Crystal Load Capacitance

CL = (C1×C2)/(C1+C2) + Cstray

Quick answer

This calculator solves CL = (C1 × C2) / (C1 + C2) + Cstray for CL. With C1 100 pF, C2 0.22 nF, Stray Capacitance 0.47 nF, the result is 0.5387 nF.

Calculator

Result
—

Formula

CL = (C1 × C2) / (C1 + C2) + Cstray

Description

A crystal oscillator requires external load capacitors (C1 and C2) to match the crystal's specified load capacitance for accurate frequency operation. The effective load seen by the crystal is the series combination of C1 and C2 (since they are in series from the crystal's perspective through ground) plus any stray capacitance from PCB traces and IC pins. If the load capacitance does not match the crystal specification, the oscillation frequency shifts: too little capacitance raises the frequency, too much lowers it. Typical load capacitance values range from 6 to 20 pF.

Variables

  • CL — Effective load capacitance seen by the crystal (F)
  • C1 — Load capacitor on one crystal pin (F)
  • C2 — Load capacitor on the other crystal pin (F)
  • Cstray — Stray capacitance from PCB traces and IC pins (F)

Practical Notes

When C1 = C2 (the typical case), CL = C1/2 + Cstray. For a crystal specifying CL = 12pF with Cstray = 5pF, you need C1 = C2 = 2 × (12-5) = 14pF. Round to standard 15pF values. Stray capacitance is typically 2-7pF depending on trace layout. Mismatched CL causes frequency errors of a few to tens of ppm.

Symbols and units

Symbol Meaning Default unit Accepted units
C1 C1 pF pF, nF, µF, mF, F
C2 C2 pF pF, nF, µF, mF, F
Cstray Stray Capacitance pF pF, nF, µF, mF, F

Step by step

  1. 1. What you know: C1 100 pF, C2 0.22 nF, Stray Capacitance 0.47 nF
  2. 2. The relation: CL = (C1 × C2) / (C1 + C2) + Cstray
  3. 3. Substitute: C1 = 100 pF, C2 = 0.22 nF, Cstray = 0.47 nF
  4. 4. Result: CL = 0.5387 nF

Worked examples

Minimum range

Given: C1 1 pF, C2 2.2 pF, Stray Capacitance 4.7 pF

Result: CL = 5.388 pF

Low range

Given: C1 10 pF, C2 22 pF, Stray Capacitance 47 pF

Result: CL = 53.88 pF

Typical

Given: C1 100 pF, C2 0.22 nF, Stray Capacitance 0.47 nF

Result: CL = 0.5387 nF

High range

Given: C1 1 nF, C2 2.2 nF, Stray Capacitance 4.7 nF

Result: CL = 5.388 nF

Maximum range

Given: C1 10 nF, C2 22 nF, Stray Capacitance 47 nF

Result: CL = 53.87 nF

How CL changes with C1

C1 (input) CL (result)
10 pF 0.4796 nF
25 pF 0.4924 nF
50 pF 0.5107 nF
75 pF 0.5259 nF
100 pF 0.5387 nF
150 pF 0.5592 nF
0.2 nF 0.5748 nF
0.4 nF 0.6119 nF
0.7 nF 0.6374 nF
1 nF 0.6503 nF

Embed this calculator

Paste this into any page. It keeps working when this page updates, and credits the source.

<iframe src="https://vbatpower.com/embed/crystal-load-cap/" width="100%" height="460" style="border:1px solid #1f2a37;border-radius:12px" loading="lazy" title="Crystal Load Capacitance"></iframe>
<p><a href="https://vbatpower.com/formulas/crystal-load-cap/">Crystal Load Capacitance</a> Calculator by vbatpower.com</p>

Frequently asked questions

How do you calculate Crystal Load Capacitance?

Use CL = (C1 × C2) / (C1 + C2) + Cstray. With C1 100 pF, C2 0.22 nF, Stray Capacitance 0.47 nF, CL works out to 0.5387 nF.

How do you calculate crystal load capacitance calculation?

Rearrange for CL: with C1 10 pF, C2 22 pF, Stray Capacitance 47 pF, CL is 53.88 pF.

What is the formula for Crystal Load Capacitance?

The formula is CL = (C1 × C2) / (C1 + C2) + Cstray. A crystal oscillator requires external load capacitors (C1 and C2) to match the crystal's specified load capacitance for accurate frequency operation.

Related Concepts

All Timing & Oscillators formulas →
555 Timer Astable 555 Timer Monostable Crystal Load Capacitance RC Oscillator Frequency Crystal Frequency Stability PLL Lock Time VCO Tuning Sensitivity Jitter from Phase Noise

Need more features?

Save calculations, import telemetry data, simulate battery discharge, and collaborate with your team.

Try the App