Ethernet Overhead
Calculate Ethernet framing overhead and its impact on effective throughput.
About Ethernet Overhead
Every Ethernet frame carries fixed overhead beyond the payload: an 8-byte preamble for clock synchronization, a 14-byte header (destination MAC, source MAC, type/length), a 4-byte Frame Check Sequence for error detection, and a 12-byte Inter-Frame Gap that separates consecutive frames on the wire — totaling 38 bytes per frame. This overhead is constant regardless of payload size, so it represents a larger percentage loss for small frames than for large ones, directly reducing the effective throughput available for actual data below the link's rated speed.
Formula
Ethernet framing overhead
overhead = preamble(8) + header(14) + FCS(4) + IFG(12) = 38 bytes/frame
- h
- Total fixed overhead per Ethernet frame
Worked example
With the default values below, Ethernet Overhead returns Total Overhead per Frame of 38. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Payload Sizeaccepts 1–9,000 bytes
- 1,460 bytes
- Link Speedminimum 1 Mbps
- 1,000 Mbps
Results
- Total Overhead per Frame
- 38 bytes
- Total Frame Size on Wire
- 1,498 bytes
- Link Efficiency
- 97.46%
- Effective Throughput
- 974.63 Mbps
Step by step
- 1Values used
- 2Ethernet framing overhead
- 3Total Overhead per Frame= 38 bytes
- 4Total Frame Size on Wire= 1,498 bytes
- 5Link Efficiency= 97.46
- 6Effective Throughput= 974.63 Mbps
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or compounding frequency into account.
- Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
What are the four components of Ethernet frame overhead?
The 8-byte preamble (plus start frame delimiter) synchronizes the receiver's clock before the frame proper begins; the 14-byte header holds destination MAC, source MAC, and EtherType/length; the 4-byte Frame Check Sequence is a CRC used to detect transmission errors; and the 12-byte Inter-Frame Gap is idle time enforced between frames, totaling 38 bytes of non-payload data per frame.
Why does this overhead matter more on a link full of small packets?
The 38-byte overhead is fixed per frame regardless of payload size, so a 64-byte minimum-size frame loses over 37% of its wire time to overhead, while a 1500-byte frame loses only about 2.5%. Workloads dominated by small packets (VoIP, gaming, many small API calls) therefore see proportionally lower effective throughput than large, bulk transfers.
Is the Inter-Frame Gap actually part of the frame?
No — it's idle time (minimum 96 bit-times) enforced on the wire between the end of one frame and the start of the next, not data within the frame itself. It is still counted as overhead because it consumes link time that cannot carry payload, reducing achievable throughput exactly like the other components.