequal capacitors.
The common rule of thumb of 2 to 5 pF of stray capacitance covers chip pin capacitance plus short traces; measure the actual frequency on a prototype and adjust if accuracy matters.
How to use the crystal load capacitor calculator
- 1Look up the crystal’s load capacitance CL on its datasheet.
- 2Estimate stray capacitance — 2–5 pF for short traces and typical MCU pins.
- 3Read the exact capacitor value and the nearest standard E12 part.
- 4Optionally enter C0 and C1m from the datasheet to estimate the frequency error.
- 5Use the check mode to evaluate an existing design or unequal capacitors.
Formula and method
In a Pierce oscillator the crystal sees the two load capacitors in series, plus stray capacitance from the IC pins and PCB traces in parallel. With equal capacitors the series combination is C/2, so each capacitor must be twice the difference between the crystal’s rated load capacitance and the stray capacitance. The calculator rounds that to the nearest standard E12 value and recomputes the load actually presented.
A crystal runs at its marked frequency only when loaded with its rated CL. The fractional offset from the series-resonant frequency is approximately C1m/(2(C0 + CL)), so a difference between the actual and rated load shifts the frequency by the difference of two such terms. Less load capacitance pulls the frequency up; more pulls it down. The trim sensitivity −C1m/(2(C0 + CL)²) shows how many ppm each picofarad of error costs.
- CL
- Load capacitance the crystal is specified for (pF)
- C1, C2
- External load capacitors from each crystal pin to ground
- Cstray
- Pin and PCB stray capacitance (pF)
- C0
- Crystal shunt (holder) capacitance (pF)
- C1m
- Crystal motional capacitance (fF)
Worked examples
18 pF crystal with 5 pF stray
Each capacitor should be 2 × (18 − 5) = 26 pF; the nearest E12 part is 27 pF, giving 27/2 + 5 = 18.5 pF of load. With C0 = 5 pF and C1m = 10 fF the crystal runs about 4.6 ppm low.
12.5 pF 32.768 kHz watch crystal, 3 pF stray
C = 2 × (12.5 − 3) = 19 pF; the nearest E12 value is 18 pF, which gives 9 + 3 = 12 pF of load. Slightly under-loaded, the crystal runs about 4.0 ppm fast — around 10 seconds per month.
Checking 22 pF and 33 pF caps on a 20 pF crystal
22 and 33 pF in series make 22 × 33 ÷ 55 = 13.2 pF; adding 4 pF stray gives 17.2 pF, 2.8 pF below the rating. With C0 = 5 pF and C1m = 10 fF that pulls the frequency about 25 ppm high — likely outside a ±20 ppm budget, so larger capacitors (33 pF each) would be a better fit.
Frequently asked questions
How do I calculate crystal load capacitors?+
Use C = 2 × (CL − Cstray), where CL is the crystal’s rated load capacitance and Cstray is pin plus board capacitance. For an 18 pF crystal with 5 pF stray, each capacitor should be 26 pF, so fit 27 pF.
Why is the load capacitor not equal to CL?+
The two capacitors are effectively in series across the crystal, so their combined value is half of each. Stray capacitance adds in parallel. That is why each capacitor is roughly twice CL minus the stray contribution.
What happens if the load capacitors are wrong?+
The oscillator usually still starts, but the frequency shifts by a few to tens of ppm. Too little capacitance makes it run fast, too much makes it run slow, and far too much can stop oscillation by reducing loop gain.
What stray capacitance should I assume?+
Most designs use 2–5 pF, combining the oscillator pin capacitance from the IC datasheet with 1–2 pF of PCB traces. Some MCU vendors give a recommended value; the best approach is to measure the frequency on a prototype.
Can I use different values for C1 and C2?+
Yes. The crystal sees C1·C2/(C1 + C2) plus stray. Unequal values are sometimes used to adjust drive level or gain; use the check mode to see the resulting load capacitance and frequency error.