PCB Trace Width Calculator
PCB Trace Width Calculator
The copper trace width a current needs for a chosen temperature rise, on an outer or inner layer, from the IPC-2221 curve fit — or the temperature rise of a width you already have — with the trace’s resistance, voltage drop and power loss over its length.
Trace width for current
3 A on an outer layer of 1 oz copper, 10 °C rise, 50 mm long, 25 °C ambient
IPC-2221 trace width
- k
- 0.048 for an external layer, 0.024 for an internal layer
- ΔT
- temperature rise of the trace above ambient, °C
- A, W, t
- cross-section (mil²), width (mil) and thickness: 1 oz/ft² = 1.378 mil = 35 µm
- T
- trace temperature = ambient + rise, for the resistance
Worked example
3 A on an outer layer of 1 oz copper, 10 °C rise, 50 mm long, 25 °C ambient
A = (3 ÷ (0.048 × 100.44))1/0.725 = 74.2 mil²
W = 74.2 ÷ 1.378 = 53.8 mil = 1.367 mm
At 35 °C, ρ = 0.018257 Ω·mm²/m; R = 0.018257 × 0.05 ÷ (1.367 × 0.035) = 19.08 mΩ
Drop = 3 × 19.08 mΩ = 57.24 mV; loss = 3² × 19.08 mΩ = 171.7 mW
On an inner layer the same current needs 3.556 mm
Width for common currents, 1 oz copper
| Current | External, 10 °C | External, 20 °C | External, 30 °C | Internal, 10 °C | Internal, 20 °C |
|---|---|---|---|---|---|
| 0.5 A | 0.12 mm | 0.08 mm | 0.06 mm | 0.30 mm | 0.20 mm |
| 1 A | 0.30 mm | 0.20 mm | 0.15 mm | 0.78 mm | 0.51 mm |
| 2 A | 0.78 mm | 0.51 mm | 0.40 mm | 2.03 mm | 1.33 mm |
| 3 A | 1.37 mm | 0.90 mm | 0.70 mm | 3.56 mm | 2.33 mm |
| 5 A | 2.77 mm | 1.82 mm | 1.42 mm | 7.19 mm | 4.72 mm |
| 10 A | 7.19 mm | 4.72 mm | 3.69 mm | 18.71 mm | 12.29 mm |
| 15 A | 12.59 mm | 8.26 mm | 6.46 mm | 32.74 mm | 21.50 mm |
| 20 A | 18.71 mm | 12.29 mm | 9.61 mm | 48.69 mm | 31.97 mm |
Sizing a trace for its current
A PCB trace is a thin strip of copper, and the current in it heats it. The width you need depends on how much temperature rise you will accept, the copper thickness and whether the trace is on an outer layer, where it can shed heat to the air, or buried inside the board. The formula almost every trace-width calculator uses is a curve fit to the conductor-sizing charts in IPC-2221: I = k × ΔT0.44 × A0.725, with the cross-section A in square mils and k = 0.048 outside and 0.024 inside. Solve it for A, divide by the copper thickness, and you have the width.
The example. 3 A on an outer layer of 1 oz copper with a 10 °C rise needs 74.2 mil² of copper, which at 1.378 mil thick is 53.8 mil, or 1.367 mm. Accept a 20 °C rise and it falls to 0.898 mm; use 2 oz copper and it halves to 0.683 mm. On an inner layer the same current needs 3.556 mm, because IPC-2221 allows inner traces only half the current of outer ones. The second mode works the other way: give it a width and it returns the temperature rise, so a 1.5 mm trace carrying 3 A rises 8.58 °C and a 0.5 mm one 52.4 °C.
Resistance and loss. The trace also drops voltage. 1 oz copper is 35 µm thick, so its sheet resistance is about 0.493 mΩ per square at 20 °C: a trace 50 mm long and 1 mm wide is 50 squares. The page works out the resistance at the trace’s working temperature (ambient plus rise); the example’s 50 mm trace measures 19.08 mΩ, drops 57.24 mV and dissipates 171.7 mW. That drop matters for sense lines, low-voltage rails and high-current returns more often than the temperature rise does. Plated copper is a little more resistive than the annealed-copper figure used here.
IPC-2221 and IPC-2152. IPC-2221’s charts date back to measurements made in the 1950s on one kind of board. IPC-2152 (2009) replaced them with new measurements and charts that take in board thickness, copper planes nearby and the board material. Its main finding for designers is that inner traces run nearly as cool as outer ones, so IPC-2221’s inner-layer widths are usually conservative, while outer traces on a thin board with no planes can run hotter than IPC-2221 predicts. IPC-2152’s charts are the standard’s own and are not reproduced here: for a tight design, look them up or use your fabricator’s tool. Keep the IPC-2221 limits in mind too: its charts stop at 35 A, 400 mil wide and 3 oz copper, and above those the formula is extrapolating.
Practical margins. Components, vias and neck-downs at pads have their own resistance and heating: a trace sized for 3 A that squeezes through a single small via is limited by the via. For wiring off the board, use the wire size calculator; for the resistance of a round wire, the AWG wire size calculator; for the voltage and power in any resistance, the Ohm’s law calculator.
Frequently asked questions
How wide should a PCB trace be for 1 A?
About 11.8 mil (0.30 mm) on an outer layer of 1 oz copper for a 10 °C rise, or 30.8 mil on an inner layer, by the IPC-2221 formula.
What is the IPC-2221 trace width formula?
I = k × ΔT^0.44 × A^0.725, with I in amperes, ΔT in °C, A in square mils, and k = 0.048 for external or 0.024 for internal layers. Width = A ÷ (1.378 mil × copper weight in oz).
How thick is 1 oz copper?
1.378 mil, which is 35 µm. 0.5 oz is 17.5 µm and 2 oz is 70 µm. Outer layers are often plated up, so the finished copper can be thicker; ask your fabricator.
Why do internal traces need to be wider?
IPC-2221 assumes a buried trace cannot shed heat as well, and allows it half the current of an outer trace. IPC-2152’s newer measurements show inner traces usually run nearly as cool, so the IPC-2221 inner widths are conservative.
Should I use IPC-2152 instead?
For a tight or high-current design, yes: it supersedes the IPC-2221 charts and accounts for board thickness and nearby copper planes. IPC-2221 remains a quick, generally safe first estimate for outer layers on typical boards.
Related calculators
References
- IPC-2221B. Generic Standard on Printed Board Design. IPC, 2012. Section 6.2, conductor current-carrying capacity charts.
- IPC-2152. Standard for Determining Current Carrying Capacity in Printed Board Design. IPC, 2009.
- Advanced Circuits. Trace Width Calculator: A = (I ÷ (k ΔT0.44))1/0.725, k = 0.048 external and 0.024 internal, 1.378 mil/oz; valid to 35 A, 400 mil, 10 to 100 °C, 0.5 to 3 oz.
- Sierra Circuits. Optimize your PCB trace using IPC-2152 standard (internal traces carry currents close to external traces).