# How do PCB trace width calculators work? Why is it asking me to input the width of it?

I need to calculate the minimum trace width on my PCB for around 25 amperes (a few steppers.) When I go to a calculator, it asks me for its thickness. Here's one from Digikey, and another from 4pcb. They both ask me to enter the thickness. Isn't that what they are supposed to calculate given the amperes and temperature rise?

I think I'm missing something.

Could you explain what I'm supposed to add, or what they mean by "thickness"?

Is the "Trace Length" the distance between the power terminals and the GND+VCC of the stepper drivers? That is, how long the trace will be.

• Assume 35 um unless your order something special. If you need thicker, talk to your OCB supplier. Commented Mar 6, 2023 at 20:36
• As an example, PCBWay offers 35μm or 70μm or 105μm of outer layer copper thickness for its PCBs. For inner layers they offer 35μm or 50μm. Commented Mar 7, 2023 at 8:40

The thickness is not the same as the width. You're asked for the thickness of the copper, as measured in a cross-section of the circuit board. Once you provide that and the other info, the calculator will give you a trace width (i.e. measured from a birds-eye view of the board).

Here's a quick, not-to-scale, sketch done by hand (I don't have easy access to an image editor at the moment):

This thickness depends on what PCB fabrication service you pay for; oftentimes it's indicated in ounces (e.g. "1 oz copper"), which really means "copper thick enough that 1 sq ft of it will weigh 1 oz". The thickness in microns is proportional to the copper weight you choose; according to this resource 1 oz copper corresponds to 35 microns, or around 1.4 mils, of copper thickness. Both of the calculators you linked already accept oz/sq ft, so you can just input the thickness that you intend to pay for.

The calculator can then calculate the width once it knows the current, thickness, and allowable temperature rise.

You are correct about trace length. Knowing the length allows the calculator to compute voltage drop and heat dissipation.

• And where can I find the thickness? In kicad I can't seem to find any data. Also, I just visited JLCPCB and indeed they got an "Outer Copper Weight" of 1 oz. At PCBWAY they have "Finished Copper" of 1 up to 13oz. Is this the "Thickness" we are talking about?
– Mito
Commented Mar 6, 2023 at 20:45
• @Mito Kicad isn't going to know, because it's just drawing traces and only knows about 2D geometry as far as board design itself. You can take those gerbers and use them to order a 1oz board, a 2oz board, etc. The copper weight you saw at JLCPCB and PCBWAY (on the quote/order form) is the thickness you care about. Commented Mar 6, 2023 at 21:17
• @Mito: The KiCad trace width calculator tool allows a choice of thickness units including oz/ft^2. Find what thicknesses are available from PCBWAY, JLBCB, et cetera. Cloose one. Enter into the calculator with other parameters like allowable temperature rise. It will calculate the width tjat you need. Commented Mar 7, 2023 at 2:37
• @Mito JLCPCB let's you choose "Outer Copper Weight" between 1 and 2oz, at least for the generic PCBs, it may be limited for some special cases. Commented Mar 7, 2023 at 20:40

"Thickness" refers to the height of the copper plating provided on your board by your board manufacturer.

Manufacturers usually quote plating thickness in "ounces" or "oz," referring to ounces per square foot of copper. 1 oz is a very common choice, working out to about 35 μm or 1.37 mil, but many manufacturers can go thicker (for a price).

The current-carrying ability of any copper conductor depends on its cross-sectional area, i.e. width times height. You want to know the width, so you need to provide the height.

• Sometimes you encounter 0.5 oz copper, too, particularly on inner layers. Commented Mar 7, 2023 at 4:04

No, they calculate the width, but you need to know the height, i.e. the copper thickness.

You can have different thickness copper PCBs, so the PCB track width calculator does not know how thick copper plating you intend to use.

• At JLCPCB I have this option of "Outer Copper Weight" of 1 or 2 oz. At PCBWAY I have "Finished Copper" of 1 up to 13 inch. Is this the "thickness" we're talking about?
– Mito
Commented Mar 6, 2023 at 21:10