Why does gigabit internet only test at about 940 Mbps?

A speed test on a gigabit plan usually shows about 930 to 945 Mbps, and that's a perfect result. A 1 Gbps Ethernet port sends 1,000 Mbps of raw signal, but about 6% of it is taken up by packet headers and the gaps between packets. A speed test only counts the data inside those packets. There's nothing to fix unless your wired result is well under about 900 Mbps.

Where the missing 60 Mbps goes

Data travels in packets. On a standard home network, each packet can carry up to 1,500 bytes, and the network adds extra bytes around it. Here's what one full-size packet costs on a gigabit Ethernet link:

Part	Bytes
Preamble and start-of-frame marker	8
Ethernet header	14
IP header (IPv4)	20
TCP header, including timestamps	32
Your actual data	1,448
Frame check sequence	4
Required gap before the next packet	12
Total time on the wire	1,538

Only 1,448 of every 1,538 bytes are your data. 1,448 ÷ 1,538 = 0.941, so the most a speed test can show on a 1 Gbps link is about 941 Mbps. If the connection doesn't use TCP timestamps, the data part grows to 1,460 bytes, and the ceiling rises to 1,460 ÷ 1,538 ≈ 949 Mbps. That's why gigabit results almost always land between 930 and 950.

The same 94% applies at every speed. A 100 Mbps Ethernet link tops out around 94 Mbps for the same reason. For more on the gap between raw capacity and what you can use, see bandwidth vs throughput.

Things that take a little more

Some connections add a few more bytes to each packet, which lowers the ceiling slightly:

• PPPoE. Many DSL and some fiber ISPs use PPPoE, which uses 8 bytes of every packet. That leaves 1,440 bytes of data, so the ceiling is 1,440 ÷ 1,538 ≈ 936 Mbps.
• IPv6. The IPv6 header is 40 bytes instead of 20, so the data part drops to 1,428 bytes. That gives 1,428 ÷ 1,538 ≈ 928 Mbps.
• VPNs. Encryption adds its own headers, and your computer's processor has to encrypt everything, which can cost much more than the headers. Turn the VPN off before testing.

So a result of 925 to 950 Mbps is normal for gigabit service, depending on how your ISP's connection is set up.

When a lower result is a real problem

Below roughly 900 Mbps on a wired connection, something other than overhead is getting in the way. Check these in order:

1. The port's link speed. If your computer or router port has connected at 100 Mbps instead of 1 Gbps, you'll see about 94 Mbps. That's the same 94% overhead on a link ten times slower. On Windows, the adapter's status window shows the link speed. On a Mac, it's in the Hardware section of the network settings for that port.
2. The cable. A damaged cable, or an old one with only four of its eight wires connected, can drop the link to 100 Mbps. Use Cat5e or better. Cat5e vs Cat6 explains when the category matters.
3. The router. Some routers can't route a full gigabit, especially with traffic shaping, parental controls or QoS turned on. Test with those turned off.
4. The computer and browser. A browser test needs plenty of processing power at gigabit speed. Older laptops, USB Ethernet adapters and busy browsers with lots of tabs can cap the result. Try another device.
5. Wi-Fi. Most Wi-Fi connections can't deliver 940 Mbps of real throughput even near the router. A Wi-Fi test well under that isn't a fault in the plan. See why Wi-Fi is slower than Ethernet.

If your wired test is still low after all that, why your speed test is lower than advertised covers what to say to the ISP.

What about results above 950 Mbps?

A result over about 950 Mbps means the device is connected through a port faster than 1 Gbps, such as 2.5 Gbps Ethernet. On a regular gigabit port, it's physically impossible. If you pay for a multi-gig plan, your modem, router and computer all need ports and cables that are faster than gigabit, or every test will stop at about 940.

How to test a gigabit connection properly

Use a computer connected by Ethernet directly to the router, close other apps and pause downloads on other devices. Then run the SpeedMetric speed test two or three times. If the download lands between 925 and 950 Mbps, you're getting everything a gigabit link can carry. For whether you need that much speed at all, see is 1 Gbps good?