MTU Calculator

Work out MTU, MSS and the overhead each tunnel adds.

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Link MTUEncapsulation

Link MTU

1500

Tunnel overhead

0 bytes

Effective MTU

1500

TCP MSS

1460

what to advertise

Plain Ethernet — the standard 1500 byte MTU.

The MSS your machine should advertise is 1460 — the MTU less the 20-byte IP header and the 20-byte TCP header.

This leaves a healthy payload; nothing should fragment.

How to Use the MTU Calculator Online

MTU is the largest packet a link will carry, and tunnels eat into it: IPsec, GRE, PPPoE and WireGuard each add their own header. Get it wrong and the symptom is maddening — small requests work, large ones hang, and the connection looks fine. This works out the effective MTU and MSS once the overhead of each tunnel is subtracted.

No software to install Free Works on any device

Step-by-Step Guide to Using the MTU Calculator

1

1. Fill in the fields with your own values
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2. The answer updates as you type — there is no button to press
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3. The reasoning is explained underneath, not just the number

Try it now

What is the MTU Calculator?

Why Use IP and Network Calculators?

Subnet masks, CIDR ranges and download times all come down to binary maths that is easy to slip on. These calculators do the sums in the page, so you can plan an address range or check a speed claim with confidence.

“One wrong subnet mask can take a whole office offline.”

Subnets worked out instantly
CIDR ranges made clear
Download times estimated
Browser details revealed

Usable host counts shown
IPv4 and IPv6 conversions
MAC addresses checked
No commands to install

Popular Uses for Subnet Calculators

Network jobs at home and at work often start with one of these sums.

Planning an office network Split an address block into subnets for each floor, team or guest WiFi.
Reading CIDR notation See the first address, last address and host count behind a range like /24.
Download estimates Work out how long a large file will take on a given connection speed.
Speed claims Turn megabits per second into megabytes to see what a plan really delivers.
Device lookups Find the maker of a network card from its MAC address prefix.
Browser checks See your user agent and browser details when a support team asks.
WebSocket tests Connect to a WebSocket endpoint and watch messages go back and forth.
IPv6 planning Convert an IPv4 address into IPv6 form while preparing a migration.

What You Can Do with the MTU Calculator

The calculation happens on your own machine, at your machine's speed

Shows the network, broadcast and usable host range together, which is the whole answer

Handles IPv6 at full length rather than truncating it for display

Why Use ToolXkit for MTU Calculator?

Your files stay with you

Everything runs inside your browser. Nothing you open or type is uploaded to a server.

Network Tools for Different Users

Network administrators

Plan subnets, check CIDR ranges and size links for expected bandwidth.

Networking students

Practise subnetting for CCNA and other exams, and check answers quickly.

Developers

Test WebSocket servers, parse user agents and inspect TCP header fields.

IT support staff

Collect browser details and estimate transfer times while helping users.

Small office owners

Understand router settings and IP ranges without calling a technician every time.

Streamers and gamers

Check upload speed, latency and packet loss figures before a stream.

Best Times to Work Out a Subnet

Network calculators help when you are:

Setting up a new router
Studying for a networking exam
Adding a guest WiFi network
Estimating a large upload
Comparing broadband plans
Identifying an unknown device
Moving to IPv6
Debugging a WebSocket connection

Frequently Asked Questions

Answers to common questions about this tool.

Why do small requests work and large ones hang?

That is the signature of an MTU problem. Small packets fit through, large ones need fragmenting, and if the ICMP messages that request fragmentation are blocked by a firewall the sender never learns and simply retries forever. It is called a path MTU discovery black hole, and it looks exactly like an intermittent fault.

What MTU should I use with a VPN?

Lower than 1500, and how much lower depends on the tunnel. WireGuard typically needs about 1420, IPsec varies with its configuration, and PPPoE takes 8 bytes before any tunnel is added. Setting it too low wastes throughput and setting it too high breaks large transfers.

Is MTU Calculator accurate enough for real network work?

The maths behind addresses and ranges is exact, so the results match what a router expects. Double-check the input, especially the prefix length, since one wrong digit changes the whole range.

Do the calculators scan my network?

No. The subnet and speed calculators only work with the numbers you type in. They send no traffic into your network, and the only tool that connects anywhere is the WebSocket Tester, to the address you enter.

Why is my real download slower than the estimate?

The calculator assumes the full connection speed is available. In practice, WiFi, other devices, server limits and protocol overhead eat into it, so real downloads often take somewhat longer.

Is MTU Calculator free and private?

Yes. It costs nothing and needs no account. What you type stays in your browser and is never sent to ToolXkit, which matters when it describes a real company network.

What is the difference between Mbps and MBps?

Mbps means megabits per second, the unit broadband plans use. MBps means megabytes per second, the unit most download windows show. One byte is eight bits, so 100 Mbps is about 12.5 MBps.

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