How does a speed test work?

What a speed test actually measures

A speed test measures four key properties of your internet connection in a matter of seconds. Understanding what each metric means is the first step to making sense of your results.

Download speed

Download speed is the rate at which data travels from a remote server to your device, expressed in megabits per second (Mbps). This is the figure most people associate with internet speed because it governs almost everything you consume online: web pages, videos, music, software updates and cloud file syncing.

• Basic browsing & email: 5–10 Mbps
• HD video streaming (one screen): 15–25 Mbps
• 4K / HDR streaming: 40–50 Mbps
• Busy household (multiple streams + gaming): 100+ Mbps

Upload speed

Upload speed is the rate at which data travels from your device to the internet, also expressed in Mbps. Most residential connections are asymmetric — upload is slower than download — but upload matters greatly for video calls, live streaming, cloud backups and sending large files.

• Standard video call (720p): 2–3 Mbps
• HD video call (1080p) or group call: 5–8 Mbps
• Remote work / cloud backups: 10–25 Mbps
• Live streaming (1080p60): 10–20 Mbps

Ping (round-trip latency)

Ping measures how long it takes for a tiny packet of data to travel from your device to a server and back, expressed in milliseconds (ms). A low ping means the server responds quickly — critical for real-time activities like online gaming, video calls and financial trading.

• Excellent: < 10 ms
• Good: 10–30 ms
• Acceptable: 30–60 ms
• Poor (noticeable lag): 60–100 ms
• Very poor: > 100 ms

Jitter

Jitter is the variation in ping over time. Even if your average ping is acceptable, large swings between measurements cause choppy audio, frozen video frames and lag spikes in games. Jitter above 30 ms is typically problematic for real-time communication.

Loaded latency (latency under load)

Standard ping tests measure latency when the connection is idle. Loaded latency measures ping while a large download or upload is in progress — a much more realistic scenario. A phenomenon called bufferbloat occurs when router buffers become oversized, causing latency to spike to hundreds of milliseconds under load even on otherwise fast connections. speedtest.it measures loaded latency so you can detect bufferbloat.

The two measurement engines

speedtest.it supports two independent measurement engines so you can choose the one that best suits your needs or cross-check results.

Cloudflare Speed

Powered by Cloudflare's global network, this engine uses HTTP/3 and QUIC to transfer test data to and from Cloudflare edge nodes. Because Cloudflare operates in over 300 cities worldwide, the test server is almost always geographically close to you, which tends to yield results that closely reflect real-world performance for everyday browsing and streaming.

Cloudflare Speed also runs multiple parallel streams simultaneously, mimicking the behaviour of a modern browser loading a complex web page. This means it is good at saturating high-bandwidth connections (1 Gbps+) that a single stream might fail to fill.

NDT7 (Network Diagnostic Tool 7)

NDT7 is an open-source protocol developed and maintained by Measurement Lab (M-Lab), a non-profit organisation backed by Google, OTI and academic institutions. It uses a single WebSocket stream over HTTPS (port 443) and is designed to make scientific-grade measurements of your connection.

NDT7's strength is transparency: all results are published to M-Lab's open data repository and can be independently verified. It also measures TCP-level metrics such as round-trip time variance and retransmission rate, giving you a deeper view into connection quality. NDT7 is the preferred engine for ISP benchmarking and regulatory reporting.

How the measurement is performed

Whether you use Cloudflare or NDT7, the test follows the same broad phases:

• Latency probe: A series of small ICMP or HTTPS requests are sent to the test server to establish a baseline ping and detect initial packet loss.
• Download phase: The test server pushes data to your device as fast as possible for a fixed duration (typically 10–20 seconds). The engine samples throughput multiple times per second and reports the median or 90th-percentile value rather than the peak, to avoid over-reporting.
• Upload phase: Your device pushes data to the test server. The same sampling approach applies.
• Loaded latency phase: Ping is measured again while a saturating data stream is active, exposing bufferbloat.

The entire test typically completes in under 30 seconds on a fast connection, and under 60 seconds on slow connections.

How to get accurate results

Many factors beyond your ISP can reduce the speed you measure. Follow these steps for a fair reading:

• Use a wired connection. Wi-Fi adds its own variables: interference from neighbouring networks, distance from the router, and signal reflections. An ethernet cable eliminates all of these.
• Close other applications. Anything that uses bandwidth in the background — cloud sync, streaming, automatic updates — will steal capacity from the test.
• Test on multiple devices.If one device shows low speeds and another does not, the problem may be the device's network card or driver, not the connection.
• Test at different times of day. Residential broadband often slows during evening peak hours when many neighbours are online simultaneously.
• Disable VPNs and proxies. These route your traffic through an extra server, adding latency and potentially capping throughput.
• Restart your modem/router. Consumer routers can develop degraded performance over time; a cold reboot (power off for 30 seconds) often restores rated speeds.

Interpreting your results

Am I getting what I pay for?

ISPs advertise speeds with the phrase “up to” for a reason: the headline figure is the theoretical maximum under ideal conditions. In practice, 80–90% of the advertised speed over a wired ethernet connection is considered normal. If you consistently measure less than 70% of your contracted speed over several days and times, contact your ISP with a log of your test results as evidence.

Why is my Wi-Fi speed so much lower than my plan?

Wi-Fi throughput depends on the 802.11 standard (Wi-Fi 5 / 6 / 6E / 7), the frequency band (2.4 GHz vs 5 GHz vs 6 GHz), the distance from the access point, and interference. A device connected on 2.4 GHz in a crowded apartment building may see a fraction of the speed available on a wired connection, even with a 1 Gbps fibre plan.

Is my bufferbloat a problem?

If your loaded latency is more than 50–100 ms higher than your idle ping, you have significant bufferbloat. The fix is usually to enable Smart Queue Management (SQM) or a similar active queue management algorithm on your router. Routers running OpenWrt or pfSense can enable CAKE or FQ-CoDel, which dramatically reduce bufferbloat without sacrificing throughput.

What ping do I need for gaming?

For first-person shooters and other latency-sensitive games, aim for idle ping below 30 ms and loaded ping below 50 ms. Jitter should be below 10 ms. Most modern games are tolerant of up to 80 ms idle ping, but jitter and packet loss are far more disruptive than raw latency.

The difference between Mbps and MB/s

Speed tests report speeds in megabits per second (Mbps), while download managers and operating systems typically show megabytes per second (MB/s). Since there are 8 bits in a byte, divide your Mbps figure by 8 to get the approximate file download rate. A 100 Mbps connection delivers roughly 12.5 MB/s of effective throughput.

Frequently misunderstood metrics

Why does the test show a higher speed than my download manager?

Speed tests use multiple parallel connections and are optimised to measure the raw capacity of your line. A single-threaded file download from a distant server is limited by that server's upload speed, CDN routing, TCP congestion control and the number of hops between you and the server. The speed test measures your line; a download manager measures the slowest link in a chain.

Does running the test use a lot of data?

A standard test transfers approximately 100–500 MB depending on your connection speed and the test duration. On very fast connections (1 Gbps+) it can exceed 1 GB. If you are on a metered mobile plan, keep this in mind before running the test.