USB-C is the small, oval plug that now turns up on almost every new laptop, phone, tablet and monitor. It is a genuine improvement: it goes in either way up, it is strong, and one kind of socket can do the work that used to need three or four. The catch is that the plug only tells you the shape. What actually travels down it, how fast, how much power and whether a screen will work, depends on the laptop, the cable and whatever is at the other end. This guide explains, in plain terms, what USB-C can do, what the names on the box mean, how charging and video work through it, and why two cables that look identical can behave completely differently.
One plug, many jobs
For years a laptop had a separate socket for each job: a round or rectangular socket for the charger, the larger rectangular USB-A sockets for a mouse or a memory stick, an HDMI or VGA socket for a screen, and sometimes a network socket. USB-C was designed so that one connector could do all of it. A single USB-C socket can carry data to and from a drive, power to charge the laptop, power out to charge a phone, and a picture to a monitor, sometimes all at the same time down one cable.
The important word is "can". USB-C is the name of the connector, the physical shape of the plug and socket. It is not a promise of what the socket does. Two laptops can each have a USB-C socket on the side, and one of them may run a pair of monitors, charge from a 100-watt charger and copy files at 40 gigabits a second, while the other charges, copies files more slowly and will not show a picture at all. Both sockets are USB-C. What sits behind the socket is different.
That is why the first step with any USB-C accessory is to find out what your laptop's own sockets support.
Finding out what your sockets can do
Start by looking closely at the side of the laptop. Many makers print a small symbol beside each USB-C socket:
- A lightning bolt usually means a Thunderbolt socket, which does data, video and charging at high speed.
- A D-shaped symbol with a P inside is the DisplayPort mark, which means the socket can send a picture to a screen.
- A battery or plug symbol usually marks the socket the laptop charges from.
- A number such as 10 or 20 beside the USB symbol is the data speed in gigabits per second.
Not every maker prints symbols, so the reliable answer is the laptop's specification page. Find the exact model, which is usually printed on a sticker underneath, and search the maker's own website for the specifications. Look for the lines about ports, display output and charging. Apple has a support page called "Identify the ports on your Mac" that shows each Mac's sockets and what they do.
Once you know what each socket can do, choosing the right USB-C accessories becomes much simpler.
The names on the box: USB 3.2, USB4 and Thunderbolt
USB names have been confusing for a long time, partly because the same speed has been renamed more than once. The organisation that looks after the USB standards, the USB Implementers Forum (USB-IF), now asks makers to put the speed itself on the box and the product, in plain numbers, rather than version names. So you may see either kind of name, and it helps to know how they line up.
| Name on the box now | Older technical name | Top data speed |
|---|---|---|
| Hi-Speed USB | USB 2.0 | 480 megabits a second |
| USB 5Gbps | USB 3.2 Gen 1 (earlier called USB 3.0) | 5 gigabits a second |
| USB 10Gbps | USB 3.2 Gen 2 | 10 gigabits a second |
| USB 20Gbps | USB 3.2 Gen 2x2 | 20 gigabits a second |
| USB 40Gbps | USB4 | 40 gigabits a second |
| USB 80Gbps | USB4 Version 2.0 | 80 gigabits a second |
A gigabit is a measure of bits, not bytes, and there are eight bits in a byte, so 10 gigabits a second is at most about 1.25 gigabytes a second, and real copying is slower than the headline figure. What matters more is the rule that the slowest part sets the speed. A fast drive in a fast enclosure, plugged into a slow socket through a slow cable, copies at the speed of the slowest of the three.
The speed names are not tied to the plug shape. The older rectangular USB-A plug can also carry 5 or 10 gigabits a second, while some USB-C cables carry only Hi-Speed USB 2.0. So a USB-C plug on its own tells you nothing about speed.
USB4
USB4 is the newer generation of USB that works only through USB-C. As well as fast data, it can carry a picture for a screen and the kind of connection that docks and fast external drives use. Intel lists 20 gigabits a second as the minimum for a USB4 computer, so a socket marked USB4 is not always a 40 gigabit one. The packaging name tells you which.
Thunderbolt
Thunderbolt is a connection developed by Intel that uses the same USB-C plug. Its generations, as Intel describes them, are:
- Thunderbolt 3: up to 40 gigabits a second through USB-C, with support for two 4K screens at 60Hz from one socket.
- Thunderbolt 4: also 40 gigabits a second, but with stricter minimums every laptop must meet, including support for at least two 4K screens and 15 watts of power for accessories.
