PCB Trace Width Calculator
Introduction
A PCB trace looks simple on the finished board, but electrically it is a conductor with finite resistance and a limited ability to shed heat. When current rises, the copper warms up. If the trace is too narrow, that temperature rise can become excessive, causing avoidable voltage drop, hot spots, unreliable performance, or in severe cases damaged copper. If the trace is much wider than necessary, the board still works, but you give up routing space that could have been used for components, ground return paths, or cleaner signal placement. This calculator is meant to help with that early design balance. It estimates a practical minimum width for an external copper trace from three inputs you usually know at schematic or layout time: current, copper thickness, and the temperature rise you are willing to allow.
That result should be read as a design starting point, not a final certification. Real boards live inside enclosures, sit near hot regulators, route through vias, and share heat with nearby copper pours. Even so, a quick width estimate is extremely useful because it tells you whether a trace is ordinary signal routing, a modest power line, or a path that deserves intentional planning. If the output is tiny, you know standard routing rules may be enough. If the output is large, you can react early by widening the route, using heavier copper, shortening the path, or moving to a copper pour rather than discovering the problem after the board is already crowded.
Why Trace Width Matters
Current through copper produces heat because every trace has resistance. The narrower the trace, the less cross-sectional area the current has to flow through, and the higher the resistance becomes. More resistance means more I²R heating and more voltage drop along the path. In a low-current sensor circuit, that may be barely noticeable. In a motor driver, battery input, LED rail, or DC-DC converter path, it can be the difference between a cool, efficient board and one that runs unexpectedly hot. Designers often think first about whether a trace can survive the current, but width also affects performance. A wider trace usually lowers resistive loss, reduces wasted power, and keeps the delivered voltage closer to what the load expects.
There is a second side to the tradeoff: board area is valuable. Oversizing every route can make the layout harder, especially on compact boards with fine-pitch components. That is why trace-width estimation matters so much in practice. It helps you reserve width where it truly buys safety and efficiency while keeping less demanding nets compact. Once you know the rough requirement, you can decide whether to widen one trace, switch the whole design to 2 oz copper, or change the placement so the high-current path becomes shorter and easier to cool.
Understanding Copper Weight
PCB fabricators usually describe copper thickness by weight, most commonly ounces per square foot. The shorthand can feel odd at first, but it is standard in board design. One ounce copper is approximately 35 micrometers thick, which is about 1.378 mils. Two ounce copper is roughly twice as thick, so the same current can be carried by a narrower trace because the conductor has more area. That is why heavy-copper boards are common in power electronics, automotive modules, battery-management circuits, and industrial control hardware.
Heavier copper does not solve every problem by itself. It can increase cost, affect manufacturability, and still needs enough width to spread heat well. But it is a powerful lever. If the calculator tells you a 1 oz trace needs to be very wide and that width does not fit your layout, increasing copper thickness is often the first alternative to examine. The relationship is intuitive: for the same required cross-sectional area, doubling the copper thickness roughly halves the needed width. In real layout work, that can be the difference between an awkward bottleneck and a clean route.
Formula
This calculator uses a commonly cited simplified fit for external traces based on IPC-style current and temperature-rise behavior. The idea is to estimate the copper cross-sectional area needed to carry a current while staying within a chosen temperature rise above ambient. In the equation below, A is the required cross-sectional area in square mils, I is current in amps, and rise is the allowable temperature increase in degrees Celsius. The constant and exponents come from an empirical fit rather than first-principles heat transfer, which is why the formula is useful for quick planning but not a substitute for detailed thermal validation.
Once the area is known, width is found by dividing by copper thickness. Because PCB copper thickness is entered in ounces, the script first converts it to mils using about 1.378 mils per ounce. The final width is then reported in mils or millimeters depending on the output setting you choose. In other words, the calculator moves through three steps: estimate area from current and allowed heating, convert copper thickness into a linear dimension, and divide area by thickness to get width.
