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.
Inputs
Undetected Bad Frames per Year
0.00000892
FCS Field Size
4bytes
Bits Protected by the CRC
12,144bits
Errored Frames per Hour
4.37
FCS Overhead of the Frame
0.264%
Step by step
Values used
Payload size (L3 packet) = 1,500 bytes; Link bit error rate (10^x) = -12 exponent; Frame rate on the link = 100,000 frames/s
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.
Undetected error rate
P(frame in error) = frame bits × BER; undetected frames = errored frames × 2^-32.
Undetected Bad Frames per Year
= 0.00000892
FCS Field Size
= 4 bytes
Bits Protected by the CRC
= 12,144 bits
Errored Frames per Hour
= 4.37
FCS Overhead of the Frame
= 0.264
How it works
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.
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.