Internet speed: what the numbers in offers mean
Internet provider offers are full of numbers, "up to 500 Mb/s", "symmetrical line", "response under 10 ms". Not all these numbers mean the same thing and not all are equally important for everyday use. Before comparing offers, it makes sense to know what individual values actually say.
Mb/s and MB/s are not the same
Internet speed is stated in megabits per second (Mb/s, with a lowercase "b"). File sizes, on the other hand, are stated in megabytes (MB, with an uppercase "B"). One byte consists of eight bits, so the actual download speed in megabytes per second is roughly eight times lower than the figure shown in the tariff offer. A connection of "100 Mb/s" therefore downloads approximately 12.5 MB/s - this distinction explains why internet speed and file download speed appear to "disagree" at first glance, even though everything is correct.
Download, upload and why upload matters
Download is the speed of downloading data (websites, streaming, updates), upload the speed of sending (video calls, cloud backup, file uploads). For many tariffs - mainly older metallic technologies - upload is significantly lower than download. This does not matter for general browsing, but for frequent video calls or working from home with uploading large files, a more balanced ratio between download and upload is an important parameter worth checking in the offer, not just the main advertised figure.
Odezva (latence)
Latency, measured in milliseconds, indicates how quickly a device and server exchange the first signal. It is almost irrelevant for downloading files but crucial for online gaming, video calls and anything requiring an immediate response. Low latency is typical of fibre connections, while higher latency is common with satellite and some wireless technologies - more in the article Starlink in the Czech Republic: when it makes sense and when it does not.
What speed 'up to' means
The word "up to" before a number means that it is the theoretical maximum under ideal conditions, not a guaranteed value. The actual speed may be lower due to distance from the cabinet or transmitter, network congestion in the area, or the quality of home WiFi. More honest offers state the guaranteed minimum speed alongside the maximum - this is the figure you should look at rather than the marketing maximum.
Aggregation: why speed fluctuates by time of day
A number of technologies do not allocate dedicated capacity to each user but share it among multiple connections at a ratio known as aggregation (e.g. 1:20 or 1:50). During peak times, when more people are connected simultaneously, the available capacity is divided between them and the actual speed drops below the advertised maximum. This is a common phenomenon, not a fault, but it is good to know that it may mainly occur in the evening.
Packet loss and latency fluctuation (jitter)
Apart from speed and response time, technical descriptions also include less familiar terms such as packet loss (how much data is lost during transmission and must be resent) and jitter, which is the fluctuation of response time over time. Both values are not advertised in standard offers but significantly affect the smoothness of video calls and online games. A stable connection with slightly lower speed is often more pleasant for these purposes than a faster connection with fluctuating quality.
How much speed do you really need
- Browsing the web and email - even low tens of Mb/s are sufficient.
- Streaming video in HD/4K - units to tens of Mb/s per device, depending on quality.
- Home office with video calls - besides download speed, upload speed and low latency also matter.
- Household with multiple devices and simultaneous streaming - a higher buffer is useful so that capacity is not divided to everyone's detriment.
The highest tariff in the offer is not automatically the one you need; on the contrary, it makes no sense to overpay for speed that you will never use. The article How to choose an internet provider describes how to compare offers comprehensively.
How to find out the actual speed of your connection
The only reliable way to find out what your connection actually provides is to measure it. You can do this in the speed test - ideally via a cable directly to the router, as WiFi results are commonly distorted. If the measured values consistently do not match the contract, the next steps are described in the article Slow internet: systematic procedure before calling your provider.