Documentation sections

PCB Trace Width Calculator (IPC-2221 / IPC-2152)

POSThttps://rftools.io/api/py/v1/calculate

Slug trace-width-current. Calculate minimum PCB trace width for current capacity per IPC-2221 and IPC-2152’s conservative Figure 5-1. Get resistance, voltage drop, and power dissipation at the width you draw. Free, instant results.

Inputs

Send every one of these in inputs, in the units shown. Most are required: a call without one is refused (400) and costs nothing; one the calculator can do without takes the default shown. An input outside its range is still computed and flagged in the provenance.

Inputs of trace-width-current
Name	Unit	Range	Default	Description
currentnumber	A	0.01 … 100	1	RMS current through the trace
copperWeightnumber	oz	0.5 … 4	1	Copper thickness (1 oz = 35 μm = 1.378 mil)
tempRisenumber	°C	1 … 100	10	Allowed temperature rise above ambient
traceLengthnumber	mm	1 … 10000	100	Length of the trace (for resistance/voltage drop)
isExternalnumber	none	0 … 1	1	Internal layers carry less current than external ones for the same width and temperature riseOne of: 1 External (outer) layer · 0 Internal layer
traceWidthnumber	mm	0.01 … 100	0.5	The width as drawn: the standards check judges it, and the DC resistance, voltage drop and power are computed for it; the minimum widths do not use it
boardThicknessnumber	mm	0.2 … 6	1.6	The whole board’s thickness. Only the board-specific width (rftools thermal model) reads it; the model is validated from 1.4 to 3.2 mm
planeDistancenumber	mm	0 … 6	0	The dielectric between the trace and the nearest solid copper plane under it; 0 means no plane. Only the board-specific width reads it
planeCopperWeightnumber	oz	0.5 … 2	0.5	The copper weight of that plane (35 µm per ounce). Only the board-specific width reads it, and only when there is a plane
standardnumber	none	0 … 1	0	The chart the current is checked against: IPC-2221A’s external or internal curve, or IPC-2152’s conservative chart for every conductorOne of: 0 IPC-2221A Fig. 6-4 · 1 IPC-2152 Fig. 5-1

Outputs

Returned in values, by key. A value that is unbounded or undefined is null.

Outputs of trace-width-current
Field	Unit	Description
width2221mm	mm	IPC-2221 Figure 6-4 through its published fit: chart A (k = 0.048) on an external layer, chart C (k = 0.024) on an internal one
width2152mmheadline	mm	IPC-2152 Figure 5-1, the conservative chart IPC-2152 recommends for internal and external conductors: the same on either layer
resistance	Ω	At the drawn trace width and a 25 °C ambient plus the allowed rise
voltageDrop	V	Current × the DC resistance of the drawn trace width
powerDiss	W	Current² × the DC resistance of the drawn trace width
inChartRange	none	1 when the rise is between 10 and 45 °C and the cross-section at most 700 sq mil, the range Figure 5-1 draws
widthBoardMm	mm	rftools thermal model (board-specific): rftools’ own model of a long trace on this FR-4 board in still air, with the plane, if any, under it, and a 15 % margin on the rise. It is not IPC-2152; inside its validated range it is never wider than the Figure 5-1 width
inModelRange	none	1 when the board-specific width lies inside the range the thermal model was validated over: Width 0.25–5.1 mm (internal from 1.27 mm), copper ½–3 oz, board 1.4–3.2 mm, a plane from 0.1 mm under the trace to where it fits, plane copper ½–2 oz, rise 5–100 °C, ambient 0–85 °C, the rise before the margin
limitCurrent	A	The most current the drawn trace may carry at the allowed temperature rise, read on the chosen chart
limitPercent	%	The current as a percentage of the chart limit; above 100 % does not comply
minTraceWidth	mm	The width at which the current equals the chart limit; at or above it the trace complies

Calling it

Calculate: one call, the request and response in full, and its errors.
Batch: this calculator with others, or a sweep, in one request.
Solve for a target: the value of one input that brings an output to a target.

