PCB Trace Width: Sizing Copper for Current and Heat

A trace is a resistor you draw. Too narrow for its current and it heats, darkens, and eventually fails like a fuse. Sizing by rule of thumb works until it does not IPC-2221 gives the engineering answer.
> At a glance: 7 minute guide · part 5 of 10 in the PCB design complete guide track · includes a worked example and a quick-reference table.

## The governing equation

Here is the working theory in one pass. IPC-2221 relates current, temperature rise and cross-sectional area: Area = [I ÷ (k × ΔT^0. 44)]^(1/0. 725) mil², with k = 0. 048 for external layers. Wider traces, thicker copper or greater permitted rise all increase capacity.

| Width (1 oz, ext.) | Current @ 10 °C rise | Typical use |
| — | — | — |
| 0.25 mm | ~0.9 A | Signals |
| 0.5 mm | ~1.4 A | Low-power rails |
| 1.0 mm | ~2.4 A | LED strips, small loads |
| 2.5 mm | ~4.5 A | Battery feeds |
| Polygon | Higher | Motor / supply nets |

## Copper weight and layer differences

What this means at the bench: Standard 1 oz copper is 35 µm thick. 2 oz doubles capacity for the same width. Internal layers dissipate into warmer air derate roughly by half versus external for the same geometry.

## Practice with the calculator

Feed current, permitted temperature rise and copper weight; take the width with margin. Power paths also benefit from polygon fills and multiple parallel layers stitched with vias thermal relief on pads is for solderability, not current.

## How to apply this in your build

Work through the sequence below each step assumes the previous one passed. For numbers that need calculating, the linked tools at the end of this guide do the arithmetic instantly.
1. List every power net with its worst-case current
2. Choose an acceptable temperature rise (10 °C conservative)
3. Compute widths per layer and copper weight
4. Add margin for connectors, vias and reflow tolerances

### Worked example

A 3 A LED rail on 1 oz outer copper needs ≈ 1. 4 mm at 10 °C rise. Drawing 0. 5 mm because it fit worked until summer, when the trace discoloured and dropped brightness. Run the numbers yourself with the PCB Trace Width and the result should agree to within rounding.

> Practical note from the bench. Every power net on our boards carries its width rationale in the design notes current, rise, width auditable at review.

## Field mistakes we see again and again

– Using signal-width traces for supply runs
– Forgetting vias in a power path are resistors too arrays share current
– Sizing by average instead of peak current for pulsed loads

## Key takeaways

– The governing equation the foundation of this guide; revisit it if any measurement here surprises you.
– Copper weight and layer differences the foundation of this guide; revisit it if any measurement here surprises you.
– Practice with the calculator the foundation of this guide; revisit it if any measurement here surprises you.

## Who this guide is for

Beginners get a single focused topic instead of a whole textbook chapter. It assumes the track’s earlier pages in the PCB design complete guide path. Intermediate readers use it as a reference the table, the worked example and the mistake list answer the questions that come up mid-build. If you teach, the structure (theory, application, example, failure modes) maps cleanly onto a lab session.

## What you need before starting

Nothing exotic: the parts or tools named in the guide, a multimeter. The PCB Trace Width / Wire Gauge (AWG) open in a tab. List every power net with its worst-case current before you begin the guide assumes it and keep the quick-reference table above within sight while you work through the steps.

### Quick reference card

| Aspect | Where to find it in this guide |
| — | — |
| Core theory | The governing equation |
| Application steps | How to apply this in your build |
| Worked numbers | Worked example |
| Failure modes | Field mistakes we see again and again |

## How this fits the PCB design complete guide track

This guide is one stop in the structured learning path. Start from the [PCB design complete guide](/tutorial/pcb-design-complete-guide) pillar page for the full map, or continue with [layout practices](/tutorial/pcb-layout-best-practices) and [via usage guide](/tutorial/pcb-via-types-guide). For the arithmetic, open the [PCB Trace Width](/tools/pcb-trace-width) or [Wire Gauge (AWG)](/tools/wire-gauge-awg).

## Frequently asked questions

What temperature rise is acceptable?
10 °C conservative, 20 °C common; sensitive or enclosed designs stay lower.

Does a solder mask or fill help?
Mask barely matters; extra solder on a trace measurably increases capacity on hand-modified boards.

Is there a calculator for this?
Yes the [PCB Trace Width](/tools/pcb-trace-width) and [Wire Gauge (AWG)](/tools/wire-gauge-awg) tools run the formulas from this guide instantly, client-side, with no signup.

## What to read after this

– The complete pcb design guide: [PCB Design complete guide](/tutorial/pcb-design-complete-guide)
– Read next: [rf pcb layout: rules for wifi, lora and beyond](/tutorial/rf-pcb-layout-guide)
– Also in this track: [what is vlsi design?
– Continue with: [kicad schematic capture: clean beginnings](/tutorial/kicad-schematic-tutorial)
– Calculate as you go: [PCB trace width calculator](/tools/pcb-trace-width) · [resistor value decoder](/tools/resistor-color-code) · [SMD code decoder](/tools/smd-resistor-code)
– From here, the natural continuation is the next guide in the track index. It assumes exactly the vocabulary this page built and adds the next layer of practice.

## Field notes

The fastest way to internalise this topic is to change one variable deliberately and predict the result before measuring. Wrong predictions are the curriculum, they show exactly which mental model needs revisiting, and the bench grades honestly.

Component substitution is a legitimate experiment as long as it is deliberate. Swap one part, predict the effect, measure, and record. That single habit converts a parts bin into a teaching lab and makes every future guide in this track faster to absorb.
## Formulas and checks from this guide

Verification checklist for this track: run DRC early and often, verify footprints against the datasheet drawing. Walk the return path of every fast signal before ordering. A five-minute Gerber preview has saved more fab cycles than any other habit.

Bookmark this page against your next build in the track. The checklist above is the same one used across 15 guides in this series.

## Field lessons worth keeping

Two for hobby density, four the moment ground integrity or impedance matters. The cost gap has collapsed.

Run DRC continuously, then once more after every final edit. The last small change breaks the most boards.