# How to calculate stray capacitance on PCB

I use a 0.4mm spacing between PCB traces (including 0.4mm space between crystal pads and ground) and I believe that's causing stray capacitance.

My circuit board is typical (1 ounce copper on FR4 fiberglass material with height 1.6mm). My ground plane is the largest possible without isolated planes.

I read that the distance between PCB traces has an inverse effect on stray capacitance.

Lets assume the non-ground traces are perfectly straight lines 10mm (1cm) and its thickness is 2.1mm

Given this information, how do I calculate stray capacitance between the traces?

• saturnpcb.com/pcb_toolkit – DKNguyen May 24 at 19:47
• longer traces 0.4mm from a ground plane? sounds like a coplanar waveguide to me! If you want to play with it; Kicad comes with an easy-to-use cpwg calculator. – Marcus Müller May 24 at 23:06
• how accurate must you be? for wide traces, just use C = Eo * Er * Area / Distance. Even for narrow traces, that formula gets you started, then you can estimate fringing, or even switch to wire-over-plane maths. – analogsystemsrf May 25 at 4:15
• Toor, I dont use Windows OS. and I need to be accurate enough so that the crystal attached to my micro can oscillate. I'm using 33pF caps as recommended by the micro datasheet but I think with the stray capacitance added and the supposedly bad tolerance of my caps, its causing no oscillation? (my caps are labelled as 33 only). I did order some NPO 5% tolerance caps and I'm hoping I'll get lucky. – Mike May 26 at 3:32