curl -X POST https://rftools.io/api/py/v1/calculate \
  -H "X-API-Key: $RFTOOLS_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "slug": "trace-width-current",
    "inputs": {
      "current": 1,
      "copperWeight": 1,
      "tempRise": 10,
      "traceLength": 100,
      "isExternal": 1,
      "traceWidth": 0.5,
      "boardThickness": 1.6,
      "planeDistance": 0,
      "planeCopperWeight": 0.5,
      "standard": 0
    }
  }'

import rftools

client = rftools.Client()  # reads RFTOOLS_API_KEY, or the key `rftools login` saved

result = client.calculate("trace-width-current", {
    "current": 1,
    "copperWeight": 1,
    "tempRise": 10,
    "traceLength": 100,
    "isExternal": 1,
    "traceWidth": 0.5,
    "boardThickness": 1.6,
    "planeDistance": 0,
    "planeCopperWeight": 0.5,
    "standard": 0,
})
print(result.values)
print(result.warnings, result.provenance.formula_ref)

const response = await fetch("https://rftools.io/api/py/v1/calculate", {
  method: "POST",
  headers: {
    "X-API-Key": process.env.RFTOOLS_API_KEY,
    "Content-Type": "application/json",
  },
  body: JSON.stringify({
    "slug": "trace-width-current",
    "inputs": {
      "current": 1,
      "copperWeight": 1,
      "tempRise": 10,
      "traceLength": 100,
      "isExternal": 1,
      "traceWidth": 0.5,
      "boardThickness": 1.6,
      "planeDistance": 0,
      "planeCopperWeight": 0.5,
      "standard": 0
    }
  }),
});
console.log(response.status, await response.json());

package main

import (
	"bytes"
	"fmt"
	"io"
	"net/http"
	"os"
)

func main() {
	body := []byte(`{
	  "slug": "trace-width-current",
	  "inputs": {
	    "current": 1,
	    "copperWeight": 1,
	    "tempRise": 10,
	    "traceLength": 100,
	    "isExternal": 1,
	    "traceWidth": 0.5,
	    "boardThickness": 1.6,
	    "planeDistance": 0,
	    "planeCopperWeight": 0.5,
	    "standard": 0
	  }
	}`)
	req, err := http.NewRequest("POST", "https://rftools.io/api/py/v1/calculate", bytes.NewReader(body))
	if err != nil {
		panic(err)
	}
	req.Header.Set("X-API-Key", os.Getenv("RFTOOLS_API_KEY"))
	req.Header.Set("Content-Type", "application/json")
	res, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer res.Body.Close()
	out, _ := io.ReadAll(res.Body)
	fmt.Println(res.Status, string(out))
}

