PCB Trace Width Calculator
Copper width from current and temperature rise — the classic IPC curve-fit, with the trace's own resistance, drop and loss along its length.
PCB Trace Width Calculator
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What this result does not account for
- Steady-state curve fit — no via or plane effects
- Conservative 1959 data; pulsed loads may exceed
In short: Two amperes on an external layer, allowed a ten-degree rise over one-ounce copper: the curve fit asks for about 42.4 square mils of cross-section — a trace 30.8 mils wide, call it 0.78 mm, about the short side of an 0603 resistor. Route the same current on an internal layer and the constant halves, the demand on area multiplies by about 2.6, and the trace widens past two millimetres. Copper is a budget: the rise you allow is the width you pay.
Formula
I = k · ΔT0.44 · A0.725 — A = (I ÷ kΔT0.44)1/0.725
The classic board chart's curve fit ties current, allowed temperature rise and copper cross-section: k is 0.048 for external layers and 0.024 for internal, the rise enters at the 0.44 power and area at the 0.725. Solve for area, divide by the foil thickness — one ounce of copper is 1.378 mils, about 35 µm — and the minimum width falls out. Because area sits under the 0.725 power, halving k multiplies the demanded area by about 2.6, not 2. The fit is a steady-state estimate from 1959-era measurements; short pulses can exceed it and the newer standard measures more.
Worked Example
- Choose the rise you can live with.
- Solve the curve fit for cross-section.
- Divide by foil thickness for the width.
Defaults: 2 A, ten degrees, one-ounce external — 0.78 mm minimum. Flip the constant to internal and the same current wants about 2.05 mm; double the allowed rise to twenty degrees and it relaxes to 0.58 mm.
Strengths & Limits Of This Model
Where this engine is strong
- Width, cross-section, drop and loss in one card
- Internal-layer penalty made explicit
Where it stops
- No copper pour, via stitching or airflow terms
Practical Use Cases
Power rails
width before the layout freeze
Internal layers
the 2.6× penalty made real
Drop budgets
milliohms along long runs
Methodology & Editorial Standards
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
PCB Trace Width Calculator — 8 Expert FAQs
8 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
Why are internal traces wider?
Buried copper sheds heat through FR-4 instead of air, so the fit halves its constant. Because area sits under the 0.725 power, the area demand multiplies by about 2.6 for the 2× constant — the geometric middle of the penalty.
How much rise should I allow?
Ten degrees is the conservative default when components nearby are rated for ordinary ambients; twenty is the space-pressed compromise. The rise stacks on the board's ambient, so a hot enclosure spends your margin before the trace warms.
What is a mil and an ounce?
A mil is a thousandth of an inch, 25.4 µm. One ounce of copper spread over a square foot is about 1.378 mils — 35 µm — thick. Copper weight is the foil spec every fab quotes in.
Can pulses exceed the chart?
Yes — the fit assumes steady state. Short bursts under a few seconds ride thermal inertia and can carry half again the chart current; long pulses settle toward the steady number.
How accurate is the resistance line?
The resistivity arithmetic is exact for the computed cross-section; the width is a minimum, and etch trapezoids and plating nudge the real value a few percent. Treat the drop as a floor estimate.
Does solder mask change it?
Marginally — it insulates the surface slightly, which the 1959 data pre-dates. The standard conservative answer is to keep the computed width and let mask be a rounding error.
When do I need a copper pour instead?
Past about ten amperes the widths stop being traces — a two-ampere-class fit at ten amperes and twenty degrees wants nearly nine millimetres. Pours, stitches and busbars take over where geometry gives up.
Is the newer standard different?
The later IPC 2152 work measured real boards with modern geometries and generally allows a bit more current than the old chart. The classic fit remains the conservative, universally quoted baseline — a sane place to start.