OctaCalc / PCB Trace Width Calculator

PCB Trace Width Calculator

Calculate the minimum required copper trace width for your PCB design using the IPC-2221 standard. Enter your current load, copper weight, and maximum allowable temperature rise to get an instant trace width recommendation in both millimetres and mil.

10 °C is a safe general-purpose target. High-reliability designs often use 5 °C.

Used to calculate absolute trace temperature (ambient + rise).

Minimum trace width —
Minimum trace width (mil) —
Trace cross-sectional area —
Copper thickness —

For educational and reference use only. Always verify results before use in real-world designs or safety-critical applications. For more information, see Calculation Assumptions and Disclaimer.

How to Use This Calculator

Enter the maximum continuous current your trace will carry, select the copper weight used in your PCB stack-up, and choose whether the trace is on an external (outer) or internal layer. Set an allowable temperature rise — 10 °C above ambient is a common conservative starting point.

The calculator instantly shows the minimum required trace width in both millimetres and mil. Enable the advanced inputs to also calculate trace resistance, voltage drop, and power dissipation for a specific trace length.

Use the copper weight presets (0.5 oz, 1 oz, 2 oz, 3 oz) to quickly compare how heavier copper reduces the required trace width. The oz/ft² and µm fields stay synchronised — editing one automatically updates the other.

Formula

This calculator uses the IPC-2221 empirical formula for current-carrying capacity:

I = k × ΔT0.44 × A0.725

Rearranged to solve for the required trace cross-sectional area (in mil²):

A = ( I / ( k × ΔT0.44 ) )1 / 0.725

Trace width from area and copper thickness:

W (mil) = A / T

Where k = 0.048 for external layers and k = 0.024 for internal layers; T is copper thickness in mil; ΔT is allowable temperature rise in °C.

Optional resistance and power calculations:

R = ρ × L / (W × t)    Vdrop = I × R    P = I² × R

Where ρ = 1.724 × 10⁻⁸ Ω·m (copper resistivity at 20 °C), L is trace length in metres, and dimensions are in SI units.

Example

Given: 2 A current, 1 oz copper, 10 °C temperature rise, external layer, 100 mm trace length.

Convert copper weight: 1 oz ≈ 1.378 mil thickness.

Calculate area: A = (2 / (0.048 × 100.44))1/0.725 = (2 / 0.1322)1.379 ≈ 42.1 mil²

Trace width: W = 42.1 / 1.378 ≈ 30.6 mil

Required trace width ≈ 0.78 mm (30.6 mil)

For 100 mm trace length, resistance ≈ 0.064 Ω, voltage drop ≈ 0.13 V, power dissipation ≈ 0.26 W.

Frequently Asked Questions

What is IPC-2221 and why is it used for trace width calculations?
IPC-2221 is the generic standard for printed board design from the IPC (Association Connecting Electronics Industries). It provides empirical formulas for minimum trace widths based on current, copper weight, and temperature rise. It is the industry-standard reference because its formulas have been validated against real-world measurements.

What is the difference between an external and internal PCB layer?
External (outer) layers are exposed to air and dissipate heat more effectively. Internal layers are sandwiched between FR4 dielectric material, which acts as thermal insulation. Because internal layers cannot shed heat as easily, they require wider traces for the same current at the same temperature rise. IPC-2221 accounts for this with k = 0.048 for external and k = 0.024 for internal layers.

What does copper thickness in oz/ft² mean?
Copper weight in oz/ft² describes how much copper is spread over one square foot of PCB surface. 1 oz/ft² equals approximately 35 µm (1.378 mil). Common weights are 0.5 oz (17 µm), 1 oz (35 µm), 2 oz (70 µm), and 3 oz (105 µm). Heavier copper allows narrower traces for the same current but adds cost and complicates fine-pitch routing.

How much temperature rise should I allow?
A rise of 10 °C above ambient is a common conservative target for general electronics. High-reliability or high-frequency designs often target 5 °C or less. Power electronics may allow 20–30 °C. Remember that ambient temperature adds to trace temperature: a 10 °C rise in a 50 °C environment gives a 60 °C trace temperature, which must remain below the PCB laminate's rating.

Why is the minimum manufacturable trace width important?
Most standard PCB manufacturers reliably produce traces down to about 0.1 mm (4 mil). Traces narrower than this may require advanced processes and higher cost. This calculator warns you when the calculated width falls near or below this threshold so you can increase copper weight or reconsider the design before sending files to fabrication.