802.1Q Tag Overhead Calculator
Measure what a 4-byte 802.1Q tag or an 8-byte QinQ stack costs in frame size, MTU and bandwidth.
About 802.1Q Tag Overhead Calculator
The tag is inserted after the source MAC address, before the EtherType, which pushes the frame past the classic 1518-byte maximum and is why switches must accept 1522-byte baby-giant frames. Because the tag sits inside the MTU budget, every tag you add takes a byte of payload away. Unaccounted tags are a classic cause of intermittent large-packet loss: the frame passes on an access port and gets dropped on the trunk, so pings work while file transfers stall.
Formula
802.1Q Tag Overhead
Each 802.1Q tag adds 4 bytes — a 2-byte TPID (0x8100) and a 2-byte TCI holding priority, DEI and the 12-bit VLAN ID — so a tagged frame is 1522 bytes and a QinQ frame 1526.
- TPID
- Tag Protocol Identifier, 0x8100 for 802.1Q and 0x88A8 for 802.1ad
- TCI
- Tag Control Information — 3-bit PCP, 1-bit DEI, 12-bit VLAN ID
- on wire
- Tagged frame plus the 20 bytes of preamble, SFD and interframe gap
Worked example
With the default values below, 802.1Q Tag Overhead Calculator returns Tagged Frame Size of 1,522. Change any input above and every figure here is recalculated in your browser.
Inputs used
- IP packet sizeaccepts 20–9,000 bytes
- 1,500 bytes
- Tag stackone of Untagged, Single 802.1Q tag, QinQ — 802.1ad double tag
- Single 802.1Q tag
- Frame rateminimum 0 frames/s
- 81,274 frames/s
- Port MTUaccepts 576–9,216 bytes
- 1,500 bytes
Results
- Tag Overhead
- 4 bytes
- Tagged Frame Size
- 1,522 bytes
- Bytes on the Wire
- 1,542 bytes
- Overhead vs Untagged
- 0.264%
- Bandwidth Consumed by Tags
- 2.601 Mbps
- Payload Left Inside the MTU
- 1,496 bytes
Step by step
- 1Values used
- 2802.1Q Tag Overhead
- 3Tagged Frame Size= 1,522 bytes
- 4Tag Overhead= 4 bytes
- 5Bytes on the Wire= 1,542 bytes
- 6Overhead vs Untagged= 0.264
- 7Bandwidth Consumed by Tags= 2.601 Mbps
- 8Payload Left Inside the MTU= 1,496 bytes
Assumptions and limits
- The result is derived only from the 4 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 3 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 802.1Q Tag Overhead calculated?
Each 802.1Q tag adds 4 bytes — a 2-byte TPID (0x8100) and a 2-byte TCI holding priority, DEI and the 12-bit VLAN ID — so a tagged frame is 1522 bytes and a QinQ frame 1526. The tag is inserted after the source MAC address, before the EtherType, which pushes the frame past the classic 1518-byte maximum and is why switches must accept 1522-byte baby-giant frames. Because the tag sits inside the MTU budget, every tag you add takes a byte of payload away.
Why does 802.1Q Tag Overhead matter?
Unaccounted tags are a classic cause of intermittent large-packet loss: the frame passes on an access port and gets dropped on the trunk, so pings work while file transfers stall.
What values do I need to enter?
This calculator takes 4 inputs: IP packet size, Tag stack, Frame rate, Port MTU. 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 the tag reduce the IP MTU?
Not on a compliant switch — 802.1Q says the tag rides on top, so the port must carry 1522 bytes to keep a 1500-byte IP MTU. It only costs you payload when an intermediate device enforces a hard 1518-byte limit.