How download speed is measured
Download speed is the rate at which data travels from the internet to your device. Speed tests normally report it in megabits per second (Mbps). A higher number means more data can be transferred during the same period.
A browser speed test downloads a known amount of data and measures how long the transfer takes. The result is throughput from the test server to that browser at that moment, not a permanent property of the line.
What uses download capacity
Video streaming, software updates, cloud-file downloads, web browsing and game downloads all use downstream capacity. Several devices can share that capacity at the same time, so a connection that is comfortable for one person may become congested in a busy household.
Why results change
Wi-Fi quality, server distance, provider routing, local congestion, device performance and background traffic can all change the result. Very small tests may finish too quickly to reach the connection’s stable throughput, while extremely large tests can use unnecessary data.
Mbps is not MB/s
Internet connections are usually advertised in megabits per second, while file sizes are often shown in megabytes. Eight bits make one byte, so a 100 Mbps connection has a theoretical maximum of about 12.5 MB/s before protocol overhead and other real-world effects.