PCB Trace Resistance Calculator (and Voltage Drop)

Background.

Copper is a good conductor, not a perfect one, and on a printed board the difference shows up in three places: the voltage that never arrives at the load, the heat the track produces, and the error a sense line adds to a measurement. This page works out the DC resistance of a trace from the inputs a board designer actually has — a width in mils, a length in millimetres, and a copper weight in ounces per square foot — and reports the voltage drop and the dissipation that follow from it.

The generic physics is R = ρL/A and there are calculators for that everywhere. What makes this a different page is the units. Nobody laying out a board knows their trace's cross-sectional area in square metres; they know it is 1 oz copper and 10 mil wide, and the conversion between those two is precisely the step that goes wrong. Copper foil is specified by weight rather than thickness, and one ounce per square foot works out at 34.798 µm — 1.37 mil — at the IPC-4562 nominal.

The most useful number on the page is probably the sheet resistance, quoted in milliohms per square. Divide resistivity by thickness and you get the resistance of one square of that copper, and it is the same whatever size the square is: a 5 mil square and a 100 mil square of the same foil have identical resistance, because making it wider and making it longer cancel exactly. For 1 oz copper at 20 °C the figure is 0.495 mΩ per square. That is where the rule of thumb that experienced designers carry around — about half a milliohm per square — comes from, and once you have it you can estimate any track by counting how many squares long it is.

Temperature matters more than people expect. Copper gains about 0.393 % of resistance for every degree Celsius, referred to 20 °C, so a track running at 85 °C inside a sealed enclosure has roughly a quarter more resistance than the same track on a bench. That figure and the reference resistivity both come from the 1914 Bureau of Standards copper wire tables that defined the International Annealed Copper Standard, adopted by the IEC in 1913 and still the reference today. The circular is explicit that the coefficient is tied to its reference temperature — it gives 0.00427 at 0 °C, 0.00393 at 20 °C and 0.00385 at 25 °C for the same copper — which is why this page names the reference rather than quoting a bare number.

The resistivity is an editable field rather than a buried constant, and deliberately so. 17.241 nΩ·m is the standard reference for annealed copper; electrodeposited PCB foil is not necessarily exactly 100 % IACS, and some references quote 16.8 nΩ·m for pure copper instead, which is 2.6 % lower and will move every answer here by the same proportion. If your fabricator publishes a figure, use theirs.

Three scope limits, stated here rather than in a collapsed answer. This is DC resistance: there is no skin effect, and above roughly a megahertz the current stops using the whole cross-section and the AC resistance rises above this figure. It is the trace only: vias, connectors, solder joints and the resistance of whatever is at each end are not included, and on a short track they can easily dominate. And the cross-section is treated as a rectangle, while a real etched trace is trapezoidal, so the true area is slightly smaller and the true resistance slightly higher.

Finally, this page does not grade its own answer. There is no published standard for an acceptable trace resistance or an acceptable voltage drop on a printed board, because it depends entirely on what the circuit can tolerate — a hundred millivolts is nothing on a 12 V motor supply and catastrophic on a 100 mV current-sense signal. Inventing a threshold would be guessing, so the page reports the numbers and leaves the judgement where it belongs.

What is pcb trace resistance calculator?

The resistance of a uniform conductor is its resistivity times its length divided by its cross-sectional area. For a printed trace the cross-section is the width times the copper thickness, and the copper thickness comes from the foil weight rather than from a direct measurement.

Sheet resistance is that expression rearranged into the form board designers use. Divide the resistivity by the thickness alone and you get a resistance per square — the resistance measured between opposite edges of any square patch of that foil. It has units of ohms, but it is conventionally written as ohms per square to make clear that it is a property of the sheet rather than of a particular piece of it. The resistance of a track is then simply the sheet resistance multiplied by the number of squares, where the number of squares is the length divided by the width. A 50 mm track that is 10 mil wide is 196.85 squares long, and that is true regardless of how thick the copper is.

Resistivity itself is temperature dependent. To first order, R at temperature T is R at 20 °C multiplied by one plus the coefficient times the difference from 20 °C. The 1914 Bureau of Standards circular that established the International Annealed Copper Standard defines that coefficient for 100 % conductivity copper as 0.00393 per degree at a 20 °C reference, and notes explicitly that the value depends on which reference temperature you pick: the same copper has a coefficient of 0.00427 referred to 0 °C. It also notes that the coefficient scales with conductivity, so a lower-purity copper has a proportionally lower coefficient.

The linear model is a first-order approximation. It is good across the range a printed board actually operates in and becomes progressively less accurate a long way from room temperature; far enough below, it would predict zero resistance, which is why this page refuses temperatures below about −234 °C rather than returning nonsense.

How to use this calculator.

1. 01Choose the direction. Working out what an existing track costs you is the review question; working out how far a rail can be routed inside a voltage budget is the layout question.
2. 02Enter the width in mils and, in the resistance direction, the routed length in millimetres. Use the length the track actually takes, not the straight-line distance — a rail that snakes around mounting holes and connectors is longer than the schematic suggests.
3. 03Pick the copper weight from your stack-up. If your fabricator has quoted a finished thickness, choose custom and enter it in micrometres: outer layers finish thicker than the base foil because plating adds copper, and the difference is worth modelling on a low-resistance rail.
4. 04Enter the current. It sets the voltage drop and the dissipation, and in the budget direction it is what converts millivolts into millimetres. It does not change the resistance itself.
5. 05Set the copper temperature to what the trace will actually reach — the board ambient plus its own heating — rather than to room temperature. The difference between 25 °C and 85 °C is about a quarter of the resistance.
6. 06Read the sheet resistance and the square count as well as the answer. Together they let you estimate any other track on the board without coming back: squares times milliohms per square, done in your head.
7. 07Remember what is not included. Vias, connectors and solder joints are outside this calculation and on a short, wide track they can easily be larger than the trace itself.
8. 08Judge the result against your own circuit. There is no standard threshold; a hundred millivolts is irrelevant on a motor supply and fatal on a precision reference.

