PCB trace width calculator

A 3 A trace on a 1 oz outer layer needs 1.65 mm for a 10 °C rise. That is the IPC-2152 figure; the older IPC-2221 curve gives 1.37 mm. Current, allowed rise and copper weight set the width; a plane within 1 mm roughly halves the rise. Both assume steady DC in still air; short pulses and hot neighbours break that.

Width, current or temperature rise — whichever one you are missing — from the IPC-2221 curves or the IPC-2152 universal chart, plus what the trace drops and dissipates at that current.

Fig 1 — Conductor cross-section, 16.3 mils² of copper.

Model

Solve for

Current (A)

Temperature rise (°C)

Copper weight (oz)

Layer

Trace length (mm)

Ambient temperature (°C)

Width: 11.8 mils · 0.300 mm
Current: 1.00 A
Temperature rise: 10.0 °C
Cross-section: 16.3 mils² · 0.0105 mm²
Copper temperature: 35 °C
Resistance, 100 mm: 169 mΩ
Resistance per metre: 1.69 Ω/m
Voltage drop: 169 mV
Power in the trace: 169 mW

How this is calculated

Standard: IPC-2221A Section 6.2 (conductor sizing charts); IPC-2152 universal chart

$I = k \cdot \Delta T^{0.44} \cdot A^{0.725}$: IPC-2221, with k = 0.048 on external layers and 0.024 on internal layers.
$I = 0.0454 \cdot \Delta T^{0.5453} \cdot A^{0.6533}$: IPC-2152, a curve fit of the universal chart to within ±10 %.
$I \times 2^{0.5453}$: The IPC-2152 correction for a plane within 1 mm, which halves the temperature rise.
$\text{width} = \frac{A}{\text{oz} \cdot 1.378}$: Copper weight in oz, cross-section in mils², width in mils.
$R = \frac{\rho \cdot L}{A} \cdot \bigl(1 + 0.00393 \cdot (T - 20)\bigr)$: Copper resistivity is 1.68e-8 Ω·m, and T is the ambient temperature plus ΔT.
$V = I \cdot R, \quad P = I^2 \cdot R$: Voltage dropped along the trace and the power it dissipates.

Assumptions

Still air, no forced cooling.
No adjacent copper pours acting as a heatsink.
No derating for solder mask or conformal coating.
DC or low-frequency current only — skin effect is ignored.
IPC-2221 internal is very conservative; IPC-2152 found internal and external traces run within a few per cent of each other.
The IPC-2152 fit is for 1 oz on a 70 mil board. Heavier copper and thinner boards shift the chart; the standard has correction charts for both.

What sets a trace's width for a given current

How wide a copper trace has to be so that a given DC current heats it by no more than a chosen amount above ambient. Two data sets are on offer, and they do not agree.

IPC-2221 (Generic Standard on Printed Board Design, Section 6.2, conductor sizing charts) is the one every older calculator uses. The curves were drawn in the 1950s from a small set of measurements; the closed form everyone fits to them is the first expression under How this is calculated above, with current in amps, ΔT in °C and cross-section in mils². Solving it for area rather than current is a matter of raising both sides to 1/0.725, and the width follows from the copper weight: 1 oz/ft² is 1.378 mil, or 34.79 µm.

IPC-2152 (Standard for Determining Current Carrying Capacity in Printed Board Design, 2009) replaced those charts with a much larger measured data set. It is published as charts, not equations. The calculator uses the common curve fit of the universal chart — 1 oz external copper, still air, 70 mil board, no plane nearby — which tracks the chart to within about ±10 % — the second expression above, in the same units, inverted the same way through its own exponent.

IPC-2152's most useful finding is that a copper plane within about 1 mm of the trace roughly halves the temperature rise for the same current. The optional plane correction applies exactly that: the predicted rise is halved, which is the same as solving the fit with 2·ΔT. It is a rule of thumb lifted from the standard's correction charts, not a separate fit.

