PCB Trace Width Calculator
Enter the current, copper thickness (oz/ft²) and allowable temperature rise to get the minimum trace width for internal and external PCB layers.
Reviewed by the ToolNestr Editorial Team — July 2026
How PCB trace width is calculated
PCB trace width is calculated using the IPC-2221 standard formula: I = k × ΔT0.44 × A0.725, where I is current in amperes, ΔT is the temperature rise in °C, A is the cross-sectional area in square mils, and k is 0.048 for external layers and 0.024 for internal layers.
The calculator solves for area A given current I, then divides by the copper thickness (1 oz = 1.37 mils) to find the minimum trace width. External traces dissipate heat better than internal traces and can carry approximately 40–50% more current for the same width.
Worked example
What different trace widths mean
Minimum trace width increases with current. These examples assume 1 oz copper on an external layer with a 10°C temperature rise.
PCB Designer
Route power traces with confidence, ensuring your PCB meets current requirements without excessive heating or voltage drop.
Hardware Engineer
Validate your board design against IPC standards early in the process, avoiding costly respins due to inadequate trace widths.
Hobbyist
Design your own PCBs at home with proper trace sizing for power supplies, motor drivers, and other high-current circuits.
Student
Learn the relationship between current, copper thickness, temperature rise, and trace geometry in practical PCB design.
| Trace width (mm) | 1 oz external | 1 oz internal | 2 oz external | 2 oz internal |
|---|---|---|---|---|
| 0.25 mm | 0.8 A | 0.6 A | 1.4 A | 1.0 A |
| 0.50 mm | 1.5 A | 1.0 A | 2.5 A | 1.8 A |
| 1.00 mm | 2.8 A | 2.0 A | 4.8 A | 3.4 A |
| 2.00 mm | 5.0 A | 3.5 A | 8.5 A | 6.0 A |
| 3.00 mm | 7.0 A | 5.0 A | 12.0 A | 8.5 A |
| 5.00 mm | 11.0 A | 7.5 A | 18.5 A | 13.0 A |
How to use the PCB Trace Width Calculator
Enter current and parameters
Type the maximum current the trace must carry. Select copper weight and expected temperature rise for your application.
Choose layer type
Select external for outer PCB layers (better heat dissipation) or internal for buried traces inside the board stackup.
Read minimum trace width
The calculator shows the minimum trace width in mm and mils, plus the cross-sectional area. Apply a safety margin of 20–30% for reliability.
Tips for PCB trace width design
Always add safety margin
The IPC-2221 formula provides the minimum trace width for a given temperature rise. Add 20–30% to account for manufacturing tolerances, ambient temperature, and nearby heat sources.
Use copper pours for high currents
For currents above 10 A, consider using copper pours or polygons instead of traces. They provide lower resistance, better heat spreading, and higher current capacity.
Consider thermal reliefs
When connecting high-current traces to through-hole pads, use thermal relief spokes to prevent heat sinking during soldering. Standard thermal reliefs reduce effective trace width at the connection point.
Keep traces short for high currents
Long traces have higher resistance and voltage drop. For high-current paths, minimize trace length or use thicker copper (2 oz or more) to reduce losses and heating.
The IPC-2221 formula
The IPC-2221 standard provides the formula: I = k × ΔT0.44 × A0.725, where I is current in amperes, ΔT is temperature rise in °C, A is cross-sectional area in square mils, and k is 0.048 for external layers and 0.024 for internal layers. The calculator solves this equation for area A given current I, then divides by copper thickness to find trace width.
Internal vs external traces
External traces are exposed to ambient air and can dissipate heat through convection and radiation. Internal traces are buried in the PCB substrate (typically FR-4), which is a poor thermal conductor. As a result, internal traces can carry only about 60–70% of the current that the same width trace on an external layer can carry. The calculator accounts for this with different k constants.
Copper weight explained
Copper weight is the average thickness of copper on a PCB layer. Standard 1 oz/ft² copper is approximately 35 µm (1.4 mils) thick. Heavier copper such as 2 oz (70 µm) or 3 oz (105 µm) provides lower resistance and higher current capacity. However, finer trace widths and spacings are more difficult to etch with thicker copper, so use heavier copper only when needed for power circuits.
Thermal considerations
Trace heating is caused by I²R losses. A trace carrying 10 A with 0.01 Ω resistance dissipates 1 W of heat. If the trace cannot radiate this heat effectively, its temperature rises. Excessive temperature rise can damage the PCB laminate, weaken solder joints, and change the resistance of nearby components. Always design for the worst-case ambient temperature and allow adequate cooling around high-current traces.
Frequently asked questions
What is the IPC-2221 standard?
IPC-2221 is the industry standard for PCB design that includes formulas for calculating trace width based on current, copper thickness, and allowable temperature rise. The formula used is I = k × ΔT^0.44 × A^0.725, where I is current, ΔT is temperature rise, A is cross-sectional area, and k is a constant (0.048 for external, 0.024 for internal layers).
What is the difference between internal and external PCB traces?
External traces are on the outer layers of the PCB and are exposed to air, allowing better heat dissipation. Internal traces are buried in the PCB and surrounded by insulating material, which traps heat. As a result, external traces can carry more current than internal traces of the same width.
What is copper weight in PCBs?
Copper weight refers to the thickness of the copper layer on the PCB, measured in ounces per square foot (oz/ft²). Standard copper weights are 0.5 oz (17.5 µm), 1 oz (35 µm), and 2 oz (70 µm). Heavier copper allows higher current capacity but makes etching finer traces more difficult.
What temperature rise is safe for PCB traces?
A 10°C to 20°C temperature rise is typical for most designs. A 10°C rise is conservative and recommended for sensitive circuits or high-reliability applications. Up to 30°C may be acceptable for power boards if the PCB material can handle the temperature. FR-4 is typically rated for 130–140°C continuous.
How do I convert trace width from mm to mils?
1 mil = 0.001 inch = 0.0254 mm. To convert mm to mils, divide by 0.0254. For example, 0.5 mm = 19.7 mils. The calculator shows both units for your convenience.
Can I use the calculator for high-frequency designs?
For high-frequency designs, trace width also affects impedance. The IPC-2221 formula only considers current-carrying capacity. For RF designs, use an impedance calculator in addition to this tool to ensure controlled impedance.
What happens if the trace is too narrow?
A trace that is too narrow for its current will overheat, potentially causing the copper to delaminate from the PCB substrate. The trace may act like a fuse and burn open, or the excessive heat can damage nearby components and solder joints.
Should I use a safety margin on trace width?
Yes. IPC-2221 gives the minimum width for a given temperature rise. It is good practice to add 20–30% margin for reliability, especially in high-temperature environments or when the trace passes near heat-sensitive components.
How does trace length affect current capacity?
The IPC-2221 formula does not account for trace length directly. However, longer traces have higher resistance and thus more voltage drop. For high-current traces, keep them as short as possible and consider using copper pours or bus bars for very long runs.
What is cross-sectional area and why does it matter?
Cross-sectional area is trace width multiplied by copper thickness. It determines the trace's resistance and current-carrying capacity. A larger cross-section means lower resistance, less voltage drop, and better heat dissipation. The calculator displays the minimum area required.
Sources & references
This tool uses standard formulas and reference values from:
- • NFPA 70 — National Electrical Code (NEC), conduit fill, box fill and conductor ampacity. nfpa.org
- • IPC-2221, Generic Standard on Printed Board Design (trace width / current). ipc.org
- • NIST reference constants and unit definitions; IEEE standards where applicable. nist.gov
For educational and preliminary use. Verify against full engineering calculations and the governing standard before any real design.