application/json

{
  "slug": "trace-width-current",
  "values": {
    "width2221mm": 0.30038652083330486,
    "width2152mm": 0.7814374629086279,
    "resistance": 0.10433004926108375,
    "voltageDrop": 0.10433004926108375,
    "powerDiss": 0.10433004926108375,
    "limitCurrent": 1.4468880626650513,
    "limitPercent": 69.11384686926509,
    "minTraceWidth": 0.30038652083330486,
    "inChartRange": 1,
    "widthBoardMm": 0.3021150755977321,
    "inModelRange": 1
  },
  "warnings": [],
  "errors": [],
  "provenance": {
    "method": "calculator:trace-width-current",
    "version": "api@3f2c1a9b04de",
    "formulaRef": "IPC-2221A (May 2003) §6.2, Figure 6-4 (the chart of IPC-2221, February 1998), through its published fit; IPC-2152 (August 2009) §5, Figure 5-1; the board-specific width: rftools thermal model (board-specific), fitted to its own 2-D solver and checked against Brooks & Adam (2015), Jouppi (2010) and Olson (2010)",
    "assumptions": [
      {
        "code": "ipc-2221-fit",
        "text": "The IPC-2221 width is IPC-2221 Figure 6-4 through its published fit A = (I/(k·ΔT^0.44))^(1/0.725), with A in mil², k = 0.048 for an external layer (chart A) and 0.024 for an internal one (chart C). Chart A draws 10 to 100 °C and chart C 10 to 45 °C, both to 700 sq mil; the curves include IPC-2221’s 10 % derating, and its note 2 (a further 15 % for boards 0.8 mm or thinner or copper 108 µm or thicker) is not applied."
      },
      {
        "code": "ipc-2152-figure-5-1",
        "text": "The IPC-2152 width is read on IPC-2152 Figure 5-1, the chart IPC-2152 recommends for internal and external conductors in all environments, through the same fit with k = 0.024 on either layer. It is the conservative answer for a board about which nothing else is known: IPC-2152’s detailed method, which narrows the trace for the board’s thickness and a copper plane under it, is not applied."
      },
      {
        "code": "steady-state-dc",
        "text": "Steady DC (or RMS) current in a single trace: pulsed currents, skin effect and heating from neighbouring traces are not modelled."
      },
      {
        "code": "copper-weight",
        "text": "Copper is 35 µm (1.378 mil) thick per ounce, and the trace cross-section is a rectangle of that thickness."
      },
      {
        "code": "standards-check-chart",
        "text": "The standards check reads the drawn width on IPC-2221A Figure 6-4’s curve for the layer, or on IPC-2152’s conservative Figure 5-1, which is IPC-2221’s internal curve, for either layer. It applies neither the further 15 % derating Figure 6-4 suggests for boards 0.8 mm thick or less and copper 108 µm thick or more, nor IPC-2152’s detailed charts and modifiers."
      },
      {
        "code": "resistance-at-temperature",
        "text": "Resistance, voltage drop and power are for the drawn trace width, with ρ = 1/58 Ω·mm²/m (IEC 60028, 1.7241×10⁻⁸ Ω·m) at 20 °C and α = 0.00393 /°C, evaluated at a 25 °C ambient plus the allowed rise."
      },
      {
        "code": "thermal-model-own-method",
        "text": "The board-specific width is the rftools thermal model (board-specific): the site's own steady-state model of a long trace on an FR-4 board in still air, with an optional copper plane under it. It is not IPC-2152 and does not read IPC-2152's charts; it is a closed form fitted to a committed 2-D solver, which is checked against published results (Brooks & Adam 2015; Jouppi 2010; Olson 2010)."
      },
      {
        "code": "thermal-model-fr4",
        "text": "The laminate is FR-4, conducting 0.8 W/(m·K) in the plane of the board and 0.3 W/(m·K) through it (Graebner 1995; Azar & Graebner 1996). A laminate that conducts less runs hotter."
      },
      {
        "code": "thermal-model-still-air",
        "text": "The board is horizontal, in still air at sea level. Its faces lose heat by natural convection (the McAdams horizontal-plate correlations, over the 51.2 mm length of the IPC-TM-650 2.5.4.1A test board) and by radiation, with an emissivity of 0.9 for the laminate and 0.1 for the bare copper trace. Forced air, an enclosure and other orientations are not modelled."
      },
      {
        "code": "thermal-model-long-trace",
        "text": "The trace is long and alone. Heat drawn off through pads, vias and leads at its ends, and heat from neighbouring traces and components, are not modelled; a short trace with heat-sunk ends runs cooler."
      },
      {
        "code": "thermal-model-plane",
        "text": "An internal trace lies at the middle of the board's thickness. A plane is one solid copper layer under the trace, across a 76.2 mm (3 in) wide board, the stated distance of dielectric below the trace. A plane above the trace, a second plane, or a plane cut away under the trace is not modelled."
