Undersized PCB traces are a frequent source of board failures. When a trace is too thin for the current, it gets hot, can cook the substrate, and often leads to cracked solder joints. Use this PCB Trace Width Calculator to find the minimum copper width needed to handle your current, using IPC-2221 standard calculations. You'll enter current, allowable temperature rise, copper weight, and trace length. This matters most in motor drives, power supplies, and embedded controls—anywhere current isn't just a few milliamps. Below, you’ll find the IPC-2221 formula, a step-by-step example, design practices that actually work, and a real FAQ.
What is PCB Trace Width?
PCB trace width is simply how wide your copper trace needs to be so it doesn’t overheat with the current you plan to run. Make it too narrow and you’ll see heat, possible board damage, or full failure.
Simple Explanation
Think of trace width like pipe diameter: the more current you need to move, the wider it has to be so the heat doesn’t build up and cause problems. If you push a lot of current through a narrow trace, heat builds up rapidly—like too much water through a too-small pipe. The IPC-2221 standard outlines the rules for getting this right, specifically for the temperature rise you can tolerate.
📐 Browse all 384 free engineering calculators
PCB Trace Current Flow Diagram
PCB Trace Width Current Interactive Calculator
This tool lets you see how changes to current, allowed temperature rise, or copper weight affect the minimum trace width you’ll need, using IPC-2221 methods. Watch the cross-section grow as current is increased—this is what you have to do to keep heating and failures in check.
TRACE WIDTH
34.8 mil
RESISTANCE
4.2 mΩ
VOLTAGE DROP
8.4 mV
FIRGELLI Automations — Interactive Engineering Calculators
How to Use This Calculator
- Enter the current in amperes that the trace must carry.
- Enter the allowable temperature rise in °C and select the copper weight from the dropdown.
- Enter the trace length in millimeters.
- Click Calculate to see your result.
PCB Trace Width Calculator
This calculator is intended for education, concept evaluation, and preliminary design. Results are based on the equations and assumptions described on this page, but cannot account for every real-world load case, tolerance, material property, environmental condition, installation detail, safety factor, code, or regulatory requirement. Verify all inputs, assumptions, units, and results independently before selecting components or using the result in a real application. Safety-critical, structural, medical, lifting, transportation, or regulated applications must be reviewed by a qualified engineer.
📹 Video Walkthrough — How to Use This Calculator
Mathematical Formulas
IPC-2221 Current Capacity Formula
Here's how you work out the cross-sectional area needed for your PCB trace:
Where:
- A = Cross-sectional area (square mils)
- I = Current (amperes)
- ΔT = Temperature rise (°C)
- k = 0.048 (constant for external traces)
- b = 0.44 (constant)
- c = 0.725 (constant)
Trace Width Calculation
Once you have the area, here's how to get the minimum trace width by using the copper thickness:
Where:
- W = Trace width (mils)
- A = Cross-sectional area (square mils)
- T = Copper thickness (mils) = Copper weight (oz) × 1.378
Resistance and Voltage Drop
Calculate resistance and voltage drop for the finished trace as follows:
Vdrop = I × R
Where:
- R = Resistance (ohms)
- ρ = Resistivity of copper (0.0169 Ω·mm²/m)
- L = Trace length (meters)
- Amm = Cross-sectional area (mm²)
- Vdrop = Voltage drop (volts)
Simple Example
Suppose you want to carry 2 A with a 10°C temperature rise, using 1 oz copper and a 25 mm trace.
Cross-sectional area: A = (2 / (0.048 × 100.44))1/0.725 ≈ 47.9 square mils
Copper thickness: T = 1 × 1.378 = 1.378 mils
Minimum trace width: W = 47.9 / 1.378 ≈ 34.8 mils (0.88 mm)
Voltage drop: about 8.5 mV
Understanding PCB Trace Width Design
Fundamentals of Current-Carrying Capacity
In practice, trace width comes down to how much heat a copper strip can shed as current flows. Resistance in the copper turns some of that current into heat (Joule heating—P = I²R), and the heat needs somewhere to go. If you undersize the trace, it will overheat the PCB, sometimes damaging layers, causing solder joint cracks, or even pushing board temperature above component limits.
The IPC-2221 standard is the widely referenced guide for minimum trace width versus temperature rise. It’s an empirical system—these are not “worst-case” or theoretical values, but what’s survived in lab tests, mostly assuming room temperature, normal air flow, “average” FR4, and no extra heat sinking beyond the copper itself.
