![[Formua for rectangular capacitor with edge effect]](../images/rectangular-cap-formula.gif)
The capacitance of a rectangular or square parallel plate capacitor including edge effect, can be calculated using the following formula.
Where l = Length in cm
w = Width in cm
εr= Relative Dielectric Constant
d = Dielectric thickness in cm
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. Please use with caution and give us some feedback, good or bad.
The capacitance is also given without edge effect for comparison purposes and uses the formula.
![[Formula for capacitor without edge effect]](../images/rectangular-cap-formula2.gif){kind=small}
Where l = Length in cm
w = Width in cm
εr= Relative Dielectric Constant
d = Dielectric thickness in cm
William J Highton
28/8/09
This calculator is provided free by Chemandy Electronics in order to promote the FLEXI-BOX
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