Why You Are Only Getting 400 Mbps on a Gigabit Plan

What Does 400 Mbps on a Gigabit Plan Mean?

Getting around 400 Mbps on a gigabit plan means your measured download speed is below the plan's advertised maximum, but it does not immediately prove that the broadband service is defective. A gigabit plan typically refers to speeds near 1,000 Mbps under suitable network conditions. Actual results can vary because of Wi-Fi signal quality, device hardware, router capacity, Ethernet negotiation, server limits, and ISP network conditions.

The first step is to determine whether 400 Mbps is the maximum speed available to your home connection or only the result from one device, app, room, or test server.

Common Cause: Testing Over Wi-Fi

Wi-Fi is one of the most common reasons for only getting 400 Mbps on a gigabit plan. Distance from the router, walls, interference, channel congestion, and the connected device's Wi-Fi generation can all reduce throughput. Even a modern Wi-Fi connection may deliver substantially less than its theoretical link rate because wireless capacity is shared and affected by overhead.

Test beside the router using a recent device connected to a 5 GHz or 6 GHz network when available. Avoid testing through a mesh satellite, range extender, or crowded 2.4 GHz band. If a wired test reaches much higher speeds, Wi-Fi is the limiting factor rather than the broadband line.

Common Cause: Ethernet Cable or Port Limitations

A damaged, poorly terminated, or older Ethernet cable can prevent the connection from negotiating at the expected speed. A device or router port limited to 100 Mbps will create a much lower ceiling, while some hardware, adapters, or powerline products may perform well below gigabit rates.

Use a known-good Cat5e or newer cable and connect directly to a gigabit-capable LAN port on the router. Check the link speed shown by the computer. It should normally report 1 Gbps or higher for a gigabit wired test. Replace USB Ethernet adapters that are limited to 100 Mbps or that rely on unstable drivers.

Common Cause: Router or Modem Throughput Limits

The router or modem may not have enough processing capacity to route traffic at gigabit speeds, especially when features such as traffic inspection, parental controls, VPN services, advanced firewall rules, or bandwidth management are enabled. Older equipment may also include slower WAN or LAN ports.

Review the hardware specifications and confirm that the WAN and LAN ports support gigabit or faster Ethernet. Temporarily disable advanced processing features for a controlled test, then re-enable them individually. Restarting the modem and router can also clear temporary faults, but it will not overcome a hardware speed limit.

Common Cause: Device Performance and Software

The test device itself may be limiting results. Older laptops, low-power processors, overloaded systems, outdated network drivers, security software, VPN clients, and background downloads can reduce measured speed. Browser-based tests may also vary depending on the browser, extensions, and available system resources.

Run the test on a current computer with background applications closed. Update the operating system and network drivers, disconnect active VPNs, and compare results in another supported browser or with a reputable speed-test application. Test more than one device to see whether the same limit appears across the network.

Common Cause: Speed-Test Method or Server Limits

A speed test measures the path between your device and a selected test server, not an abstract maximum for the entire internet. A distant, busy, or poorly connected server may produce a lower result. Single-connection tests can also understate capacity when the route has latency or packet loss.

Choose a nearby test server and repeat the test at different times. Use several reputable testing services, but keep the device and connection method consistent. Compare single-device results with multi-connection downloads from a reliable, well-provisioned service. A stable 400 Mbps result on one server is less conclusive than similar results across multiple nearby servers.

Common Cause: ISP Network Conditions or Plan Configuration

Peak-time congestion in the access network, maintenance, line-quality problems, provisioning errors, or an incorrect service profile can reduce performance. Cable broadband may experience local capacity sharing, while fiber connections can still be affected by equipment, authentication, or upstream network issues.

Compare wired results during busy and quiet periods. If multiple wired devices remain near 400 Mbps while using different test servers, contact the ISP and provide timestamps, test locations, device details, and results. Ask the provider to verify the subscribed speed profile, signal levels where applicable, and whether there are known local faults.

How to Diagnose the Limiting Factor

1. Use a wired connection: Connect a capable computer directly to the router with a tested Cat5e or newer cable.
2. Confirm link speed: Check that the computer and router negotiate at 1 Gbps or above.
3. Run several tests: Use nearby servers and repeat tests at different times.
4. Compare devices: Test another computer to separate network limits from device limits.
5. Check router usage: Pause downloads, streaming, VPNs, backups, and security scans.
6. Record results: Note connection type, time, server, download speed, upload speed, and latency.

Practical Ways to Improve Gigabit Speed

• Use direct wired Ethernet for devices that need consistent high throughput.
• Place the router in an open, central location away from thick walls and interference.
• Prefer 5 GHz or 6 GHz Wi-Fi for high-speed local connections when supported.
• Update router firmware, device drivers, and operating systems.
• Replace damaged cables and confirm that all adapters and ports support gigabit speeds.
• Reduce unnecessary router processing features during troubleshooting.
• Use a suitable mesh system with wired backhaul if coverage, rather than broadband capacity, is the problem.

When to Contact the ISP

Contact the ISP when a direct wired test on capable equipment remains close to 400 Mbps across multiple nearby servers and at different times. Share clear evidence instead of only reporting a single result. Include the plan name, test method, router model, wired link speed, measured download and upload speeds, latency, and the times of testing.

If wired performance is high but Wi-Fi remains near 400 Mbps, the broadband service is probably working normally and the next improvement should focus on wireless coverage, channel conditions, client hardware, or router placement.