The equation used is A = (I / (0.024 * rise0.44))(1/0.725), where A is the cross-sectional area in square mils, I is the current in amps, and rise is the allowed temperature increase in Celsius. This expression roughly matches external trace data for ordinary convection. Once the area is known, the width is simply A divided by the copper thickness in mils. If you choose millimeters as the output, the calculator converts from mils to millimeters at the end. Internal layers usually need more width because they lose heat less easily than outer layers.
Formula: A = I÷0.024×rise^0.44^1/0.725
Formula: w = A / t
Here, w is trace width and t is copper thickness in mils. A smaller allowed temperature rise makes the denominator smaller in the first equation, which increases the required area and therefore widens the trace. Thicker copper does the opposite in the second equation: it reduces the width you need for the same area. That is exactly the tradeoff the calculator is designed to expose.
How to Use the Calculator
Start with the highest continuous or expected peak current that the trace will carry during normal operation. If a power rail feeds several loads, use the current in the portion of the route that sees the sum of those loads, not the smaller branch current farther downstream. Then choose the copper thickness in ounces. If your board house is supplying a standard two-layer prototype board, 1 oz is a common default. If you already know the design uses heavier copper, enter that value directly. Next, choose the allowed temperature rise. A smaller number is more conservative because it limits how hot the copper is allowed to become. Designers often start with 10 °C for general-purpose work, then tighten or relax that target depending on enclosure temperature, reliability goals, and available board space.
After you click the button, the result shows the estimated minimum width. If the number looks surprisingly large, that usually means one of three things: the current is substantial, the copper is relatively thin, or the temperature-rise target is cautious. At that point, you have options. You can widen the trace, increase copper thickness, shorten the route, split current across multiple parallel paths, or accept a larger temperature rise if the application allows it. The tool is most useful when you try a few combinations and compare how sensitive the width is to each design decision.
Example Calculation
Imagine a board that must carry 2 A to a motor driver on an external layer. With 2 oz copper and an allowed 10 °C rise, the estimated width is about 28 mils, or roughly 0.70 mm. That may be easy to route on a medium-size board. If you keep the same current and temperature-rise goal but switch to 1 oz copper, the required width roughly doubles to about 56 mils. The electrical requirement did not change; only the conductor thickness changed. This is a good example of how copper weight can trade directly against route width when you are fitting a power path into a limited area.
The example also shows why the result should be interpreted in context. If the route is very short, close to a ground pour, and only sees that current intermittently, you may decide the design has extra practical margin. If the same route sits inside a sealed enclosure near a hot regulator, you may widen it even beyond the calculated value. The calculator gives you the thermal starting line; engineering judgment finishes the job.
Interpreting the Result
A calculator output is easiest to use when you treat it as the minimum width for the specific assumptions entered. It is not a promise that every board, every enclosure, and every trace shape will behave exactly the same way. For example, long traces create more voltage drop than short ones even when both meet the same temperature-rise target. Narrow neck-downs near connector pins or component pads can become local hot spots if they carry the full rail current. Similarly, vias between layers can become the real bottleneck, so a generous trace width on the surface does not help much if the current must squeeze through a single small via in the middle of the path.
If the result is close to your fabricator's normal routing width, you probably have an easy design. If the result is tens of mils wider than the rest of the board, that is a signal to plan for copper pours, dedicated power routing, or a different mechanical arrangement. A wide trace is not a failure; it is simply information. Many successful boards deliberately devote significant area to current paths because that copper pays back in lower loss, lower heat, and easier compliance with performance goals.
Sample Trace Width Comparison Table
This table gives quick reference values for typical external traces. Use it for rough intuition, then rely on the calculator for a specific estimate based on your exact inputs.
| Current | Copper Weight | Temp Rise | Estimated Width |
|---|---|---|---|
| 1 A | 1 oz | 10 °C | 24 mils |
| 3 A | 1 oz | 10 °C | 55 mils |
| 5 A | 2 oz | 10 °C | 60 mils |
If your design uses internal layers, higher ambient temperature, restricted airflow, or long via transitions, choose more width than the quick-reference values suggest.
