Engineering Reference
PCB Copper Weight and Thickness Reference
Quick-reference PCB copper weight and nominal thickness, mil and metric conversions, trace cross-section, resistance, voltage drop, power loss, and IPC-2221-style current model conditions.
- Reading Time
- 12 min
- Format
- Copper and trace lookup
- Updated
- September 25, 2026
Quick PCB Copper Reference
| Parameter | Symbol | Typical unit | Meaning | Common mistake |
|---|---|---|---|---|
| Trace width | W | mm, mil | Finished conductor width | Width alone does not define current capacity |
| Copper thickness | t | µm, mm, mil | Finished conductor thickness | Nominal foil and finished copper can differ |
| Copper weight | — | oz/ft² | Area-weight convention used for copper foil | It is not total copper mass on the PCB |
| Trace length | L | mm, m | One-way conductor length in a resistance model | Include neck-downs, vias and return paths separately |
| Cross-sectional area | A | mm², mil² | W × t after unit normalization | Do not multiply mm by µm without converting |
| Copper resistivity | ρ | Ω·m | Material property at a stated temperature | It increases with temperature |
| Trace resistance | R | Ω | ρL/A for a uniform conductor | Geometry and temperature assumptions matter |
| Current | I | A | DC or stated RMS current through the trace | Peak, RMS and average current are not interchangeable |
| Current density | J | A/mm² | I/A | Not a universal safe-current criterion |
| Voltage drop | Vdrop | V | I × R | The relevant current and temperature must be stated |
| Power loss | Ploss | W | I²R electrical loss | Loss alone does not determine temperature rise |
| Temperature rise | ΔT | °C | Modelled rise above the defined reference condition | It is not automatically absolute trace temperature |
Copper Weight to Nominal Thickness
PCB copper weight means ounces per square foot. This table uses the same nominal conversion as the ECParts calculators: 1 oz/ft² ≈ 34.79 µm. It is a conversion reference, not a guarantee of finished copper thickness.
| Copper weight (oz/ft²) | Thickness (µm) | Thickness (mm) | Thickness (mil) |
|---|---|---|---|
| 0.5 | 17.395 | 0.017395 | 0.6848 |
| 1.0 | 34.790 | 0.034790 | 1.3697 |
| 1.5 | 52.185 | 0.052185 | 2.0545 |
| 2.0 | 69.580 | 0.069580 | 2.7394 |
| 3.0 | 104.370 | 0.104370 | 4.1091 |
| 4.0 | 139.160 | 0.139160 | 5.4787 |
PCB Units and Conversions
| Reference | Inch | Millimeter | Micrometer or mil |
|---|---|---|---|
| 1 mil | 0.001 in | 0.0254 mm | 25.4 µm |
| 10 mil | 0.010 in | 0.254 mm | 254 µm |
| 1 mm | 0.0393701 in | 39.3701 mil | 1000 µm |
| 35 µm | 0.001378 in | 0.035 mm | 1.378 mil |
Base copper versus finished copper
Copper weight is an area convention
Trace Geometry, Resistance and Loss
| Quantity | Formula | Conditions |
|---|---|---|
| Trace cross-section | A = W × t | Width and thickness in compatible units |
| Uniform-trace resistance | R = ρL/A | Uniform geometry and resistivity at a stated temperature |
| Temperature-adjusted resistivity | ρ(T) ≈ ρ20[1 + α(T − 20°C)] | Linear approximation over an appropriate temperature range |
| Voltage drop | Vdrop = I × R | DC or stated RMS current compatible with the loss model |
| Copper loss | Ploss = I²R | Electrical loss only; a thermal model is still required |
| Current density | J = I/A | Descriptive quantity, not an independent ampacity limit |
| Legacy current estimate | I = k × ΔT^0.44 × A^0.725 | ECParts IPC-2221-style model; A in mil², k = 0.048 external or 0.024 internal |
Copper reference constants
Current density needs thermal context
Current Capacity and Temperature-Rise Model
Model used by ECParts
What the estimate means
IPC-2221 and IPC-2152 differ
Temperature rise is conditional
Worked Reference Examples
1 oz nominal conversion
Trace cross-section
Trace resistance
Voltage drop at 2 A
Copper loss at 2 A
Fabrication and Model Boundaries
- Use finished width and copper thickness for released calculations.
