PCB Trace Width Calculator

Find the required copper trace width for a given current using the IPC-2221 standard. Enter your current, copper weight, allowable temperature rise and layer type, and the tool returns the minimum trace width in mils and millimeters, the cross-sectional area, and the resulting resistance and voltage drop over your trace length.

🔌Real Design Presets

📝Trace and Current Inputs

Trace current (A)

Continuous current the trace must carry, in amperes.

Copper weight (oz/ft²)

Finished copper thickness of the layer, in ounces.

Layer location

Outer traces cool better, so they need less width.

Allowable temp rise ΔT (°C)

Trace heating above ambient, typically 10 to 30 °C.

Ambient temperature (°C)

Board surroundings; sets the final conductor temperature.

Trace length (mm)

Conductor run length, used for resistance and voltage drop.

Conductor material

Resistivity used for the resistance calculation.

Result decimal places

Controls rounding on every result card.

Required Trace Width 0 mils minimum copper width

Width in Millimeters 0 mm same width, metric units

Cross-Sectional Area 0 mils² copper section width x thickness

Resistance & Voltage Drop 0 mV over the trace length

🔢Formula Snapshot

A(I / k ΔT^0.44)^1.379

WArea / thickness

0.048k external layer

0.024k internal layer

📋Current to Width, 1oz External at 10°C Rise

Current	Area (mils²)	Width (mils)	Width (mm)
0.5 A	6.3	4.5	0.11
1 A	16.3	11.8	0.30
2 A	42.4	30.8	0.78
3 A	74.2	53.8	1.37
5 A	150.0	108.9	2.77
7 A	238.6	173.2	4.40
10 A	390.3	283.2	7.19
15 A	682.8	495.5	12.59

📊Copper Weight to Thickness Reference

Copper Weight	Thickness (mils)	Thickness (µm)	Typical Use
0.5 oz	0.689	17.5	Fine-pitch signal
1 oz	1.378	35	General purpose
2 oz	2.756	70	Power and heavy current
3 oz	4.134	105	Motor drives, bus bars
4 oz	5.512	140	High-current backplanes
0.25 oz	0.345	8.75	RF and dense routing

📏Unit Conversions

Unit	Equals	In Base Unit	Note
1 mil	0.0254 mm	0.001 inch	One thousandth of an inch
1 mm	39.37 mils	0.03937 inch	Metric width
1 oz copper	1.378 mils	35 µm	Standard 1 oz layer
1 mil²	0.000645 mm²	645.16 µm²	Area conversion
1 A	1000 mA	1 C/s	Ampere of current
1 Ω	1000 mΩ	1 V/A	Ohm of resistance

🗃External vs Internal Width Comparison Grid

Current	1oz Ext (mils)	2oz Ext (mils)	1oz Int (mils)	2oz Int (mils)	Area (mils²)
0.5 A	4.5	2.3	11.8	5.9	6.3 / 16.3
1 A	11.8	5.9	30.8	15.4	16.3 / 42.4
2 A	30.8	15.4	80.4	40.2	42.4 / 110.8
3 A	53.8	26.9	140.5	70.3	74.2 / 193.7
5 A	108.9	54.4	284.3	142.1	150.0 / 391.7
7 A	173.2	86.6	452.2	226.1	238.6 / 623.1
10 A	283.2	141.6	739.5	369.7	390.3 / 1019
15 A	495.5	247.7	1294	647	682.8 / 1783
20 A	736.8	368.4	1924	962	1015 / 2651
25 A	1002	501.2	2617	1309	1381 / 3607

⚙Formula Breakdown (IPC-2221)

Area A = (I / (k ΔT^0.44))^(1/0.725)The IPC-2221 curve fit gives the required copper cross-section in square mils from current I in amps and allowable temperature rise ΔT in °C. Note 1/0.725 = 1.379.

k = 0.048 external, 0.024 internalExternal traces sit on the surface and shed heat to air, so k is larger and the trace can be narrower. Internal traces are buried, so they need roughly 2.6 times the area.

Thickness t = oz × 1.378 milsCopper weight sets the layer thickness. 1 oz of copper spreads to 1.378 mils, which is 35 micrometers, so 2 oz is 2.756 mils.

Width W = A / tDivide the required cross-section by the copper thickness to get the minimum trace width in mils. Multiply mils by 0.0254 to convert to millimeters.

Resistance R = ρ L / (W × t)With copper resistivity ρ = 1.72e-8 Ω·m, trace length L and the section area W × t all in meters, this gives the DC resistance of the finished trace.