IPC-2221 Standard Implementation
The constants in the standard (k, b, c) aren’t magic—they just match test data where traces were run until the temperature rose by so many degrees above ambient. The heat leaves the copper mainly sideways (into the PCB), upward into the air if it’s an external layer, and a bit by radiation. External traces cool faster; internal ones can't dump much heat except through the PCB, so they always need to be wider for the same current and rise.
This calculator uses the external trace constants; these are more forgiving than for internal traces, but for anything buried in the board, you’ll want to make traces significantly wider—about 1.5x to 2x is common practice.
Copper Weight Considerations
Copper weight (oz/ft²) really just tells you the thickness of the copper. 1 oz copper is 1.378 mils thick. Here’s what actually ends up on boards:
- 0.5 oz (17.5 μm): Used for finer pitch, low current, or where you’re squeezing routes
- 1 oz (35 μm): What most standard boards come with unless you’ve asked for more
- 2 oz (70 μm): Used for higher currents (like power supplies or motor drivers)
- 3+ oz (105+ μm): Needed for serious current or for sinking heat, but can make fine pitch work difficult
Practical Design Example
Say you need a trace to carry 5 A, allow up to 15°C of temperature rise, on 2 oz copper, over 50 mm:
Given:
- Current (I) = 5 A
- Temperature rise (ΔT) = 15°C
- Copper weight = 2 oz
- Trace length = 50 mm
Calculation:
Cross-sectional area: A = (5 / (0.048 × 150.44))1/0.725 = 152.4 square mils
Copper thickness: T = 2 × 1.378 = 2.756 mils
Minimum width: W = 152.4 / 2.756 = 55.3 mils (1.4 mm)
Resistance: R = 1.52 mΩ
Voltage drop: V = 5 × 1.52 = 7.6 mV
Design Considerations and Best Practices
There’s no substitute for understanding a few real-world constraints beyond the IPC-2221 numbers:
Manufacturing Tolerances: Every fab has a lower limit for how narrow a trace can realistically be—often 4-6 mil for common boards, tighter for high-density, but yields drop fast when you go narrower. Always confirm with your PCB house before finalizing width or clearance.
Current Density: Lower current density (amps per mm² or mil²) helps with reliability and reduces electromagnetic noise. IPC’s formula gives you a minimum; for stress-free operation, many designers pick widths to keep well below 1-2 A/mm².
Voltage Drop Considerations: The trace may be wide enough thermally, but if voltage drop along the length is too high, your load could starve. In power and drive circuits, keep voltage drop below 1-5% of your supply voltage, or sensitive parts may malfunction.
Impedance Control: For high-speed digital, RF, or controlled impedance signals, the trace width you calculate for current might be overridden by the geometry needed for correct impedance. Always check routing for signal integrity, not just heat.
Applications in Motion Control Systems
When working with motion or actuator controllers, plenty of traces will see more than a couple amps. Power traces for the motors (5-30 A) need heavy copper and extra width; logic and control traces (0.1-2 A) can be narrower, and signal traces for feedback can be narrower still. Use the calculator for each class, but always sanity-check with respect to what the subsystem actually draws.
Advanced Considerations
Pulse Current Handling: With pulsed loads (such as PWM-driven motors), the trace will generally tolerate short high-current pulses (microsecond to millisecond) provided the average power/temperature stay within specs. Accurately modeling this isn’t trivial—thermal time constants and duty cycle both matter, so do the math if it’s borderline.
Multi-layer Thermal Modeling: If you’re using more complex board stackups, copper thickness, and nearby planes will affect cooling. If you’re pushing the limits, use a thermal analysis tool to model heat flow in 3D, especially for dense or high-power assemblies.
Solder Mask Effects: A solder mask only makes a minor difference to cooling in most cases, but open exposed copper does let heat out more freely—some high-current designs intentionally leave traces unmasked for this reason.
Environmental Factors: If your board runs in a hot enclosure, has no airflow, or sits in a harsh environment, your actual safe current may be much lower than IPC-2221 predicts. Adjust your numbers conservatively if in doubt.
Related Design Tools
Trace width is just one parameter out of many. You’ll often need other calculators too, like those for power dissipation, controlled impedance, or cable voltage drop. Check out our engineering calculators section for tools that fill in the rest of the picture.
Frequently Asked Questions
📐 Explore our full library of 384 free engineering calculators →
About the Author
Robbie Dickson
Chief Engineer & Founder, FIRGELLI Automations
Robbie Dickson brings over two decades of engineering expertise to FIRGELLI Automations. With a distinguished career at Rolls-Royce, BMW, and Ford, he has deep expertise in mechanical systems, actuator technology, and precision engineering.
🔗 Explore More Free Engineering Calculators
Need to implement these calculations?
Explore the precision-engineered motion control solutions used by top engineers.
