Calculator Inputs
Example Data Table
| Layer | Width | Copper | Rise | Length | Load | Estimated Current |
|---|---|---|---|---|---|---|
| External | 50 mil | 1 oz | 10 °C | 50 mm | 1 A | About 2.1 A |
| Internal | 50 mil | 1 oz | 10 °C | 50 mm | 1 A | About 1.0 A |
| External | 100 mil | 1 oz | 20 °C | 100 mm | 2 A | About 4.7 A |
| External | 40 mil | 2 oz | 15 °C | 75 mm | 2 A | About 3.7 A |
Formula Used
The calculator uses the common IPC-2221 style current estimate:
I = k × ΔT0.44 × A0.725
I is current in amperes. ΔT is allowed temperature rise in degrees Celsius. A is copper cross-sectional area in square mils. The external trace constant is 0.048. The internal trace constant is 0.024.
Cross-sectional area equals trace width multiplied by copper thickness. The tool converts millimeters, inches, ounces, micrometers, and mils before calculation.
Resistance is calculated with:
R = ρ × L ÷ A
Voltage drop equals current multiplied by resistance. Power loss equals current squared multiplied by resistance.
How To Use This Calculator
- Enter the planned trace width.
- Select the matching width unit.
- Enter copper thickness or copper weight.
- Choose external or internal layer placement.
- Add allowed temperature rise.
- Enter trace length and expected load current.
- Set a derating percentage for safety margin.
- Press the calculate button to review results.
- Download the CSV or PDF report if needed.
Understanding PCB Trace Current
A PCB trace carries current like a flat copper wire. Its safe current depends on width, copper thickness, temperature rise, and layer placement. Wider traces carry more current. Thicker copper also helps. Outer layers usually cool better because air can remove heat faster.
Why Trace Sizing Matters
Small traces can overheat when current is high. Heat raises resistance. Higher resistance creates more voltage drop. That drop can disturb sensitive circuits. It can also waste power. Good sizing reduces heat stress and improves reliability. It also protects nearby parts from unwanted temperature rise.
External And Internal Layers
The calculator separates external and internal traces. External traces use a larger constant in the current equation. Internal traces use a smaller constant because buried copper loses heat more slowly. This makes internal traces need more area for the same current and temperature limit.
Copper Thickness And Width
Copper weight is often listed in ounces. One ounce copper is about 34.8 micrometers thick. Two ounce copper is thicker and lowers resistance. Width is entered in mils, millimeters, or inches. The tool converts each value before applying the equation.
Voltage Drop And Power Loss
Current capacity is only one design check. A trace may stay cool but still drop too much voltage. This calculator estimates resistance from copper resistivity, length, width, thickness, and temperature. It then calculates voltage drop and power loss for your entered load current.
Derating And Practical Design
Derating gives extra safety. A 80 percent derating limit means the design uses only 80 percent of the estimated capacity. This allows space for manufacturing variation, blocked airflow, solder mask, heat from parts, and high ambient temperature.
Best Use Cases
Use this tool during early board layout. Check power rails, motor lines, LED strings, charging paths, and connector traces. Compare several widths before routing. Review both current margin and voltage drop. Final safety may still need lab testing, thermal review, and manufacturer guidance.
Reading The Result
The result lists estimated capacity, derated capacity, resistance, voltage drop, power loss, copper temperature, and required width. A positive margin is usually preferred. A negative margin means the trace is undersized for the chosen limit. Save exports for documentation, checking, and team review later.
FAQs
What does this PCB trace current calculator estimate?
It estimates current capacity, derated safe current, resistance, voltage drop, power loss, and required width using trace geometry, copper thickness, layer type, length, and temperature rise.
What is the difference between internal and external traces?
External traces usually cool better because they contact air. Internal traces are buried inside board layers, so they normally need more copper area for the same current.
Why does copper thickness affect current capacity?
Thicker copper increases cross-sectional area. More area lowers resistance and heat generation. This helps the trace carry more current with less voltage drop.
What is temperature rise?
Temperature rise is the allowed increase above ambient temperature. A lower rise is more conservative. A higher rise allows more current but creates more heat.
Why should I use derating?
Derating adds safety margin. It helps cover real board effects, manufacturing tolerance, airflow limits, nearby heat sources, solder mask, and operating changes.
Can this replace thermal testing?
No. It is useful for planning and comparison. Final designs may still need lab testing, thermal simulation, and board manufacturer review.
Why is voltage drop included?
A trace can have enough current capacity but still lose too much voltage. Voltage drop helps check power rail stability and load performance.
Which copper unit should I choose?
Use ounces when your board stackup lists copper weight. Use micrometers or mils when your manufacturer provides actual copper thickness.