      },
      {
        "code": "thermal-model-margin",
        "text": "The board-specific width includes a 15 % margin for the model's measured disagreement with published data: it is the width at which the model's rise is the allowed rise divided by 1.15."
      },
      {
        "code": "thermal-model-heat",
        "text": "The model's Joule heat uses the copper resistivity of IPC-TM-650 2.5.4.1A, 10.827 Ω·cmil/ft at 25 °C for 1 oz and heavier copper and 11.529 Ω·cmil/ft for lighter, rising 0.393 % per °C. The resistance and power outputs keep 1/58 Ω·mm²/m."
      },
      {
        "code": "thermal-model-ambient",
        "text": "The board-specific width is computed at a 25 °C ambient, the temperature IPC-TM-650 2.5.4.1A tests at; the allowed rise is above that ambient."
      },
      {
        "code": "thermal-model-range",
        "text": "Outside the model's validated range (Width 0.25–5.1 mm (internal from 1.27 mm), copper ½–3 oz, board 1.4–3.2 mm, a plane from 0.1 mm under the trace to where it fits, plane copper ½–2 oz, rise 5–100 °C, ambient 0–85 °C) the board-specific width is still reported, flagged with a warning naming the bound. On boards thinner than 1.4 mm, IPC-2152's published thickness data show traces running hotter than the model predicts, so the IPC-2152 Figure 5-1 width is the one to use there."
      }
    ],
    "validRange": {
      "status": "inside",
      "bounds": {
        "current": {
          "min": 0.01,
          "max": 100,
          "unit": "A"
        },
        "copperWeight": {
          "min": 0.5,
          "max": 4,
          "unit": "oz"
        },
        "tempRise": {
          "min": 1,
          "max": 100,
          "unit": "°C"
        },
        "traceLength": {
          "min": 1,
          "max": 10000,
          "unit": "mm"
        },
        "isExternal": {
          "min": 0,
          "max": 1,
          "unit": ""
        },
        "traceWidth": {
          "min": 0.01,
          "max": 100,
          "unit": "mm"
        },
        "boardThickness": {
          "min": 0.2,
          "max": 6,
          "unit": "mm"
        },
        "planeDistance": {
          "min": 0,
          "max": 6,
          "unit": "mm"
        },
        "planeCopperWeight": {
          "min": 0.5,
          "max": 2,
          "unit": "oz"
        },
        "standard": {
          "min": 0,
          "max": 1,
          "unit": ""
        }
      },
      "outside": [],
      "model": {
        "description": "IPC-2152 Figure 5-1 through I = 0.024·ΔT^0.44·A^0.725, checked against IPC-2221 (1998) Figure 6-4 chart C (the same chart) at 10 to 45 °C and up to 700 sq mil: the cross-section is within −3.5 % to +4.4 % of 12 chart readings, and 11.3 % short at 10 °C and 700 sq mil",
        "worstCaseError": 0.12,
        "inside": true
      },
      "models": [
        {
          "description": "IPC-2152 Figure 5-1 through I = 0.024·ΔT^0.44·A^0.725, checked against IPC-2221 (1998) Figure 6-4 chart C (the same chart) at 10 to 45 °C and up to 700 sq mil: the cross-section is within −3.5 % to +4.4 % of 12 chart readings, and 11.3 % short at 10 °C and 700 sq mil",
          "worstCaseError": 0.12,
          "inside": true
        },
        {
          "description": "rftools thermal model (board-specific), a closed-form thermal network fitted to rftools' own committed 2-D solver of the trace's cross-section: Width 0.25–5.1 mm (internal from 1.27 mm), copper ½–3 oz, board 1.4–3.2 mm, a plane from 0.1 mm under the trace to where it fits, plane copper ½–2 oz, rise 5–100 °C, ambient 0–85 °C (the rise before the margin). Inside that range its rise is within 2.81 % of 3,168 solver results",
          "worstCaseError": 0.029,
          "inside": true
        }
      ]
    },
    "computedAt": "2026-09-24T14:05:11.003Z",
    "inputs": {
      "current": 1,
      "copperWeight": 1,
      "tempRise": 10,
      "traceLength": 100,
      "isExternal": 1,
      "traceWidth": 0.5,
      "boardThickness": 1.6,
      "planeDistance": 0,
      "planeCopperWeight": 0.5,
      "standard": 0
    },
    "seed": null,
    "elapsedSeconds": 0.003
  }
}

{
  "detail": [
    {
      "type": "float_parsing",
      "loc": [
        "body",
        "inputs",
        "current"
      ],
      "msg": "Input should be a valid number, unable to parse string as a number",
      "input": "abc"
    }
  ]
}

An input sent as text. Nothing was counted.

{
  "detail": "More than 120 requests in 60 seconds from this address.",
  "errorKind": "rate_limited",
  "limit": 120,
  "windowSeconds": 60,
  "upgradeUrl": "https://rftools.io/pricing/"
}

Too many requests in a minute, with Retry-After.

Same inputs on the calculator pagePCB Trace Width Calculator (IPC-2221 / IPC-2152), at these values