MTU & Bandwidth Calculator
Compute MTU overhead, fragmentation and transfer time at various bandwidths.
Overview
The MTU & bandwidth calculator computes how a chosen Maximum Transmission Unit interacts with a transfer: payload bytes available after IP and TCP/UDP headers, the fragmentation behaviour at common MTU boundaries, and the wall-clock time to move a given file size at typical bandwidths. Enter an MTU, a file size, and a link rate, and the table fills in.
Network engineers tuning a VPN tunnel, cloud architects sizing jumbo frames in a storage VLAN, and home-lab tinkerers debugging a slow link all need MTU and bandwidth math without setting up a spreadsheet. Long-tail keywords covered: calculate MTU overhead for VPN, jumbo frame MTU 9000 vs 1500, and file transfer time at different bandwidths.
How it works
The MTU is the largest IP packet that a link can carry without fragmentation. Ethernet defaults to 1500 bytes. PPPoE drops 8 bytes for the header (1492). IPsec tunnels lose another 50–60 bytes depending on the cipher. Jumbo frames at 9000 bytes are common in data centres but only work end-to-end if every hop supports them.
From an MTU, subtract 20 bytes for the IPv4 header (40 for IPv6) and 20 for TCP (or 8 for UDP) to get the maximum payload per packet. Transfer time is the file size divided by the link rate, with a small adjustment for protocol overhead (typically 3–5% on TCP at modest loss rates).
Examples
- Ethernet MTU 1500: 1460 bytes of TCP payload per packet, 1472 for UDP.
- IPv4 GRE tunnel over Ethernet: MTU 1476, payload 1436 for TCP.
- Jumbo frame 9000: 8960 bytes of TCP payload, six times fewer packets per megabyte transferred.
- 1 GB file at 100 Mbit/s: roughly 80 seconds of transfer plus a few seconds of TCP overhead.
FAQ
How do I find the real path MTU?
Run ping -M do -s <size> on Linux (or ping -f -l <size> on Windows) with progressively larger sizes until packets stop arriving. The largest size that succeeds, plus 28 bytes for IP+ICMP headers, is the path MTU.
Why do VPNs reduce my throughput so much?
Each VPN packet is the original payload plus the tunnel headers, encryption framing, and integrity check. If the encapsulation pushes you over the link MTU, the underlying network fragments — usually catastrophic for performance.
Do jumbo frames always help?
Only if every hop end-to-end supports them. A single 1500-byte hop in the middle of an otherwise 9000-byte path forces fragmentation or path-MTU discovery to bring everyone down to 1500.
What is path MTU discovery?
A TCP stack starts with the local interface MTU and sends DF (don't fragment) packets. Routers that cannot forward them reply with ICMP "fragmentation needed" and the new MTU; the stack lowers its segment size accordingly.