Any of the three variables can be the unknown — width, current or temperature rise — the equation is just rearranged. Given a trace length the tool also reports DC resistance, voltage drop and dissipated power at the operating temperature, which is the ambient plus the rise — the last two expressions above.

IPC-2221 trace width chart, 10 °C rise

Minimum trace width for a 10 °C temperature rise, computed by the calculator above from the IPC-2221 formula for the two copper weights a board house quotes by default. Internal layers get half the current of external ones at the same width, because the standard assumed they cool only through the laminate.

Current	1 oz external	1 oz internal	2 oz external	2 oz internal
0.5 A	0.12 mm (5 mil)	0.30 mm (12 mil)	0.06 mm (2 mil)	0.15 mm (6 mil)
1 A	0.30 mm (12 mil)	0.78 mm (31 mil)	0.15 mm (6 mil)	0.39 mm (15 mil)
2 A	0.78 mm (31 mil)	2.03 mm (80 mil)	0.39 mm (15 mil)	1.02 mm (40 mil)
3 A	1.37 mm (54 mil)	3.56 mm (140 mil)	0.68 mm (27 mil)	1.78 mm (70 mil)
5 A	2.77 mm (109 mil)	7.19 mm (283 mil)	1.38 mm (54 mil)	3.60 mm (142 mil)
10 A	7.19 mm (283 mil)	18.71 mm (737 mil)	3.60 mm (142 mil)	9.36 mm (368 mil)

IPC-2152 trace width chart, 10 °C rise

The same currents through the IPC-2152 fit. The standard measured internal traces within a few per cent of external ones on the same board, so the layer column is gone; what replaced it is the copper plane, which the standard found roughly halves the rise when it sits within about a millimetre of the trace.

Current	1 oz, no plane	1 oz, plane within 1 mm	2 oz, no plane	2 oz, plane within 1 mm
0.5 A	0.11 mm (4 mil)	0.06 mm (2 mil)	0.05 mm (2 mil)	0.03 mm (1 mil)
1 A	0.31 mm (12 mil)	0.17 mm (7 mil)	0.15 mm (6 mil)	0.09 mm (3 mil)
2 A	0.89 mm (35 mil)	0.50 mm (20 mil)	0.44 mm (17 mil)	0.25 mm (10 mil)
3 A	1.65 mm (65 mil)	0.92 mm (36 mil)	0.82 mm (32 mil)	0.46 mm (18 mil)
5 A	3.60 mm (142 mil)	2.02 mm (79 mil)	1.80 mm (71 mil)	1.01 mm (40 mil)
10 A	10.40 mm (410 mil)	5.83 mm (230 mil)	5.20 mm (205 mil)	2.92 mm (115 mil)

Worked example: 3 A at a 10 °C rise in 1 oz copper

3 A, 10 °C rise, 1 oz copper, external layer, 100 mm long, 20 °C ambient.

IPC-2221 external
  ΔT^0.44      = 10^0.44        = 2.754
  k · ΔT^0.44  = 0.048 · 2.754  = 0.1322
  I / that     = 3 / 0.1322     = 22.70
  A            = 22.70^(1/0.725) = 22.70^1.379 = 74.2 mil²
  W            = 74.2 / 1.378   = 53.8 mil = 1.37 mm

IPC-2221 internal (k = 0.024)
  A = (3 / (0.024 · 2.754))^1.379 = 45.40^1.379 = 192.9 mil²
  W = 192.9 / 1.378 = 140.0 mil = 3.56 mm

IPC-2152 universal chart, no plane
  ΔT^0.5453        = 10^0.5453        = 3.510
  0.0454 · 3.510   = 0.1594
  I / that         = 3 / 0.1594       = 18.83
  A                = 18.83^(1/0.6533) = 18.83^1.531 = 89.4 mil²
  W                = 89.4 / 1.378     = 64.9 mil = 1.65 mm

