MTU Calculator

Calculate the optimal Maximum Transmission Unit (MTU) and Maximum Segment Size (MSS) for your network connection. Select your connection type to see the correct MTU value, understand the header overhead, and prevent packet fragmentation that causes slowdowns and connection issues.

What Is MTU?

The Maximum Transmission Unit (MTU) is the largest packet size (in bytes) that a network interface can transmit without fragmenting the data. The standard Ethernet MTU is 1500 bytes, but different connection types like PPPoE, VPN tunnels, and GRE encapsulation add overhead headers that reduce the effective MTU.

When a packet exceeds the MTU, it must be fragmented into smaller pieces — or dropped entirely if the Don't Fragment (DF) flag is set. Fragmentation increases latency, wastes bandwidth, and can cause connection failures. Getting your MTU right eliminates these problems. Learn more about how packets travel through your network with our gateway explainer.

MTU Values by Connection Type

Different network technologies have different maximum MTU values due to their header overhead:

Pro Tip: If you're on a PPPoE (DSL) connection, set your router's MTU to 1492 at 192.168.1.1. This prevents the "websites won't load" issue caused by oversized packets being silently dropped. You can also enable MSS clamping on your router to automatically adjust packet sizes for all devices on your network.

MSS vs MTU Explained

While MTU is the total packet size, MSS (Maximum Segment Size) is the maximum amount of actual data (payload) in a TCP segment. The difference is the protocol headers:

MSS = MTU - IP Header - TCP Header
MSS = 1500 - 20 - 20 = 1460 bytes (standard Ethernet, IPv4)
MSS = 1500 - 40 - 20 = 1440 bytes (standard Ethernet, IPv6)
MSS = 1492 - 20 - 20 = 1452 bytes (PPPoE, IPv4)

MSS is negotiated during the TCP three-way handshake. If your router supports MSS clamping, it automatically adjusts the MSS to prevent fragmentation — this is often the best solution for VPN and PPPoE connections. Check your subnet configuration with our Subnet Calculator.

How to Find Your Optimal MTU

Use the ping command with the Don't Fragment flag to discover the largest packet your path supports:

Windows

ping -f -l 1472 8.8.8.8
REM If this fails, reduce size: 1472, 1464, 1452, 1440...
REM Optimal MTU = largest working size + 28 (IP+ICMP headers)

macOS / Linux

ping -D -s 1472 8.8.8.8
# Reduce size until pings succeed
# Optimal MTU = largest working size + 28

The value 1472 = 1500 (Ethernet MTU) - 20 (IP header) - 8 (ICMP header). If you're on PPPoE, start with 1464. Use our Ping Test to verify connectivity after making changes.

MTU and VPN Performance

VPN tunnels are the most common cause of MTU problems because they encapsulate your original packets inside additional headers:

If websites fail to load or connections stall when connected to a VPN, reduce your MTU. Most VPN clients allow you to set this in the configuration file. For network monitoring while troubleshooting, use our traffic monitoring guide.

How to Change MTU on Your Router

Most routers allow MTU configuration through the admin panel. Access yours at 192.168.1.1 or your gateway address:

1. Log in to your router's admin panel.
2. Navigate to WAN/Internet settings.
3. Find the MTU setting (usually under Advanced).
4. Set the optimal value from the calculator above.
5. Save and reboot the router.

For per-device MTU changes, use these commands:

# Windows (Admin Command Prompt)
netsh interface ipv4 set subinterface "Wi-Fi" mtu=1440 store=persistent

# Linux
sudo ip link set dev eth0 mtu 1400

# macOS
sudo ifconfig en0 mtu 1440

Common MTU Problems and Solutions

MTU mismatches cause subtle but frustrating issues. Here are the most common symptoms and fixes:

• Websites partially load — Small requests work but large pages fail. Fix: Lower MTU or enable MSS clamping.
• VPN connection drops — Fragmented packets get reassembled out of order. Fix: Set VPN MTU to 1400 or lower.
• Slow large file transfers — Excessive fragmentation wastes bandwidth. Fix: Use the ping test above to find optimal MTU. Use our File Transfer Calculator to verify expected speeds.
• SSH works but HTTPS fails — SSH uses small packets; HTTPS sends large ones. Fix: Set correct MTU for your connection type.
• Gaming lag spikes — Fragmentation adds latency. Fix: Match MTU to your connection type. Check with our Latency Test.

Video: Understanding MTU and Fragmentation

Related Tools & Guides

• Subnet Calculator
• Bandwidth Calculator
• Ping Test
• Network Latency Test
• What Is NAT?
• What Is a Gateway?
• Speed Test
• What Is Port Forwarding?

Frequently Asked Questions

What MTU should I use for PPPoE?

Use 1492 for PPPoE connections. The PPPoE header adds 8 bytes of overhead to the standard 1500-byte Ethernet MTU. Most DSL routers default to this, but check your router settings at 192.168.1.1 to confirm.

What is the best MTU for gaming?

For most gaming, the default 1500 MTU works fine on Ethernet connections. If you're gaming over a VPN, reduce to 1400-1440. The key is avoiding fragmentation — use the ping test above to verify your optimal size.

Does MTU affect internet speed?

Not directly, but wrong MTU causes packet fragmentation or drops, which severely impacts throughput and latency. Correct MTU ensures packets flow without fragmentation, maximizing your effective speed. Test with our Speed Test before and after changes.

What is MSS clamping?

MSS clamping is a router feature that intercepts TCP connections and reduces the MSS value in SYN packets to match your network's MTU. This prevents fragmentation without requiring each device to set its MTU manually — it's the recommended approach for PPPoE and VPN networks.

Should I use jumbo frames?

Jumbo frames (MTU 9000) only benefit local networks with supported hardware. They reduce per-packet overhead for large transfers like NAS backups. Never use them on WAN connections. All switches and NICs in the path must support the same jumbo frame size.

Why do some websites not load with wrong MTU?

When packets exceed the path MTU and have the Don't Fragment flag set, they're silently dropped. Small requests (DNS, SYN) work fine, but large responses (HTML, images) fail. This makes sites appear to "hang" after initial connection. Fixing MTU or enabling MSS clamping resolves this.

How does MTU relate to fragmentation?

If a packet exceeds the MTU of any link in its path, it must be fragmented into smaller pieces. Each fragment has its own header overhead, reducing efficiency. Fragments can arrive out of order or get lost, requiring retransmission. Proper MTU configuration eliminates fragmentation entirely.

About Tommy N.

Tommy is the founder of RouterHax and a network engineer with 10+ years of experience in home and enterprise networking. He specializes in router configuration, WiFi optimization, and network security. When not writing guides, he's testing the latest mesh WiFi systems and helping readers troubleshoot their home networks.