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Capacitance of a rectangular capacitor, with edge effect, calculator

  This calculator can be used for finding the capacitance between PCB layers. It uses a formula derived at Chemandy Electronics which is unconventional but produces very useful results. Use it at your own risk and see the notes below.

Relative Dielectric Constant (εr): 
Width:   cm
Length:   cm
Dielectric Thickness:   mm
Capacitance without edge effect:   pF
Capacitance including edge effect:   pF

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  After a long unsuccessful search for a suitable accurate formula, I derived the formula above mainly for use when designing PCB,s and measuring Er. It assumes that there is a mirror ground plane on the bottom of the board and dielectric extending outside the edges of the (copper) plates. I have found the formula to be reasonably accurate on FR4 and it 'tracks' well to Kirchhoff's formula for a circular capacitor with an equivalent area, thickness and Er. It also appears to work fairly well on PCB tracks (using Eeff). I make no claim that this formula will work for anything other than a PCB and reserve the right to change it as and when more valid data is available to me. Hopefully this calculator will stimulate some discussion which could lead to a more accurate formula. It has been pointed out that it is not possible to take the log of L*w by two separate people (because it is a dimensioned number) but as yet no solution has been offered for this. Please use with caution and give us some feedback, good or bad.

  The capacitance of a rectangular or square parallel plate capacitor including edge effect, can be calculated using the following formula.

[rectangular capacitor formula]


L is the Length in cm

w is the Width in cm

εr is the Relative Dielectric Constant

d is the Dielectric thickness in cm

 The capacitance is also given without edge effect for comparison purposes and uses the formula.

[capacitor formula ignoring edge effect]

This calculator is provided free by Chemandy Electronics in order to promote the FLEXI-BOX

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William J Highton 28/8/2009