Tools / PCB Trace Current Calculator
PCB Trace Current Calculator (IPC-2221)
How much current can a PCB trace carry? Enter trace width, copper weight and allowed temperature rise to get the IPC-2221 current capacity — or work backwards to find the trace width your current needs.
Max current a trace can carry
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Trace width needed for your current
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The IPC-2221 formula
I = k × ΔT0.44 × A0.725I = current in amps · ΔT = allowed temperature rise in °C · A = trace cross-section in square mils (width × copper thickness) · k = 0.048 for external layers, 0.024 for internal layers.
Note: IPC-2152 is the newer, experimentally validated standard and supersedes these charts, but the IPC-2221 formula remains the industry-standard quick estimator. For critical high-current designs, confirm with your PCB fabricator.
Current-capacity reference chart
Amps at ΔT = 10 °C, computed live with the IPC-2221 formula above.
| Width | 1 oz external | 2 oz external | 1 oz internal | 2 oz internal |
|---|
Worked examples
Example 1 — the classic 100 mil power trace.
Width 100 mil, 1 oz copper (1.37 mil thick), external layer, ΔT = 10 °C.
Cross-section A = 100 × 1.37 = 137 mil². I = 0.048 × 100.44 × 1370.725
= 0.048 × 2.754 × 35.41 ≈ 4.68 A.
Example 2 — sizing a 2 A USB power trace.
Target 2 A, 1 oz, external, ΔT = 10 °C.
W = (2 / (0.048 × 100.44))1/0.725 / 1.37 ≈ 30.9 mil (0.79 mm).
With a 25% safety margin, use ≈ 40 mil (1.0 mm).
Example 3 — internal layer, 200 mil, 2 oz.
Buried trace, 200 mil wide, 2 oz (2.74 mil), ΔT = 20 °C.
A = 200 × 2.74 = 548 mil². I = 0.024 × 200.44 × 5480.725
= 0.024 × 3.736 × 96.74 ≈ 8.68 A — notice it is far below the external-layer equivalent because internal traces cool poorly.
Example 4 — thin 10 mil signal trace.
Width 10 mil, 1 oz, external, ΔT = 10 °C → A = 13.7 mil² →
I = 0.048 × 2.754 × 6.67 ≈ 0.88 A. Plenty for logic signals; size it for impedance, not current.
Frequently asked questions
What is the IPC-2221 trace current formula?
The IPC-2221 current-capacity formula is I = k × (ΔT)0.44 × A0.725, where I is current in amps, ΔT is the allowed temperature rise in °C, A is the trace cross-section in square mils, and k is 0.048 for external (outer) layers or 0.024 for internal layers.
What does ΔT (temperature rise) mean, and which value should I use?
ΔT is how much hotter the trace is allowed to get above ambient temperature. A common, conservative choice is 10 °C. Power traces sometimes use 20 °C or 30 °C, but higher rise means shorter board life and hotter nearby parts, so 10 °C is the safest default for long-term reliability.
What is the difference between external and internal layer current ratings?
External (outer-layer) traces can shed heat into the air, while internal traces are buried in FR-4 and can only conduct heat through the board. IPC-2221 models this with k = 0.048 for external layers and k = 0.024 for internal layers, so an internal trace of the same size carries exactly half the current for the same temperature rise.
How thick is 1 oz copper in mil and mm?
1 oz copper is 35 micrometers thick, which equals 1.37 mil or 0.0348 mm. Other standard weights scale linearly: 0.5 oz = 0.69 mil, 2 oz = 2.74 mil, 3 oz = 4.11 mil and 4 oz = 5.48 mil.
How do I calculate the required trace width for a target current?
Solve the IPC-2221 formula for width: W = (I / (k × ΔT0.44))1/0.725 / thickness, where thickness is the copper thickness in mil. For example, 2 A on 1 oz copper, external layer, 10 °C rise needs about 31 mil (0.79 mm) of width; add a safety margin on top.
Is IPC-2221 still valid, or should I use IPC-2152?
IPC-2152 is the newer, experimentally validated standard and supersedes the IPC-2221 charts, but the IPC-2221 formula remains the most widely used quick estimator and is still quoted by most PCB manufacturers. For critical high-current designs, confirm the result with your fabricator and consider IPC-2152 data.
Why do internal traces carry less current than external traces?
Heat leaves an outer-layer trace by convection and radiation into the air. An inner-layer trace is surrounded by insulating FR-4, so heat must conduct sideways through copper and laminate before it can escape. With worse cooling, the same current produces a larger temperature rise, hence the lower k factor of 0.024.
What safety margin should I add to the calculated trace current?
A common practice is to design for 75 to 80 percent of the IPC-2221 value — multiply the calculated current by 0.75, or size the width about 25 to 30 percent wider than the bare minimum. This covers manufacturing tolerances, hot spots and nearby heat sources.
Do vias and connectors need the same current treatment as traces?
Yes, and they are often the weakest link. A via barrel has far less copper cross-section than the trace feeding it, so high-current paths need multiple vias in parallel, larger annular rings, or filled and plated vias. Always check via current capacity separately from the trace.
How do I convert mil to mm for trace width?
1 mil equals 0.0254 mm. Multiply mil by 0.0254 to get mm, or divide mm by 0.0254 to get mil. For example, 100 mil = 2.54 mm and 31 mil = 0.79 mm.
Does the IPC-2221 formula apply to flexible PCBs or very thick copper?
The formula was derived from rigid FR-4 test data, so treat results for flex circuits, metal-core boards or copper above 4 oz as rough estimates only. Thermal behavior differs substantially in those constructions, so validate with your manufacturer.
Can I use this calculator for high-frequency or high-voltage traces?
This calculator covers DC and low-frequency current heating only. At high frequencies, skin effect and proximity effect concentrate current at the copper surface and raise AC resistance, while high voltage adds clearance and creepage requirements from IPC-2221B that are unrelated to heating. Use dedicated tools for those cases.
Free tool by NRTechStudio