MTU Calculator
Calculate the optimal maximum transmission unit for a network path.
About MTU Calculator
The MTU (Maximum Transmission Unit) is the largest packet size a network link can carry in a single frame — 1500 bytes for standard Ethernet, or up to 9000 bytes for 'jumbo frames' on networks that support them. If a payload plus its headers exceeds the MTU, it must be split into multiple fragments, each carrying its own header overhead, which reduces the useful data ratio and adds reassembly work at the destination. Path MTU Discovery is the mechanism modern stacks use to avoid fragmentation by finding the smallest MTU along a route.
Formula
Fragment count
fragments = ceil(payload_size / (MTU − header_overhead))
- S
- Payload size in bytes
- h
- Header overhead per packet (e.g. IPv4+TCP = 40 bytes)
Worked example
With the default values below, MTU Calculator returns Fragments Required of 4. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Payload Sizeminimum 1 bytes
- 4,500 bytes
- MTUminimum 68 bytes
- 1,500 bytes
- Header Overhead per Packetminimum 0 bytes
- 40 bytes
Results
- Fragments Required
- 4
- Usable Payload per Packet
- 1,460 bytes
- Total Bytes on Wire
- 4,660 bytes
- Header Overhead
- 3.56%
Step by step
- 1Values used
- 2Fragment count
- 3Fragments Required= 4
- 4Usable Payload per Packet= 1,460 bytes
- 5Total Bytes on Wire= 4,660 bytes
- 6Header Overhead= 3.56
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 2 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
What is the standard MTU for Ethernet?
1500 bytes is the standard Ethernet MTU, defined by the original Ethernet II frame format. This has remained the default across most LANs and the public internet for decades.
What are jumbo frames and why use them?
Jumbo frames raise the MTU to typically 9000 bytes, reducing the number of packets (and thus header overhead and CPU interrupts) needed to move the same amount of data. They're common in data center and storage networks but must be supported end-to-end to be useful.
What happens if a packet is larger than the path MTU?
Routers either fragment the packet into smaller pieces (adding overhead and reassembly work) or, if the Don't Fragment (DF) bit is set, drop the packet and return an ICMP 'Fragmentation Needed' message so the sender can reduce its packet size.
Why does fragmentation increase overhead?
Every fragment needs its own copy of the packet headers (IP, and sometimes TCP), so splitting one large packet into several smaller ones multiplies the header bytes sent, reducing the fraction of the transmission that's actual payload data.