What it does
Two everyday sums for circuit board layout:
- Trace width for current. A copper trace heats up as current flows. The tool gives the narrowest trace that stays within your chosen temperature rise, using the formula fitted to the IPC-2221 design chart, then the trace’s resistance, voltage drop and heat over its length.
- Impedance. Fast signals (USB, Ethernet, RF, fast clocks) need traces of a set characteristic impedance, often 50 Ω single-ended. Give the stack-up and width to get Z0, or the impedance to get the width, for a microstrip (outer layer over a plane) or a stripline (inner layer centred between two planes).
These formulas are approximations. They are good for planning and checking, but for a board that must hit an impedance, ask the board maker to calculate it with their field solver and real materials. For study and planning, not a substitute for professional judgement.
How to use it
- Choose millimetres or mils. Values already typed are converted when you switch.
- For current: give the current, the temperature rise you accept (10 °C is a common, cautious choice), the copper weight and whether the trace is on an outer or inner layer. Add the length to see resistance and drop.
- For impedance: pick microstrip or stripline, the dielectric height (to the plane for microstrip; between the two planes for stripline), the board εr and either the width or the impedance you want.
Worked example
A made-up 2 A supply trace on an outer layer with 1 oz copper and a 10 °C rise needs at least 0.786 mm (31 mil), a cross-section of 42.4 mil². Over 50 mm it has about 33.4 mΩ at 35 °C, so it drops about 67 mV and makes about 133 mW of heat. The same current on an inner layer needs about 2.05 mm.
A microstrip 0.35 mm wide on 0.2 mm of dielectric with εr 4.2 and 1 oz copper is Z0 = 51.2 Ω, with an effective εr of 3.05 and a delay of 5.83 ps/mm. For exactly 50 Ω it should be 0.366 mm wide.
FAQ
Why is the inner layer so much wider? Inside the board the heat must pass through the laminate; the IPC chart halves the current for the same cross-section, so the area grows by about 2.6 times.
How accurate is the IPC-2221 formula? It is based on old measurements of single traces and is usually cautious. Wide copper pours, planes underneath and airflow let traces carry more; a crowded board in a box carries less. IPC-2152 has newer, more detailed charts.
Does solder mask change the impedance? Yes, usually by lowering microstrip Z0 by 1 to 3 Ω. The formula here ignores solder mask, frequency dependence and copper roughness.
Where the numbers come from
- Trace current capacity: the curve-fit formula for the IPC-2221 design chart, I = k ΔT^0.44 A^0.725 (as widely published; the formula only, no chart data).
- Microstrip: Hammerstad, E. and Jensen, O. (1980), Accurate models for microstrip computer-aided design, IEEE MTT-S Digest, with their strip-thickness correction. Stripline: Wheeler, H. A. (1978), Transmission-line properties of a strip line between parallel planes, IEEE Trans. MTT 26 (equations only).
- Copper resistivity 1.724 × 10⁻⁸ Ω·m and temperature coefficient 0.00393 per °C (annealed copper, a general fact).
- Checked against an independent 2-D finite-volume field solve in SciPy and the exact stripline formula. Example values are made up.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.