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.
Inputs
Tagged Frame Size
1,522bytes
Tag Overhead
4bytes
Bytes on the Wire
1,542bytes
Overhead vs Untagged
0.264%
Bandwidth Consumed by Tags
2.601Mbps
Payload Left Inside the MTU
1,496bytes
Step by step
Values used
IP packet size = 1,500 bytes; Tag stack = Single 802.1Q tag; Frame rate = 81,274 frames/s; Port MTU = 1,500 bytes
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.
Tagged Frame Size
= 1,522 bytes
Tag Overhead
= 4 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
How it works
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
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.