Ethernet Trace Calculator
10/100 Mbps | Gigabit | 10 Gigabit Ethernet
Review routing guidance for Ethernet. Numeric differential calculation is temporarily unavailable; confirm pair geometry with a validated field solver and your fabricator.
What this page can and cannot tell you
Ethernet over twisted pair uses 100 Ω differential pairs; the width and spacing that achieve it depend on your stackup. This page does not compute a numeric result for this topic. Use your PHY datasheet and the fabricator's impedance stackup for tolerance and skew budgets.
Assumptions
- The 100 Ω figure is the commonly cited nominal differential target; this site has not verified it against the standard.
Limits
- Numeric differential calculation is switched off until it is validated.
Ethernet Nominal Impedance Targets
| Standard | Speed | Pairs Used | Nominal Differential Z |
|---|---|---|---|
| 10BASE-T | 10 Mbps | 2 pairs | 100Ω |
| 100BASE-TX | 100 Mbps | 2 pairs | 100Ω |
| 1000BASE-T | 1 Gbps | 4 pairs | 100Ω |
| 10GBASE-T | 10 Gbps | 4 pairs | 100Ω |
Ethernet PCB Design Guidelines
100Ω Differential
The nominal differential impedance target for Ethernet is 100Ω. Tolerance and intra-pair / inter-pair skew budgets are defined in the current interface specification and your SoC/PHY datasheet; check them there. This page does not state them.
Transformer Coupling
Ethernet uses magnetics (transformers) for isolation. Maintain 100Ω impedance from PHY to transformer, and from transformer to RJ45 connector.
Length Matching
Tolerance and intra-pair / inter-pair skew budgets are defined in the current interface specification and your SoC/PHY datasheet; check them there. This page does not state them.
Ethernet Layout Best Practices
PHY to Transformer
Transformer to RJ45
Speed-Specific Considerations
10/100 Ethernet
- • 100Ω nominal differential target
- • Uses only 2 of 4 pairs
- • Check PCB material requirements in your SoC/PHY datasheet
- • Check skew budgets in your SoC/PHY datasheet
- • Common in cost-sensitive designs
Gigabit & 10G Ethernet
- • Tolerance and intra-pair / inter-pair skew budgets are defined in the current interface specification and your SoC/PHY datasheet; check them there. This page does not state them.
- • All 4 pairs are used bidirectionally
- • Better ground plane continuity required
- • Check skew budgets in your SoC/PHY datasheet
- • Check PCB material requirements for your channel
Definitions and References
Ethernet is a packet-based local area networking technology used in industrial, commercial, and embedded systems. A cable assembly refers to terminated network or I/O cabling prepared for a specific interconnect job, while a wire harness is an organized bundle of conductors used to route signals or power through equipment and vehicles.
Review Your Ethernet Stackup
Enter your own stackup (dielectric height, Er, and copper thickness) in the impedance calculator linked below, because trace width depends on stackup. The calculator provides isolated-line single-ended estimates. Differential width and spacing calculations are temporarily unavailable; use a validated coupled-line solver.
Source note: the impedance values on this page are the nominal targets commonly cited for this interface. This site does not reproduce the standard's text and has not verified them against it. Confirm them, with tolerances and skew budgets, in the IEEE 802.3 clause for your PHY and your PHY datasheet (IEEE 802.3 and your PHY vendor) and your SoC/PHY datasheet.
Ethernet Design FAQ
What impedance does Ethernet use on a PCB?
The nominal differential impedance target is 100 ohms. Tolerance and intra-pair / inter-pair skew budgets are defined in the current interface specification and your SoC/PHY datasheet; check them there. This page does not state them.
Do I need magnetics for Ethernet?
Yes. Isolation magnetics are standard between the PHY and the cable interface to provide common-mode rejection and safety isolation.
How short should Ethernet traces be?
Keep PHY-to-transformer routing short and direct, then keep the transformer-to-RJ45 section even shorter if possible.
Can I use integrated magnetics?
Yes. Many RJ45 connectors include integrated magnetics, which can simplify placement and routing when the layout still preserves pair symmetry.
What matters besides width?
Pair spacing, reference plane continuity, return path quality, and connector transitions all affect Ethernet signal quality in addition to width.
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