PCB Trace Width Calculator — IPC-2221, External and Internal
Who this is for: For anyone routing a board with power on it: the difference between a trace that's just a wire and a trace that's a fuse you accidentally designed.
IPC-2221 trace width for your current, temperature rise and copper weight — external or internal layer — plus resistance and voltage drop when you enter the trace length.
Quick answer: The calculator applies the IPC-2221 chart formula: cross-section area (mil²) = (current ÷ (k × ΔT^0.44))^(1 ÷ 0.725), with k = 0.048 for external layers and 0.024 for internal, then width = area ÷ copper thickness (1 oz = 1.378 mil). A 1 A trace with a 10 °C rise allowance on 1 oz external copper: ≈ 0.30 mm minimum (11.8 mil). Same current on an internal layer: ≈ 0.78 mm — internal traces run about 2.6× wider because FR-4 traps the heat.
10 °C is the conservative default; 20–30 °C for short bursts
Adds resistance, voltage drop and power loss
| Current | Width (mm) | Width (mil) | Use at least |
|---|---|---|---|
| 0.5 A | 0.12 | 4.5 | 0.15 mm |
| 1 A | 0.3 | 11.8 | 0.35 mm |
| 2 A | 0.78 | 30.8 | 0.8 mm |
| 3 A | 1.37 | 53.8 | 1.4 mm |
| Standard | IPC-2221 chart formula |
|---|---|
| External k factor | 0.048 · internal 0.024 |
| 1 oz copper | 35 µm / 1.378 mil thick |
| Reference (verified in tests) | 1 A · 10 °C · 1 oz external ≈ 0.30 mm |
| Also outputs | resistance · voltage drop · power loss (with length) |
How the IPC-2221 trace width calculation works
Trace width is a thermal question, not an electrical one. A trace has some resistance, current through it makes heat, and the copper-to-air (or copper-to-FR-4) surface has to shed that heat without the trace exceeding the temperature rise you allow. The IPC-2221 chart — the industry standard since 1998 — encodes the empirically-measured relationship, and this calculator implements its formula directly with unit tests asserting the published reference values (1 A / 10 °C / 1 oz external ≈ 11.8 mil).
The two decisions that change the answer most: the layer (external traces cool through air and solder mask; internal traces are sandwiched in plastic and need roughly 2.6× the width for the same current) and the allowed temperature rise (10 °C is the conservative default; 20–30 °C is defensible for short inrush pulses but a bad idea for a continuous supply rail).
Width is only half the job — the calculator also returns the trace's resistance when you enter its length, and therefore the voltage drop and power loss. For a sensor's supply line, a 0.2 V drop across a thin trace corrupts measurements long before any heat problem appears; that's why the length field exists. For real boards, the standard practice stack is: size the trace by this formula, then add copper pours, thermal relief and stitching vias around power paths so the real board runs cooler than the calculation assumes.
Common uses
- Sizing the 5 V rail trace that feeds a 2 A LED strip header
- Checking a motor driver's output traces on 2 oz copper
- Verifying a sensor supply line won't drop enough volts to matter
- Deciding between 1 oz and 2 oz copper for a small power board
- Estimating the voltage drop on a long, thin analog trace before respinning the board
When to ignore the formula
Three cases: (1) very short traces can carry much more than the chart says because the board itself is the heatsink — a 1 mm stub to a via is not a thermal problem; (2) traces that are actually fuses on purpose (should never exist for users), and (3) high-frequency signals, where width is set by impedance (50 Ω microstrip) not current. For everything else — power rails, motor lines, LED feeds — the IPC formula with a 10 °C rise is the right default and errs conservative.
Copper weight in one line
1 oz copper = 35 µm thick = 1.378 mil. Cheapest fab default is 1 oz; 2 oz roughly halves the width needed for the same current and costs a little more. For a 5 V 8 A LED strip project, 2 oz external copper at 10 °C gives ≈ 1.9 mm — still a wide bus, which is why high-current boards use pours and multiple layers rather than wider and wider single traces.
Frequently Asked Questions
How wide should a PCB trace be for 1 amp?
Why do internal traces need to be much wider?
What temperature rise should I use?
Does trace length matter for the width calculation?
Can I just use a copper pour instead of wide traces?
How many vias do I need to carry power between layers?
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