PCB Trace Resistance Calculator (and Voltage Drop)

Background.

Send 1 A down 50 mm of 10 mil track in ordinary 1 oz copper and about a tenth of a volt never reaches the far end. That is the trace's DC resistance at work, and on a printed board it shows up in three places: a lower rail voltage at the load, heat in the copper, and an error added to anything measured across the track.

The physics is the textbook R = ρL/A. What makes a board different is the units. Layout tools give widths in mils and lengths in millimetres, and copper foil is sold by weight, in ounces per square foot, rather than by thickness. One ounce per square foot is 34.798 µm thick at the IPC-4562 nominal, or 1.37 mil, and that conversion is what turns a stack-up and a layout into ohms.

You can work in either direction. Give a width and a length to get the resistance, the voltage drop at your current and the power lost as heat. Or give a voltage drop you can live with and get the longest trace of that width that stays inside it, which is the question to ask when deciding how far a regulator may sit from its load.

The constants come from the 1914 Bureau of Standards copper wire tables, which set out the International Annealed Copper Standard adopted by the IEC in 1913: 17.241 nΩ·m at 20 °C and a temperature coefficient of 0.00393 per degree. Both are editable inputs or explicit in the result, and the answer is the copper alone, at DC. No published standard says how much drop is acceptable on a board, so the result is reported, not graded against a pass mark.

What is pcb trace resistance calculator?

A trace's resistance is its copper resistivity times its length, divided by its cross-section, the width times the foil thickness. Board designers usually split that into two factors. Sheet resistance, resistivity divided by thickness, is the resistance between opposite edges of any square patch of the foil, written in ohms per square because it describes the copper layer rather than a particular piece of it. The number of squares is the trace's length divided by its width. Multiply the two and you have the resistance. For 1 oz copper at 20 °C the sheet resistance is 0.4955 mΩ per square, the source of the old rule of thumb of about half a milliohm per square.

How to use this calculator.

1. 01Choose whether you want the resistance of a given trace or the longest trace that fits a voltage-drop budget.
2. 02Enter the width in mils (1 mm is 39.37 mil) and either the routed length in millimetres or the drop you will accept in millivolts. Use the length the track actually follows, including detours around holes and parts, not the straight-line distance.
3. 03Pick the copper weight for the layer. If your fabricator quotes a finished thickness, choose custom and enter it in micrometres; plated outer layers end up noticeably thicker than the base foil.
4. 04Enter the current. It sets the drop and the heat, and in the budget direction turns millivolts into millimetres; the resistance itself does not depend on it.
5. 05Set the copper temperature to what the trace will reach in service, board ambient plus its own heating, rather than room temperature.
6. 06Leave the resistivity at 17.241 nΩ·m unless you have a figure for your foil, then read the sheet resistance and square count along with the answer.

The formula.

Copper thickness is t = copper weight × 34.798 µm, and the cross-section is A = w × t, treating the etched trace as a rectangle. Resistivity is corrected for temperature with the linear model ρ(T) = ρ₂₀ × (1 + α(T − 20)), where ρ₂₀ is 17.241 nΩ·m and α is 0.00393 per °C. The coefficient belongs to its 20 °C reference: the same 1914 circular gives 0.00427 for a 0 °C reference and 0.00385 for 25 °C, so mixing a coefficient with the wrong reference temperature skews the correction. The resistance is R = ρ(T) × L ÷ A, which is the same as R_sheet × (L ÷ w) with R_sheet = ρ(T) ÷ t. The voltage drop is V = I × R and the heat is P = I² × R, so doubling the current doubles the drop but quadruples the heat. In the budget direction the steps run backwards: the largest allowed resistance is V_budget ÷ I, and the longest trace is that resistance × A ÷ ρ(T). The linear model reaches zero resistivity at about −234.5 °C, so temperatures at or below that are refused, as is anything above 1,000 °C. Widths, lengths, currents, thicknesses and resistivity must all be greater than zero. The answer moves in direct proportion to length, resistivity and the temperature factor, and in inverse proportion to width and thickness.

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.