The formula.

The base expression is R = ρL/A, with A the width times the copper thickness. Everything else on this page is that relation rearranged or evaluated at a different temperature.

Copper weight becomes thickness first. One ounce per square foot is 34.798 µm at the IPC-4562 nominal, so 1 oz copper 10 mil wide has a cross-section of 34.798 µm × 254 µm = 8.8387 × 10⁻⁹ m², which is 0.008839 mm².

Resistivity is then corrected for temperature by the linear model ρ(T) = ρ₂₀ × (1 + α(T − 20)), with α = 0.00393 per degree referred to 20 °C. That equation and that coefficient are exactly what the 1914 Bureau of Standards circular defines, including the reference temperature: it prints α₀ = 0.00427, α₁₅ = 0.004011, α₂₀ = 0.00393 and α₂₅ = 0.00385 for the same 100 % conductivity copper, and defines α₂₀ as (Rt − R₂₀) / (R₂₀(t − 20)). At 25 °C the factor is 1 + 0.00393 × 5 = 1.01965.

Sheet resistance is ρ(T) divided by the thickness alone. For 1 oz copper at 20 °C that is 17.241 nΩ·m / 34.798 µm = 4.955 × 10⁻⁴ Ω, or 0.4955 mΩ per square — the familiar half-milliohm figure. At 25 °C it rises to 0.5052 mΩ per square.

The square count is the length divided by the width, with both in the same units: 50 mm divided by 0.254 mm is 196.85. Multiplying gives the resistance directly — 0.5052 mΩ × 196.85 = 99.45 mΩ — and the page returns both factors so the arithmetic can be checked by eye. That identity is asserted by one of this calculator's tests rather than merely claimed.

The voltage drop is current times resistance and the dissipation is current squared times resistance, so doubling the current doubles the drop but quadruples the heat. In the budget direction the arithmetic runs backwards: the allowed drop divided by the current gives a maximum resistance, and that resistance times the cross-section divided by the resistivity gives a maximum length.

All arithmetic is exact decimal at forty significant digits, rounded once at the return boundary to ten decimal places. Nothing on this page is classified into bands, and that is a deliberate choice rather than an omission: no standard publishes an acceptable trace resistance or an acceptable voltage drop for a printed board, so any threshold shown here would be invented.

A worked example.

Frequently asked questions.

References& sources.

1. [1]Circular of the Bureau of Standards No. 31, 3rd edition, "Copper Wire Tables", United States Department of Commerce, 1 October 1914. Public domain; read directly from the NIST PDF on 2026-07-29. The document that established the International Annealed Copper Standard, and the source of both constants on this page. It states: "The International Annealed Copper Standard, in various units of mass resistivity and volume resistivity, is: 0.15328 ohm (meter, gram) at 20 °C, 875.20 ohms (mile, pound) at 20 °C, 0.017241 ohm (meter, mm²) at 20 °C, 1.7241 microhm-cm at 20 °C, 0.67879 microhm-inch at 20 °C, 10.371 ohms (mil, foot) at 20 °C" — 0.017241 Ω·mm²/m being the 17.241 nΩ·m used here. On the temperature coefficient it states that "the best value to assume for the temperature coefficient of good commercial annealed copper wire is that corresponding to 100 per cent conductivity, viz: α0 = 0.00427, α15 = 0.004011, α20 = 0.00393, α25 = 0.00385", defines α20 as (Rt − R20)/(R20(t − 20)) — the exact model implemented here — and records that "this value was adopted as standard by the International Electrotechnical Commission in 1913".
2. [2]IEC 60028:1925, "International standard of resistance for copper". The modern standard of record for the values above and the reason they remain in force. Access: gated — the IEC sells this standard and it was NOT opened for this page. Nothing here is attributed to a clause of it; every figure comes from the 1914 Bureau of Standards circular that the IEC adopted, which is public domain and was read directly.
3. [3]Eurocircuits, "Tolerances on Copper Thickness" technical guideline. Retrieved and read 2026-07-29. Source of the copper weight to thickness conversion used on this page: "1oz/ft² has a thickness of 1.37 mil (0.00137 Inch) or 34.798 µm", citing IPC-4562, together with the notes that IPC-4562 permits up to a 10 % reduction in base copper thickness and that outer layers finish thicker than the base foil because galvanic plating adds roughly 20 µm.
4. [4]IPC-2221C, "Generic Standard on Printed Board Design", December 2023. Published table of contents read directly on 2026-07-29; the standard itself is sold by IPC and was NOT opened, and no value is taken from it. Cited for the existence of Table 10-1 "Internal Layer Foil Thickness After Processing" and Table 10-2 "Thickness of External Conductor of the Finished Printed Board after Plating" — the reason this page treats the copper weight as a nominal and offers an editable finished-thickness field rather than assuming the base foil is what ends up on the board.