- Thunderbolt 5: up to 80 gigabits a second both ways, with a Bandwidth Boost that raises it to 120 gigabits in one direction when a lot of video is being sent, and support for two 8K screens.
Intel says Thunderbolt is designed to work with USB products too, so an ordinary USB-C mouse, drive or hub plugged into a Thunderbolt socket works as normal. The reverse is not true: a Thunderbolt dock plugged into a socket that is only USB will usually work in a reduced way, or not at all, depending on the dock. If a product is described as compatible with Thunderbolt ports, check whether it needs Thunderbolt or merely works with it.
Which speed do you need?
For a mouse, keyboard or webcam, any speed is plenty. For copying photographs and video, or running a laptop from an external drive, a faster socket and an M.2 SSD enclosure that matches it make a real difference, and the guide to SSD enclosures goes into that. For a desk set-up with several screens running from one cable, a Thunderbolt or USB4 socket and a docking station made for it is the combination that gives the most.
Power: how USB-C charging works and the watts
Most laptops sold in recent years charge through USB-C, using a system called USB Power Delivery, usually shortened to USB PD. Its job is to let one charger safely charge very different things, from earphones to large laptops.
The charger and the laptop agree first
When you plug a USB PD charger into a laptop, power does not simply pour down the cable. The charger and the laptop talk to each other first. The charger says which voltages and how much current it can supply, and the laptop asks for the one it wants. Only then does the charger raise the power to that level. The USB-IF puts it simply: a device can negotiate for only the power it needs. That is why a big charger does not harm a small device.
The watts
Power in watts is volts multiplied by amps. Until 2021, USB PD reached up to 100 watts, made from 20 volts at 5 amps. The USB PD 3.1 update added higher fixed voltages of 28, 36 and 48 volts, giving 140, 180 and 240 watts. The original range up to 100 watts is called the Standard Power Range, and the new range up to 240 watts is the Extended Power Range. Laptops that need more than 100 watts through USB-C use the Extended Power Range, and so do the chargers and cables that serve them.
For everyday use, the number that matters is the one your laptop asks for. A laptop sold with a 65-watt charger will charge at full speed from a 65W USB-C charger or anything larger, and more slowly from a smaller one. Larger laptops may need 100W chargers or 140W chargers. The guide to laptop chargers goes through choosing one in detail.
Power goes both ways
USB-C power is not one-way. The same socket that charges your laptop can usually send power out to charge a phone or run an accessory. Intel says a Thunderbolt 4 or 5 socket must supply at least 15 watts to accessories, and a USB4 socket at least 7.5 watts. This is how a bus-powered hub, a drive or a portable monitor can run from the laptop with no charger of its own, though anything powered this way does draw down the laptop's battery.
Charging through a hub, dock or monitor
Many USB-C hubs have a charging socket, so the charger plugs into the hub and the hub passes the power on to the laptop down the same cable that carries everything else. The hub keeps a little of that power for itself, so the laptop receives slightly less than the charger supplies. Some monitors with a USB-C input charge the laptop in the same way, and Apple says a Mac that charges through USB-C can charge from a display as well as from a power adapter. The guide to USB-C hubs and docks covers pass-through charging and what to look for.
Video over USB-C
A USB-C socket that can send a picture to a screen is doing something called DisplayPort Alternate Mode, usually written DP Alt Mode. It is a standard from VESA, the body that looks after DisplayPort, and it lets the USB-C connector and cable carry a full DisplayPort picture and sound, alongside USB data and power, down one cable.
Inside a USB-C cable there are four high-speed lanes. In DP Alt Mode, some or all of them carry the picture, and the rest carry fast USB data. A separate pair of wires always carries slower USB 2.0 data. This explains something that puzzles a lot of people: when a hub sends a high-resolution picture using all four lanes, the hub's other USB sockets can drop to USB 2.0 speed. Nothing is broken. The lanes are simply being used for the screen.
Thunderbolt and USB4 sockets carry video in a slightly different way, packing the DisplayPort picture inside their own fast connection, which is how a single Thunderbolt socket can run two screens at once through a dock.
Not every USB-C socket does video
DP Alt Mode is optional for the laptop maker. Some laptops, often lower-priced ones, have USB-C sockets that carry data and power but no picture at all. Others have two USB-C sockets and only one of them does video. Before buying a USB-C to HDMI adapter or a USB-C monitor, check the specification page for "DisplayPort", "DP Alt Mode", "video output" or Thunderbolt against the socket you mean to use.
The adapters and cables for a screen
- To an HDMI screen or TV: a USB-C to HDMI cable in one piece, or an adapter and an HDMI cable.
