Understanding results
Why speed test results change between runs and between sites
Short answer: every speed test measures a different path. Each one picks its own server, uses a different number of connections, and runs for a few seconds at a moment when your Wi-Fi, your household and your provider's network are all doing something different. Big gaps are normal; what matters is whether the result explains the problem you actually have.
Run our test (single connection, with latency under load)
Why different sites disagree
| Test | Measures to | Connections | What it is good for |
|---|---|---|---|
| Speedtest by Ookla | A host it picks, often inside or next to your provider's network | Several in parallel | The most your line can carry right now |
| Fast.com (Netflix) | Netflix's own delivery servers | Several | How well Netflix will stream |
| Google's speed test and this site | M-Lab research servers, usually outside your provider's network | One | What one ordinary transfer gets, and how latency behaves under load |
| Your provider's app or router test | The provider's own server | Varies | Showing the provider's side of the line is working |
The gaps can be large. On r/HomeNetworking, a thread titled "This is why you don't use fast.com for real results" drew more than 250 comments of people comparing numbers (thread). One commenter, u/neon_overload, put it well: "Netflix's FAST test is accurate, for measuring your connection to Netflix's servers," adding that Speedtest.net is "simply better at finding a server to measure that is closer to you." Another reported fast.com showing 1.1 to 1.2 Gbps on a line limited to 1 Gbps at the modem, and a third said their provider's own test "always shows 1/3" of what they actually got.
Research backs this up. A 2022 study from UC Santa Barbara found Ookla medians "up to two times greater than M-Lab measurements for the same subscription tier, city, and ISP" (paper, PDF), while a 2023 University of Chicago study found the two agree closely in the lab and diverge mainly on high-latency paths (paper, PDF).
Why the same test changes run to run
- Your Wi-Fi. Moving one room away, a microwave, or a neighbour's network on the same channel can halve a result. Test on a cable to remove this variable.
- Other devices. A phone backing up photos, a console updating or a 4K stream in another room shares the same line.
- Time of day. Many networks slow in the evening. Users have documented fiber lines dropping from around 947 Mbps in the day to far lower figures between 8 and 11 pm (example). Testing at the same hour each day is the only fair comparison.
- Server choice. Many tests pick the server automatically. A different server on the next run means a different path.
- Your device and browser. Above a few hundred Mbps, an older laptop, a busy browser tab or a USB network adapter can become the limit. A commenter in the thread above noted browser-based tests "aren't reliable for speeds over a few hundred Mbps" and preferred Ookla's desktop or command-line app for gigabit lines.
Which number should you trust?
Pick the test that matches your question:
- "Am I getting the speed I pay for?" Wired, straight into the router, with a multi-connection test like Ookla, at several times of day. Keep screenshots with timestamps.
- "Why does Netflix buffer?" Fast.com, because it measures to Netflix.
- "Why do calls and games lag when someone else is online?" A test that shows latency under load, such as ours. The download number will not explain this; the jump in latency will.
- "Why is this one download slow?" Compare the download's own speed in MB/s with your test result divided by eight. If the test is much faster, the server you are downloading from is the limit.
A routine that gives you a usable answer
- Run three tests in a row in the place where the problem happens, and note the spread.
- Run three more next to the router, ideally on a cable.
- Repeat both at a quiet hour and at the hour the problem usually happens.
If the room tests are low and the router tests are fine, the problem is in your home. If both drop at the same hour every day, it is likely congestion on your provider's side, and the timestamps are your evidence.