Crystal load capacitor calculator
Enter the load capacitance from the crystal's datasheet and an estimate of the stray capacitance of the pins and board. The calculator returns the value for each of the two equal capacitors, the nearest E12 part and the load it actually gives. You can also check an existing pair.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose capacitor values to size the two capacitors, or load of a pair to check existing ones.
- Enter the crystal’s load capacitance CL from its datasheet (for example 12.5 pF for many 32.768 kHz watch crystals, 18 or 20 pF for many MHz crystals).
- Enter the stray capacitance of the pins and traces.
- Read the capacitor value, the nearest E12 part and the load that part gives.
- Load seen by the crystal: CL = C1 × C2 / (C1 + C2) + C_stray.
- Equal capacitors: C1 = C2 = 2 × (CL − C_stray).
The E12 series has 12 values per decade: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82. The calculator picks the closest on a ratio scale.
Worked example
A crystal with CL = 18 pF and an estimated 5 pF of stray capacitance: C = 2 × (18 − 5) = 26 pF. The nearest E12 value is 27 pF, which gives 27 ÷ 2 + 5 = 18.5 pF, close to the specified load.
A 12.5 pF watch crystal with 3 pF stray: C = 19 pF, nearest E12 18 pF, load 12 pF.
Checking a pair: C1 = 22 pF and C2 = 33 pF with 4 pF stray give 13.2 + 4 = 17.2 pF.
Tips
- Use C0G/NP0 capacitors. Their value does not drift with temperature or voltage like X7R.
- Some chips have built-in load capacitors. Read the datasheet before adding external ones.
- Fine tune by measurement. Measure the frequency with a buffered output and move one capacitor a step up or down.
Frequently asked questions
How do I calculate crystal load capacitors?
The crystal sees C1 and C2 in series plus the stray capacitance: CL = C1 × C2 / (C1 + C2) + C_stray. With equal capacitors this becomes C = 2 × (CL − C_stray). An 18 pF crystal with 5 pF stray needs 2 × 13 = 26 pF on each side.
What stray capacitance should I assume?
It is the capacitance of the oscillator pins and the board traces. Microcontroller datasheets often give the pin capacitance; values of 2 to 5 pF in total are common estimates for a short, compact layout. Measure the frequency and adjust if you need accuracy.
Why not just use two capacitors equal to CL?
Two capacitors in series give half their value. Two 18 pF capacitors make 9 pF plus the strays, well below 18 pF, so the crystal runs a little fast. The error is small in parts per million but matters for real-time clocks and radio.
What happens if the load capacitance is wrong?
The oscillator still runs, but slightly off frequency: too little capacitance makes it run fast, too much makes it run slow. Far too much can make it slow to start or stop oscillating.