PCB Trace Current Calculator

How to use this PCB trace current calculator

What this calculator does

This calculator estimates how much current a rectangular copper PCB trace can carry for a selected allowable temperature rise using the familiar IPC-2221 current-versus-cross-section curve fit. It also derives cross-sectional area, trace temperature, temperature-adjusted copper resistance, voltage drop at the estimated maximum current, and resistive power dissipation. It is useful for early routing checks and design comparisons, but it is not a substitute for a board-specific thermal model, compliance review, prototype measurement, or manufacturer guidance. The current formula is a simplified empirical model; nearby planes, copper pours, laminate properties, airflow, enclosure temperature, and manufacturing variation can materially change real behavior.

When to use it

Use it when checking whether an existing power trace has reasonable current-carrying headroom, comparing internal and external routing, seeing how thicker copper changes capacity, or estimating the voltage-drop penalty of a long trace. It is also useful as a quick sanity check before detailed simulation. For the underlying printed-board design context, see IPC's description of the current IPC-2221C Generic Standard on Printed Board Design.

How to calculate

1. The calculator opens with a ready-to-use demonstration: an internal trace, 9 mil width, 2 oz/ft² copper, 10 °C allowable rise, 25 °C ambient, and 10 in length. The first results and the example Excel workbook are available immediately.
2. Replace the demonstration values with your trace data. Use the adjacent unit selectors instead of manually converting values; switching a unit converts the current entry and recalculates the model. The Additional inputs panel contains Ambient temperature and Trace length and may be collapsed when you do not need to edit them.
3. Read Maximum current first, then review Resistance, Voltage drop, and Power dissipation to decide whether thermal capacity alone is sufficient for the electrical design.
4. Select Download Excel to export the current validated inputs and canonical results to a real XLSX workbook. Select Reset to clear the demonstration and entered numeric data. After Reset, Excel export is disabled until a complete valid state is entered again.

Input guide

• Trace location is required and accepts External or Internal. Internal uses k = 0.024; External uses k = 0.048. With otherwise identical inputs, the external-layer estimate is higher. Do not use External merely because it produces a larger number.
• Trace width is a required positive decimal. The demonstration value is 9 mil. Width unit accepts mil or mm and converts the value when changed. Wider traces increase cross-sectional area and therefore increase the current estimate. A common mistake is confusing mil (0.001 inch) with millimetres.
• Trace thickness is required and positive. The example is 2 oz/ft². Thickness unit accepts oz/ft², mil, or µm; this implementation follows the reference convention of 1 oz/ft² = 1.378 mil. Greater thickness increases area and current capacity. Enter finished copper thickness when that differs materially from nominal foil weight.
• Max. desired temperature rise is required and must be greater than zero. The example is 10 °C. Temperature-rise unit accepts °C or °F as a temperature difference, so 10 °C converts to 18 °F. A larger allowed rise produces a larger estimated current, but that does not mean the hotter operating point is suitable for components or laminate.
• Ambient temperature is required for the resistance calculation and may be entered in °C or °F using Ambient-temperature unit. The demonstration uses 25 °C. Ambient temperature does not change the IPC current curve directly here; it changes operating trace temperature and therefore the copper resistance adjustment.
• Trace length is required and positive. The example is 10 in. Length unit accepts in, cm, or mm. Length does not alter the IPC current-capacity estimate in this model, but a longer trace raises resistance, voltage drop, and I²R power loss.

Numeric fields accept ordinary decimal numbers using a dot as the decimal separator, with optional correctly grouped commas such as 1,250.5. Scientific notation, decimal commas such as 1,5, mixed units pasted into the number field, missing values, and non-finite values are rejected rather than silently reinterpreted.

Output guide

Maximum current is the estimated amperage at the selected rise; it is an empirical estimate, not a guaranteed rating. Cross section is width × copper thickness in square mils and is an exact geometric conversion within the stated 1.378 mil/oz convention. Trace temperature is ambient plus the allowed rise. Resistance is the calculated copper resistance at that trace temperature and length. Voltage drop is maximum current × resistance, while Power dissipation is resistance × current². Zero is not valid for width, thickness, rise, or length here; unusually high voltage drop or power loss can make an otherwise thermally acceptable trace unsuitable.

Worked example

For the startup internal trace, the cross section is 9 mil × 2 oz/ft² × 1.378 mil per oz/ft² = 24.804 mil². Using k = 0.024, the curve fit gives I = 0.024 × 10^0.44 × 24.804^0.725 = 0.678 A after display rounding. The trace temperature is 25 + 10 = 35.00 °C. For 10 in of copper, the temperature-adjusted resistance is approximately 0.2804 Ω; that produces about 0.190 V drop and 0.129 W dissipation at the estimated maximum current. These values are the same values used by the first-open results and workbook checkpoints.

Learn more

The formula used here is the common IPC-2221 curve-fit interpretation also documented in DigiKey's PCB trace-width formula guide. For the resistance adjustment, the temperature dependence of copper resistance is a standard physical effect; NIST's historical Copper Wire Tables handbook provides reference data for copper resistivity and temperature coefficients.