- Check foil and plating tolerances with the fabricator.
- Review neck-downs, pads, connectors and layer transitions separately.
- Via barrel geometry and via thermal behavior require separate analysis.
- Do not infer a universal amp rating from trace width alone.
- Do not treat current density as a universal safe-current rule.
- Do not convert I²R loss directly into a fixed temperature rise.
- Do not assume internal and external traces differ by one universal physical ratio.
- Use IPC-2221-style results as empirical screening estimates.
- Use IPC-2152 guidance, thermal analysis and measurement where risk warrants it.
- Controlled impedance depends on stackup and dielectric data outside this page.
- Creepage and clearance are safety topics outside this copper lookup.
Common Interpretation Mistakes
Treating 35 µm as exact finished copper
Confusing mil with millimeter
Publishing a condition-free ampacity table
Forgetting unit normalization
Lookup and Verification Workflow
- 1Identify base and finished copper requirements.
- 2Confirm the fabricator's copper tolerance and plating process.
- 3Normalize width, thickness and length units.
- 4Calculate cross-sectional area from finished geometry.
- 5Use copper resistivity at the relevant temperature.
- 6Calculate resistance, voltage drop and I²R loss.
- 7Define current as DC, RMS, average or peak.
- 8Define the permitted temperature rise and ambient reference.
- 9Select internal or external context and identify the model.
- 10Review neck-downs, vias, connectors and return paths.
- 11Check IPC-2152 guidance or thermal analysis where appropriate.
- 12Validate high-current designs with the fabricator and measurement.
Support reference
FAQ
How thick is 1 oz PCB copper?
ECParts uses the nominal conversion 1 oz/ft² ≈ 34.79 µm, 0.03479 mm, or 1.3697 mil. Finished copper can differ because of foil tolerance and fabrication plating.
Does 1 oz copper mean the whole PCB contains one ounce of copper?
No. PCB copper weight is conventionally ounces per square foot, an area-weight description of the foil rather than the total copper mass on a board.
Is 1 oz copper exactly 35 µm thick?
No. About 35 µm is a convenient rounded nominal value. ECParts uses 34.79 µm for calculator consistency, while actual finished thickness depends on fabrication specifications and tolerances.
What is the difference between base copper and finished copper?
Base copper is the starting foil. Finished outer-layer copper may include plated copper added during fabrication. The exact relationship is process-specific, so use the released fabricator stackup.
How do I convert PCB mils to millimeters?
Multiply mils by 0.0254. For example, 10 mil equals 0.254 mm. A mil is one thousandth of an inch, not one millimeter.
How is PCB trace resistance calculated?
For a uniform trace, R = ρL/A, where A is width times thickness after unit normalization. Copper resistivity and its temperature must also be defined.
Does a wider or thicker PCB trace have lower resistance?
Yes, when length, material and temperature are unchanged. Greater width or thickness increases cross-sectional area and reduces R = ρL/A.
How much current can a 1 mm PCB trace carry?
There is no condition-free answer. Copper thickness, allowed temperature rise, internal or external location, board construction, adjacent copper, airflow and the selected model all matter.
What does PCB trace temperature rise mean?
It is a modelled conductor temperature increase above a defined reference or ambient condition. It is not a universal absolute temperature guarantee.
What is the difference between IPC-2221 and IPC-2152?
The ECParts trace calculators use the familiar legacy IPC-2221-style empirical equation. IPC-2152 provides more detailed conductor current and temperature guidance with additional board and environmental variables; the two should not be treated as equivalent.
Can trace power loss alone predict temperature rise?
No. I²R gives electrical loss, but temperature rise also depends on copper spreading, substrate, planes, airflow, enclosure, ambient and nearby heat sources.
Are internal and external trace current ratings related by one fixed ratio?
No universal physical ratio applies. The ECParts legacy equation uses different empirical coefficients, but real thermal behavior depends on the actual stackup and environment.
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