# What trace width do I need?

Works out the minimum width of a printed circuit board trace for a current, a temperature rise and a copper thickness, on an outer or inner layer, from the IPC-2221 formula.

- Page: https://www.acalculator.org/physics/trace-width-calculator
- JSON spec: https://www.acalculator.org/physics/trace-width-calculator.json
- Version: b3b195a1f917

## Default answer

Example with the default inputs (Current 1 A, Temperature rise (°C) 10, Copper weight (oz/ft²) 1, Layer Outer (external)): Use a trace at least 11.83 mils (0.3004 mm) wide.

## Inputs

| Key | Label | Description |
| --- | --- | --- |
| i | Current | The current the trace must carry. |
| dt | Temperature rise (°C) | How much hotter than the surroundings the trace may get, in °C. 10 °C is a common choice. |
| oz | Copper weight (oz/ft²) | The copper thickness as weight per square foot: 1 oz is 35 µm (1.378 mil). |
| layer | Layer | An outer layer cools in the air; an inner layer is buried in the board and needs a wider trace. |

## Outputs

| Key | Label | Description |
| --- | --- | --- |
| width | Minimum width (mil) | The narrowest trace that meets the rise, in mils (thousandths of an inch). |
| mm | Minimum width (mm) | The same width in millimetres (1 mil = 0.0254 mm). |
| area | Cross-section (mil²) | The copper cross-section area the current needs, in square mils. |
| thickness | Copper thickness (mil) | Copper weight × 1.378 mil. |

## Method

A (mil²) = (I ÷ (k × ΔT^0.44))^(1 ÷ 0.725), k = 0.048 (outer layer) or 0.024 (inner layer). Thickness (mil) = oz × 1.378. Width = A ÷ thickness; mm = mil × 0.0254.

## Assumptions

- IPC-2221 is a curve fit to old measurements of bare traces in still air; it is a starting point, often conservative for inner layers. Planes, nearby copper and airflow change the real temperature.
- The charts behind the fit cover currents up to 35 A, widths up to 400 mil, rises of 10 to 100 °C and 0.5 to 3 oz copper; outside that range treat the answer with care.

## Worked examples

1. i = 1, dt = 10, oz = 1, layer = external gives width = 11.825836, mm = 0.300376, area = 16.296001. Source: Salitronic, "PCB trace width calculator and guidelines" (IPC-2221: I = k × ΔT^0.44 × A^0.725, k = 0.048 external, 0.024 internal, A in mil²; 1 oz copper = 35 µm), https://salitronic.com/kb/pcb-trace-width/ (retrieved 2026-10-05); Advanced Circuits, "PCB trace width calculator" (width = area ÷ (oz × 1.378 mil); the IPC-2221 charts cover up to 35 A, 400 mil, 10 to 100 °C rise, 0.5 to 3 oz), https://www.advancedpcb.com/en-us/tools/trace-width-calculator/ (retrieved 2026-10-05).
2. i = 5, dt = 20, oz = 2, layer = internal gives width = 92.986118, area = 256.269741. Source: Salitronic, "PCB trace width calculator and guidelines" (IPC-2221: I = k × ΔT^0.44 × A^0.725, k = 0.048 external, 0.024 internal, A in mil²; 1 oz copper = 35 µm), https://salitronic.com/kb/pcb-trace-width/ (retrieved 2026-10-05).
3. i = 10, dt = 10, oz = 1, layer = external gives width = 283.231909, mm = 7.19409. Source: Salitronic, "PCB trace width calculator and guidelines" (IPC-2221: I = k × ΔT^0.44 × A^0.725, k = 0.048 external, 0.024 internal, A in mil²; 1 oz copper = 35 µm), https://salitronic.com/kb/pcb-trace-width/ (retrieved 2026-10-05).

## FAQ

### How do I calculate PCB trace width?

IPC-2221 relates current I (A), temperature rise ΔT (°C) and cross-section A (square mils): I = k × ΔT^0.44 × A^0.725, with k = 0.048 on outer layers and 0.024 on inner layers. Solve for A, then divide by the copper thickness: 1 oz copper is 1.378 mil. For 1 A, 10 °C and 1 oz on an outer layer the width is about 11.8 mil (0.30 mm).

### Why do inner layers need wider traces?

An inner trace is buried in the board material, which conducts heat poorly, so it cannot shed heat into the air. IPC-2221 halves the constant k for inner layers; for the same current and rise, the trace needs about 2.6 times the cross-section.

### What temperature rise should I use?

10 °C above the surroundings is a common, cautious choice; 20 °C is often used where space is tight. A larger rise allows a narrower trace but runs the copper hotter, which matters near heat-sensitive parts and in hot enclosures.

### What does copper weight in ounces mean?

It is the weight of copper spread over one square foot. 1 oz of copper on a square foot is about 35 µm (1.378 mil) thick; 2 oz is twice that. Thicker copper carries the same current in a narrower trace.

### How accurate is the IPC-2221 formula?

It is a curve fit to old charts of bare traces in still air, covering up to 35 A, widths up to 400 mil, rises of 10 to 100 °C and 0.5 to 3 oz copper. Planes, nearby copper, vias and airflow change the real temperature. Use it as a starting point and leave a margin.

### How do I convert mils to millimetres?

A mil is a thousandth of an inch, so 1 mil = 0.0254 mm. A 10 mil trace is 0.254 mm wide, and 0.1 mm is about 3.94 mil.

## Sources

- Salitronic, "PCB trace width calculator and guidelines": IPC-2221, I = k × ΔT^0.44 × A^0.725 with k = 0.048 external and 0.024 internal, A in mil²; 1 oz copper is 35 µm. https://salitronic.com/kb/pcb-trace-width/ (retrieved 2026-10-05)
- Advanced Circuits, "PCB trace width calculator": width = area ÷ (oz × 1.378 mil), and the chart ranges (up to 35 A and 400 mil, 10 to 100 °C, 0.5 to 3 oz). https://www.advancedpcb.com/en-us/tools/trace-width-calculator/ (retrieved 2026-10-05)