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📦 MTU / MSS Calculator
MTU (Maximum Transmission Unit) is the largest packet your network can carry. MSS (Maximum Segment Size) is the largest TCP payload that fits. They differ by header overhead. Get this wrong and you get fragmentation, timeouts, or that classic "works for small files, hangs on big ones" symptom. This calculator does the subtraction for you given the protocols you stack on top.
Examples
MTU 1500, IPv4/TCPMSS = 1500 - 20 (IPv4) - 20 (TCP) = 1460 bytesMTU 1492 (PPPoE), IPv4/TCPMSS = 1452 bytesMTU 1500, IPv4/TCP + IPsecMSS varies 1340-1400 depending on cipher and modeFrequently asked questions
Default Ethernet MTU?
1500 bytes. Jumbo frames extend to 9000+ but only on networks that support it end-to-end.
When does PMTU discovery fail?
When ICMP "Fragmentation Needed" gets filtered (common on misconfigured firewalls). Result: packets larger than the smallest hop's MTU get dropped silently, causing hangs.
About this calculator
MTU / MSS Calculator runs entirely in your browser using standard formulas. No data is sent to any server. We don't share your inputs with lenders, brokers, or anyone else — there's no funnel and no follow-up email.
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Why this tool
vs typical free network-tools sites
| NetToolset | Typical free site | |
|---|---|---|
| Price | Free | Free |
| Signup required | No | Often (for advanced features) |
| Ads inside the tool | No | Yes (banner + interstitial) |
| Logs your IP / queries | No | Yes (often resold) |
| Data sent to a server | Math runs in browser; lookups hit Cloudflare DoH directly | Yes (all goes through their server) |
| Open source | Yes | No |
| Bookmarkable URL per tool | Yes | Mixed |
| Multilingual | EN / ES / PT / FR / DE | Usually EN only |
| Loads in under 1s | Yes (static) | Often slow (ad tracking) |