The speed of your data depends on four main things: the type of connection (fibre, cable, 4G, 5G), the bandwidth you have, the delay (latency) on the route, and how busy the network is. The signal itself moves close to the speed of light. What you feel as "slow" or "fast" is almost always one of those four.
Below we explain each factor in plain words, show how much speed everyday apps actually need, and give you tips for when your phone feels slow abroad.
How fast does data actually travel?
There are two different "speeds" people mix up. One is how fast a signal moves. The other is how much data arrives per second.
- Signal speed. Light travels at 299,792,458 metres per second in a vacuum (NIST). In a fibre optic cable it is slower, around 200,000 km per second, because the glass slows the light down (ADTRAN). That is roughly 5 microseconds of delay for every kilometre.
- Data rate. This is how many bits arrive each second, measured in megabits per second (Mbps) or gigabits per second (Gbps). This is the number your internet plan and speed tests talk about.
So the distance alone is rarely the problem. A signal can cross a continent in a fraction of a second. Slow pages are usually caused by a thin connection, a crowded network or a weak signal.
What determines the speed at which data travels?
1. The type of connection
The connection type sets the ceiling for your speed.
- Fibre optic: data travels as pulses of light through glass. It offers the highest speeds and low delay.
- Cable (coaxial): fast, but the line is shared with your neighbours, so it can slow down at busy times.
- DSL (phone line): usually slower, and speed drops the further you are from the exchange.
- Satellite: the signal has a long way to go, so the delay is higher. Systems in lower orbits reduce it.
- Mobile (4G and 5G): radio waves between your phone and a cell tower. Speed depends on the generation, the frequency band and the signal.
For 5G, the ITU set a minimum target of 20 Gbit/s download and 10 Gbit/s upload, but only as a peak under ideal conditions (ITU-R M.2410). Real speeds on your phone are much lower. Our guide to the difference between 4G and 5G explains what to expect. If the 4G and LTE labels confuse you, read whether 4G is the same as LTE.
2. Bandwidth
Bandwidth is the maximum amount of data your connection can carry per second. Think of it as the number of lanes on a motorway. More lanes let more data through at once. It matters most when you download large files, stream in 4K or share the connection with several devices.
3. Latency
Latency (or "ping") is the time a small packet of data needs to reach its destination, measured in milliseconds (ms). It depends on the distance to the server and the number of routers on the way. Low latency matters for video calls, online games and anything live. For 5G, the ITU minimum target for the radio part is 4 ms (1 ms for specialised uses), under ideal conditions (ITU-R M.2410).
4. Congestion and speed limits
When many people use the same cell tower or cable line at once, everyone gets a smaller share. This is why speeds often drop in the evening, at stadiums, in airports and at festivals. Some plans also slow your speed once you use up a data allowance, or limit certain types of traffic. Your plan details or your provider's terms say if this applies to you.
5. Your signal, device and surroundings
Walls, floors and distance from the tower or router weaken the signal. An older phone or router may not support the latest standards, so it cannot use the full speed on offer. Full signal bars only show signal strength, not how busy the network is. See our guide on the difference between Wi-Fi and cellular data for how the two connections behave.
How is data speed measured?
Three terms cover most questions:
- Bandwidth or data rate: how much data per second, in Mbps or Gbps.
- Latency: the delay, in ms.
- Download and upload speed: data coming to your device and data going from it. Video calls and uploads need good upload speed too.
One detail that trips people up: speeds are shown in megabits (Mb), while file sizes are shown in megabytes (MB). A byte is 8 bits, so a 100 Mbps connection moves about 12.5 MB per second at best.
How much speed do you need?
Less than most people think. These are the figures published by the services themselves:
- Netflix: 3 Mbps for HD (720p), 5 Mbps for Full HD (1080p) and 15 Mbps for 4K (Netflix Help Center).
- YouTube: about 2.5 Mbps for 720p, 5 Mbps for 1080p and 20 Mbps for 4K (YouTube Help).
- Zoom: 1.2 Mbps for a 720p one-to-one video call, and 3.8 Mbps up and 3.0 Mbps down for 1080p (Zoom).
Add these up if several people or devices share one connection. To see how much data these activities use, read how to check your data usage on iPhone.
Why is my mobile data slow when I travel?
When you are abroad, your speed comes from the local network you connect to, and the same factors apply. Your phone may be on a busy tower, in a weak coverage area, or on 4G where you expected 5G. A travel eSIM does not change the physics, but you can choose a plan that uses the local networks of the country you visit and keep your home plan out of the picture. Speeds also vary a lot by destination, as our list of countries with the best and worst internet in the world shows.
Try these steps in order:
- Check the signal type. Look for 4G, LTE or 5G at the top of your screen. If you only see 3G or no data, move to a better spot, such as near a window.
- Toggle airplane mode for ten seconds to make your phone reconnect. Our airplane mode guide shows how.
- Check your data plan. Make sure you still have data left and that the right line is set for mobile data. See how cellular data works.
- Test on another network or at another time. If it is only slow in one crowded place, congestion is the likely reason.
- Still stuck? Work through our list of mobile data issues and fixes.
If you travel to the United States, a data-only eSIM lets you get online on arrival without paying roaming fees. Pick the amount of data that fits your trip. For help choosing, read our guide to eSIM data plans.
Data-only plans for the United States · 10 GB, 20 GB or 50 GB · Going somewhere else? Find your destination
FAQs
What determines the speed at which data travels?
The type of connection, the bandwidth, the latency and the congestion on the network. Your signal strength and your device also matter, especially on mobile.
How fast does data travel on the internet?
The signal travels at close to the speed of light: about 200,000 km per second in fibre optic cable. The speed you experience, in Mbps, depends on your connection and how busy the network is.
What is the speed at which data travels to and from your ISP measured in?
In bits per second, usually megabits per second (Mbps) or gigabits per second (Gbps). This is called the data rate or bandwidth.
What is the difference between bandwidth and latency?
Bandwidth is how much data can move per second. Latency is how long a single piece of data takes to arrive. You can have high bandwidth and still feel lag if latency is high.
Why is my phone slow even with full signal bars?
The bars show signal strength, not network load or quality. A busy tower, a weak upload path or a plan with a speed limit can all slow you down even with a strong signal.
Why is one website slow while others are fast?
The problem can be on the website's side, such as an overloaded server, or on the route to it. If other sites load fine, your own connection is probably not the cause.
Does my device matter for data speed?
Yes. A phone, laptop or router that does not support the latest standards cannot use the full speed of a fast network.
Is 5G always faster than 4G?
Not always. 5G is designed for higher peak speeds and lower delay, but real speed depends on your signal, the band and how many people share the tower. Read do you need a 5G SIM for more.
Is Wi-Fi faster than mobile data?
Not by default. Wi-Fi speed depends on the connection behind it and on how many people share it. A busy shared network can be slower than a good 4G or 5G signal.
