MTU Calculator
Calculate maximum transmission unit and payload size after protocol header overhead.
About MTU Calculator
The Maximum Transmission Unit (MTU) is the largest packet size an interface can send without fragmentation. Standard Ethernet uses 1500 bytes, jumbo frames raise that to 9000 for high-throughput LANs, and PPPoE reduces it to 1492 to make room for its 8-byte header. Encapsulating traffic inside a VPN or tunnel (IPsec, WireGuard, GRE, OpenVPN) adds overhead bytes that must be subtracted from the underlying interface's MTU to avoid fragmentation or silent packet drops when the Don't Fragment bit is set.
Formula
Effective MTU with tunnel overhead
effective_mtu = base_mtu − vpn_overhead
- M_b
- base interface MTU
- O
- tunnel/VPN encapsulation overhead in bytes
Worked example
With the default values below, MTU Calculator returns Effective MTU of 1,500. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Interface Typeone of Ethernet (1500), Jumbo Frames (9000), PPPoE (1492), Custom
- Ethernet (1500)
- Custom Base MTUaccepts 68–65,535
- 1,500
- VPN / Tunnel Overheadminimum 0 bytes
- 0 bytes
Results
- Base MTU
- 1,500 bytes
- Effective MTU
- 1,500 bytes
- Overhead Subtracted
- 0 bytes
- Recommendation
- MTU looks standard.
Step by step
- 1Values used
- 2Effective MTU with tunnel overhead
- 3Effective MTU= 1,500 bytes
- 4Base MTU= 1,500 bytes
- 5Overhead Subtracted= 0 bytes
- 6Recommendation= MTU looks standard.
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, fees or charges or compounding frequency into account.
- Figures are rounded for display to whole numbers, 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
Why do PPPoE connections use MTU 1492 instead of 1500?
PPPoE adds an 8-byte header (6 bytes PPPoE + 2 bytes PPP) on top of the standard 1500-byte Ethernet payload, so the usable payload for upper layers shrinks to 1492 bytes to stay within the Ethernet frame limit.
What is a safe MTU for a WireGuard or IPsec tunnel?
A common safe value is 1420 for WireGuard (60 bytes overhead under 1500 Ethernet) and roughly 1436-1446 for IPsec, though exact overhead varies with the specific encryption/authentication algorithms and whether NAT-T is in use — subtract the actual overhead from your path's base MTU.
What happens if MTU is set too high for the path?
Packets larger than the path's actual MTU get fragmented, or dropped outright if the Don't Fragment bit is set and no ICMP 'fragmentation needed' message gets through (common with firewalls blocking ICMP), causing mysterious connection stalls — Path MTU Discovery exists to detect and avoid this.