About the PCB Trace Width Calculator
This PCB trace width calculator finds the minimum copper trace width needed to carry a given current without heating more than a chosen amount above ambient. It uses the widely published curve-fit of the IPC-2221 design charts, for both external (outer) layers and internal layers, and converts the required cross-section into a width for your copper weight (0.5 oz to 3 oz and beyond).
It is aimed at hardware designers laying out power rails, motor drivers, LED strips and battery circuits, and at hobbyists choosing a trace width in KiCad, Eagle or Altium. Besides the width in mils and millimetres, it reports the trace resistance, voltage drop and power lost along the length you enter, which often matters more than heating on long or low-voltage runs.
The IPC-2221 charts were drawn for roughly 0–35 A, widths up to 0.4 in (10 mm), 10–100 °C rise and 0.5–3 oz copper; outside that range the formula extrapolates. The newer IPC-2152 standard, based on fresh test data, generally allows narrower internal traces, so IPC-2221 is a conservative starting point. Add margin for high ambient temperatures, enclosed boards and nearby hot parts.
With the default inputs, the minimum trace width is 30.76 mil. Change any value above to recalculate instantly.
How to use the pcb trace width calculator
- 1Enter the maximum continuous current the trace must carry.
- 2Choose the temperature rise you can accept (10 °C is conservative).
- 3Select the copper weight and whether the trace is on an outer or inner layer.
- 4Enter the trace length and ambient temperature for the resistance figures.
- 5Use the width shown as a minimum and round up in your PCB tool.
Formula and method
IPC-2221 gives current-carrying capacity as charts of current against conductor cross-section for several temperature rises. The industry-standard curve fit of those charts is I = k × ΔT^0.44 × A^0.725, with A in square mils, I in amps, ΔT in °C, k = 0.048 for external layers and k = 0.024 for internal layers. Solving for A gives the minimum cross-section, and dividing by the copper thickness (1 oz/ft² ≈ 1.378 mil ≈ 35 µm) gives the width.
Resistance uses annealed copper at 1.724 × 10⁻⁸ Ω·m at 20 °C, corrected by +0.393% per °C to the trace’s operating temperature (ambient + allowed rise). Voltage drop is I × R and the heat dissipated is I² × R. Plating, solder mask and nearby planes change real results slightly.
- I
- Current (A)
- ΔT
- Temperature rise above ambient (°C)
- A
- Conductor cross-section (mil²)
- k
- 0.048 external, 0.024 internal
- W
- Trace width (mil)
- ρ(T)
- Copper resistivity at operating temperature
- L
- Trace length
Worked examples
2 A on a 1 oz outer layer, 10 °C rise
A = (2 ÷ (0.048 × 10^0.44))^(1/0.725) ≈ 42.39 mil². Dividing by 1.378 mil of 1 oz copper gives about 30.8 mil (0.78 mm). A 50 mm run at 35 °C has about 0.033 Ω, dropping 67 mV and dissipating 0.13 W.
5 A motor feed on 2 oz copper, 20 °C rise
A = (5 ÷ (0.048 × 20^0.44))^(1/0.725) ≈ 98.5 mil². With 2 oz copper (2.756 mil thick) the width is about 35.7 mil (0.91 mm). Over 100 mm the trace drops roughly 0.15 V.
1 A on an internal 1 oz layer
Internal layers use k = 0.024, half the external value, so 1 A on an inner layer needs the same 42.4 mil² — about 30.8 mil — as 2 A on the outside.
Frequently asked questions
How wide should a PCB trace be for 1 amp?+
With IPC-2221, 1 oz copper and a 10 °C rise, 1 A needs about 12 mil (0.30 mm) on an outer layer and about 31 mil (0.78 mm) on an inner layer. Many designers simply use 20 mil or more for 1 A outer-layer power traces.
Why do internal traces need to be wider?+
Buried traces are surrounded by FR-4, a poor heat conductor, while outer traces can lose heat to the air. IPC-2221 therefore halves the constant k for internal layers, increasing the required width about 2.6 times for the same current.
What is the difference between IPC-2221 and IPC-2152?+
IPC-2152 replaced the old charts with new test data covering board thickness, planes and copper weight. It usually allows narrower internal traces than IPC-2221, so IPC-2221 results are conservative.
What does 1 oz copper mean?+
1 oz copper means one ounce of copper spread over one square foot, a layer about 1.378 mil (35 µm) thick. 2 oz is about 70 µm and halves the width needed for the same cross-section.
What temperature rise should I use?+
A 10 °C rise is a common conservative default. Up to 20–30 °C is often acceptable for compact boards, but check that ambient plus rise stays well below your laminate and component ratings.