Voltage drop V = I × ROhm's law turns the trace resistance into the voltage lost along the run, which matters for power rails and sense lines.

Validity limitsIPC-2221 is valid up to about 35 A, ΔT up to 100 °C and trace widths up to 400 mils. Beyond that, use bus bars, planes or thermal simulation.

💡PCB Trace Design Tips

Add margin above the minimum: The IPC-2221 width is a bare minimum for the stated temperature rise. Size traces about 20 percent wider than the calculated value to cover etch tolerance, vias and connector heating. For a 5 A rail at 1 oz external the tool gives roughly 109 mils, so route 130 mils or more.

Prefer heavier copper for power: Going from 1 oz to 2 oz copper halves the required width for the same current, because thickness doubles while area stays fixed. A 10 A trace needs about 283 mils on 1 oz but only 142 mils on 2 oz, freeing valuable board space on dense layouts.

Then again, you get the board back and discover the power rail melted before the thing powered up. Nobody wants to admit how often that happens. Copper traces appears as thick wire in CAD view, but they are actualy thin sheets of metal with exact thermal limits. Getting the trace size right isn’t an act of guesswork, or at least not most of the time, it’s an exercise in understanding how much heat needs to dissipating for the current you want to run through it.

Fortunately the tool on this page handles the heavy lifting by applying IPC-2221 standard for you, so you just need to check if computed width works within your design instead of fighting with constants and exponents.

How to Size Copper Traces Correctly

Here’s the problem: Heat is a side effect of electrical resistance. As electrons move along a narrow surface, they bump into individual copper atoms and convert some electrical energy into heat. Unless the heat dissipates quickly enough, this cause the trace to become hotter. And that’s problematic because heat can damage nearby circuit element or cause the board substrate itself to peel apart.

The calculator calculates the required width based on amount of current you require and the temperature rise you are willing to tolerate. Then it provides you with a width value that strikes best balance between these two factor. Designers who don’t use thermal information and rely only on visual estimation gets this wrong.

Two primary constants in this equation mirror laws of physics: A trace laid down on an outside layer of a board has direct exposure to ambient temperature and dissipates heat easily, needing only 1/4 the cross section as one routed internally which is sandwiched between layers of insulating material and thus traps heat within the board. This is why you’ll note that internal power plane traces is often thicker copper, or have larger traces. This is reflected in the math, it calls for about 2.5x the surface area of copper for internal traces. When you choose your layer type, the calculator will factor this in for you (so don’t forget and undervolt a hidden rail!)

The other variable that changes the game is the kind of copper used. Normal 1 oz copper comes in at around 35 micrometers thick. But if you use 2 oz copper then you get twice as thick without having to make it any wider. Since resistance depends on cross-sectional area, you can cut the trace width in half and still carry same current. When you’re working with limited board space, this is an extremely valuable trade off. Maybe there’s no room for a wide power rail, but you can always specify heavier copper with your manufacturer. With the tool, you can step from 0.5 oz all the way to 3 oz. This lets you visualize amount of space you’ll be able to save by upgrading material thickness.

Power distribution’s silent killer is its own voltage drop. Sure, a trace may not heat up but that doesn’t mean it isn’t electrically failing by dropping too much voltage en route to the load. Too much resistance in path back to the regulator means a starving sensor readout or processor core. With the resistivity of copper as a variable (not constant!), the calculator bases the voltage drop based off your inputted trace length. It then presents you with a concrete number in millivolts so that you know whether to go for a shorter route or a wider trace. This transforms an abstract thermal limit into real electrical performance measure.

There are no perfect manufacturers in the real world so there will be margins added for real world designs. Traces etched will be slightly smaller than they was drawn. Localized resistances of vias and pads aren’t included in basic linear calculations. Add another 20% to your designed width for good measure. That accounts for not only tolerances but also ambient variation and heating up of connectors. So the final design is solid instead of being right on the edge of failing. You should of added more just in case.

Using presets can speed up your workflow by providing realistic starting points. The presets are realistic starting points, which means they accelerate your work flow. Quickly seeing what a 20 A battery bus looks like versus a 1 A signal line saves retyping numbers each time. This also shows the size difference between heavy power lines and delicate logic wires. Then you adjust variables to fit conditions on your boards.

It is a little thing, but it makes all the difference when it comes to reliability. After all, you want a board that runs cool under load and provides stable voltage. The calculator provides the data to do just that; engineering, not guesswork.