- To a DisplayPort monitor: a USB-C to DisplayPort adapter or cable.
- To an older projector: a USB-C to VGA adapter.
- To a USB-C monitor: a USB-C cable that supports video, which carries the picture, data and often the charging too.
The number of screens a laptop can run, and at what resolution, is set by the laptop, not the adapter. The guide to connecting a laptop to a monitor or TV explains how to find that out and how to set the screens up once they are plugged in. If you want a second screen that travels, portable monitors often run from a single USB-C cable for both picture and power.
Cables: why two that look alike are not
This is where most USB-C trouble comes from. Every USB-C to USB-C cable has the same plug at each end, but inside they differ in three ways that matter.
Data speed
Some USB-C cables carry only Hi-Speed USB 2.0 data. They are fine for charging and for a mouse or keyboard, but they are slow for copying files and carry no video at all, because they lack the high-speed lanes. Many cables that come in the box with phones and earphones are of this kind. Faster cables are rated for 5, 10, 20, 40 or 80 gigabits a second. A cable rated for 40 gigabits a second or more is the kind that docks, fast drives and screens need.
Power
The USB-IF's labelling for certified USB-C to USB-C cables shows one of two power ratings: 60 watts or 240 watts. A 60-watt cable carries up to 3 amps at 20 volts. Anything over 60 watts needs a cable built for 5 amps, which carries a small chip in the plug that tells the charger what the cable can safely take. You will also see older cables sold as 100-watt cables: these are 5-amp cables made before the higher voltages were added, and they are fine up to 100 watts but are not the cable to use with a 140-watt charger. For the larger laptops, choose a cable rated for 240 watts. The USB-C to USB-C laptop cables range is described by the power and speed each cable is rated for.
Video
A cable that carries a picture from a laptop to a USB-C monitor needs the high-speed lanes, so a USB 2.0 charging cable will charge the laptop from the monitor but show nothing on the screen. If a USB-C monitor stays dark, the cable is the first thing to change.
Thunderbolt cables
Thunderbolt cables have the same USB-C plug and are marked with the lightning bolt. Intel describes its Thunderbolt 5 cables as universal: they work as USB-C cables for data, charging and video as well as for Thunderbolt, and come in lengths up to 2 metres. A Thunderbolt cable is a safe choice when one cable has to do everything, while an ordinary USB-C cable may not give a Thunderbolt dock its full speed.
USB-C to USB-A
Cables and adapters between USB-C and the older USB-A let older plugs work with newer laptops and the other way round. They are useful for memory sticks, receivers and older drives. They are not a way to charge a USB-C laptop properly: Microsoft says a Surface charged from a USB-A charger with an adapter cable will not fast charge, and the same holds for laptops in general, because USB PD laptop charging works between USB-C ends.
A simple way to stay on top of it
Buy each cable for its job and label it. A short, fast cable for the drive, a 240-watt cable for the charger, a video-capable cable for the monitor. A small cable clip or tie with a coloured tag on each one saves a lot of guessing later.
Questions people ask
Can I charge my laptop from any of its USB-C sockets?
Not always. Some laptops charge from every USB-C socket, others from only one or two. Look for a charging symbol beside the socket, or check the specification page.
Will a 100W charger damage a laptop that came with a 65W one?
No, as long as both use USB Power Delivery. The laptop asks for the power it needs and the charger supplies only that. Apple says a Mac that charges through USB-C can use an adapter of higher or lower wattage than the one recommended.
Why is nothing showing on my monitor through USB-C?
Either the socket does not carry video, or the cable is a USB 2.0 charging cable without the lanes a picture needs. Try another USB-C socket, then a cable rated for video.
What is the difference between USB4 and Thunderbolt 4?
Both use USB-C and can reach 40 gigabits a second. Thunderbolt 4 sets fixed minimums every laptop must meet, including two 4K screens and 15 watts for accessories, while a USB4 socket may run at 20 gigabits a second.
Why do my hub's USB sockets slow down when the screen is plugged in?
When a high-resolution picture uses all four high-speed lanes of the cable, only the slower USB 2.0 wires are left for data. A Thunderbolt or USB4 dock avoids this.
Where to go next
All the USB-C kit is gathered under USB-C laptop accessories, with USB-C laptop chargers and USB-C cables for power. For the next step:
- How to choose a USB-C hub or docking station: more sockets from one
- How to connect a laptop to a monitor or TV: cables, adapters and settings
- Laptop chargers explained: watts, USB-C charging and UK plugs
- Faster storage: SSD enclosures explained: making use of a fast socket