Other Design Considerations
Trace width is only one part of current handling. A short, wide connection from a connector to a plane may behave very differently from a long serpentine route of the same width. Solder mask coverage can slightly affect cooling, and nearby copper pours can help spread heat. Airflow matters too. A board in free air usually cools better than a board sealed inside a dense plastic enclosure. Component placement matters because a trace running beside a hot inductor or linear regulator begins with less thermal headroom than one isolated in open board area. Mechanical constraints also matter: if a trace must neck down to pass through a fine-pitch connector or IC pad, that necked region deserves special attention because it may dominate the thermal performance.
In higher-current products, designers often go beyond simple single-trace routing. They may use wide polygons, multiple vias in parallel, mirrored traces on several layers, solder-coated copper for modest reinforcement, or dedicated copper bars for very large currents. Those approaches do not invalidate the calculator. Instead, they build on the same principle that more conductor area and better heat spreading improve performance.
Limitations and Assumptions
This page assumes an external trace under typical conditions and uses a simplified empirical equation rather than the full chart-based treatment of every geometry and environment. It does not model pulsed thermal cycling, unusual stackups, forced airflow, adjacent heat sources, unusual copper roughness, or specialized board materials. It also does not calculate fusing current, safety creepage, or voltage-drop limits directly. Those are separate design checks and can be just as important as temperature rise.
For conservative work, especially safety-critical or high-reliability designs, the right workflow is to use the calculator early, build in extra margin, and then confirm the result with your board manufacturer's guidance, lab measurements, or thermal simulation. That is particularly important for internal traces, heavy-current connectors, battery paths, and anything that could fail in a hazardous way if the copper becomes too hot.
Practical Design Tips
Once you have the calculated width, a few practical habits make the result more useful in real layout work:
- Round the width up to a manufacturable design rule rather than treating the exact decimal output as sacred.
- Check the hottest branch current, not just the supply average, when sizing a critical segment.
- Avoid tiny neck-downs in pads, vias, or connector exits if that narrowed region carries the full current.
- When in doubt, spend a little more copper on important power paths because the extra margin often reduces both temperature rise and voltage drop.
These habits are simple, but they prevent a common mistake: getting a reasonable thermal answer from a calculator and then losing that benefit through one overlooked bottleneck in the actual geometry.
Conclusion
Reliable PCB power routing is mostly about matching the conductor to the load. This calculator gives you a fast estimate of that match by linking current, copper thickness, and allowable temperature rise to a recommended trace width. Use the output as a minimum planning value, compare a few scenarios, and then apply sensible margin for the real conditions your board will face. A few minutes of early width planning can prevent heat problems, voltage-drop surprises, and expensive board revisions later.
Reading the Output
When the calculator returns a required trace width, treat that number as a minimum starting point for an external layer under ordinary cooling. If your board lives in a hot enclosure, uses internal routing, carries current through vias, or must keep voltage drop especially low, it is wise to choose more copper than the raw output alone suggests. Extra width reduces resistance, spreads heat, and gives you manufacturing margin. A result that seems larger than expected is often a useful warning that the current path deserves deliberate power-layout treatment instead of ordinary thin routing.
It is also smart to compare the result with your fabrication rules and your mechanical plan. If the required width is much wider than your normal routing grid, make room for it early. Designers often widen the route near connectors, convert long current paths into polygon pours, or use multiple parallel vias when changing layers. The calculator tells you the scale of the problem quickly, which is exactly what makes it useful during placement and early routing rather than only at the end.
Mini-Game: Trace Tuner
This optional mini-game turns the same idea behind the calculator into a fast visual challenge. Current bursts race toward an inspection gate, and each burst has its own current, copper weight, and allowed temperature rise. Your job is to widen or narrow the trace before the burst arrives so the live trace thickness matches the glowing safe-width window. It is quick to learn, replayable, and surprisingly good at building intuition: higher current or tighter temperature limits usually demand a fatter trace, while heavier copper lets you get away with less width.