Resistance of the 1.65 mm trace at 30 °C
  W · T = 1.6476e-3 m · 34.79e-6 m = 5.732e-8 m²
  R20   = 1.68e-8 · 0.1 / 5.732e-8  = 29.3 mΩ
  R30   = 29.3 · (1 + 0.00393 · 10) = 30.5 mΩ
  V     = 3 · 0.0305 = 91 mV
  P     = 9 · 0.0305 = 0.27 W

So IPC-2221 external says 1.37 mm, IPC-2152 says 1.65 mm, and IPC-2221 internal says 3.56 mm. Read that carefully: at this current the 2152 universal chart is slightly more conservative than 2221 external, and 2221 internal is more than twice as wide as anything 2152 measured. Put a plane 0.2 mm under the trace and 2152 with the plane correction gives 3 A at 10 °C on 0.92 mm (solve with ΔT = 20 °C: A = 50.1 mil², W = 36.4 mil).

Where IPC-2221 and IPC-2152 stop being valid

IPC-2221 range. The charts cover up to 35 A, 100 °C rise, 400 mil width and 0.5–3 oz. Past that the equation still produces a number; the tool flags it as extrapolation.
IPC-2221 internal is not a measurement of internal traces. It is the external curve halved, a safety factor applied in the 1950s. IPC-2152 measured real internal traces and found they run at most a few percent hotter than external ones in still air, and often cooler, because the laminate conducts heat better than air does. The IPC-2152 "conservative" chart is literally the old 2221 internal curve, kept for people who need to cite it.
IPC-2152 universal chart assumptions. 1 oz, external, still air, 70 mil thick board, plane far away. Thinner boards, heavier copper and vacuum all need the standard's correction factors, which the fit does not include. The plane correction here is a single factor of two; the standard's charts show it varying with distance and plane copper weight.
Neither is a thermal model of your board. A trace under a hot regulator, inside an enclosure, or next to a dozen other current-carrying traces will run hotter than any chart. Parallel traces do not get independent heating budgets.
DC only. Skin depth in copper is about 66 µm / √f(MHz), so above a few MHz current crowds to the surface and resistance climbs. For switching-converter outputs the ripple current matters too.
Short pulses. The charts assume steady state. For fault or inrush currents lasting under a second use a fusing estimate (Onderdonk) instead; the trace can carry far more briefly. The fusing current calculator does that one, and reports when the fault is long enough that the adiabatic assumption behind it has stopped holding.
When it matters — over about 5 A, or anywhere a 20 °C rise would be a problem — use the actual IPC-2152 charts with their correction factors, or a thermal simulation, and then measure it with a thermocouple on the first article.

Common trace width mistakes

Using the 2221 internal curve because "it's safe". It costs you double the copper for no measured reason and pushes people toward 2 oz inner layers they do not need.
Trusting the fab's finished copper weight. "1 oz" outer layers start as ½ oz foil and get plated up; the finished thickness is anywhere from 30 to 45 µm depending on the shop. Inner layers are what they say. Ask for the spec, or design to the thinner figure.
Forgetting the resistance. A 3 A trace at 10 °C rise is fine thermally, but 91 mV of drop on a 3.3 V rail is 2.8 %. Long return paths through a cut-up ground fill count double.
Sizing the trace and starving the via. A 1.65 mm 1 oz trace has 0.057 mm² of copper; a single 0.3 mm via with 25 µm plating has 0.026 mm², about half of that. Use the via calculator and add vias.
Picking a 10 °C rise by habit. The rise adds to ambient. 10 °C on a board already at 70 °C inside a box is a different situation from 10 °C on a bench, and FR-4's Tg is not the limit — the solder joints and electrolytic caps next to the trace are.
Necking down at pads and through solder mask openings. The narrowest point sets the temperature; a thermal relief spoke is often the real bottleneck.