Frame Check Sequence Calculator
Size the 4-byte Ethernet FCS, count the bits it protects and estimate how many corrupt frames slip past CRC-32.
About Frame Check Sequence Calculator
A frame's error probability is its length in bits multiplied by the link bit error rate. CRC-32 catches every burst up to 32 bits and every odd number of bit errors, but a residual 1-in-4.3-billion fraction of corrupted frames still passes, which is what the undetected figure counts. The residual error rate is the number auditors ask about when Ethernet carries storage or financial traffic, because a frame that passes FCS but is corrupt becomes silent data corruption further up the stack.
Formulas
Frame Check Sequence
The FCS is a 32-bit CRC over the frame from the destination MAC to the end of the payload, so a corrupted frame escapes detection with probability 2^-32.
- frame bits
- 8 × (payload + 14-byte MAC header + 4-byte FCS), floored at the 64-byte minimum frame
- BER
- Link bit error rate
- 2^-32
- Residual probability that CRC-32 accepts a corrupted frame
Undetected error rate
P(frame in error) = frame bits × BER; undetected frames = errored frames × 2^-32.
Worked example
With the default values below, Frame Check Sequence Calculator returns Undetected Bad Frames per Year of 0.00000892. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Payload size (L3 packet)accepts 0–9,000 bytes
- 1,500 bytes
- Link bit error rate (10^x)accepts -18–-4 exponent
- -12 exponent
- Frame rate on the linkminimum 0 frames/s
- 100,000 frames/s
Results
- FCS Field Size
- 4 bytes
- Bits Protected by the CRC
- 12,144 bits
- Errored Frames per Hour
- 4.37
- Undetected Bad Frames per Year
- 0.00000892
- FCS Overhead of the Frame
- 0.264%
Step by step
- 1Values used
- 2Frame Check Sequence
- 3Undetected error rate
- 4Undetected Bad Frames per Year= 0.00000892
- 5FCS Field Size= 4 bytes
- 6Bits Protected by the CRC= 12,144 bits
- 7Errored Frames per Hour= 4.37
- 8FCS Overhead of the Frame= 0.264
Assumptions and limits
- The result is derived only from the 3 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 8 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
How is Frame Check Sequence calculated?
The FCS is a 32-bit CRC over the frame from the destination MAC to the end of the payload, so a corrupted frame escapes detection with probability 2^-32. A frame's error probability is its length in bits multiplied by the link bit error rate. CRC-32 catches every burst up to 32 bits and every odd number of bit errors, but a residual 1-in-4.3-billion fraction of corrupted frames still passes, which is what the undetected figure counts.
Why does Frame Check Sequence matter?
The residual error rate is the number auditors ask about when Ethernet carries storage or financial traffic, because a frame that passes FCS but is corrupt becomes silent data corruption further up the stack.
What values do I need to enter?
This calculator takes 3 inputs: Payload size (L3 packet), Link bit error rate (10^x), Frame rate on the link. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.
Does a bad FCS get corrected or dropped?
Dropped. Ethernet's FCS is detection-only, with no forward error correction, so the receiving NIC counts a CRC error and discards the frame — recovery is left to TCP or the application.