Tool
One cable carries your display, your external SSD, your network and your USB gear. Work out how much of it each one takes, why your drive slows down when a monitor is plugged into the same docking station, how many displays your Mac will actually drive, and which port everything should be in.
Step 1
Pick a family
Step 2
Pick a chip
Pick a family first, then the chip inside it.
Pick your Mac and the tool applies its real display limit, marks the ports that cannot run a dock, and shows you where each device should go.
Apple menu → About This Mac → System Information → Thunderbolt
Power the dock supplies
Tap anything to set it up or move it
Thunderbolt 4, 40 Gbps each way
Most explanations of Thunderbolt bandwidth miss one thing. It happens to be the thing that matters most.
40 Gbps is not one pool that your devices share. It is two separate pipes. Each one carries 40 Gbps. Each one runs in a single direction. They sit inside the same cable and point opposite ways. Traffic in one pipe cannot borrow room from the other.
That one fact changes most of the answers. DisplayPort only goes one way. Pixels travel from the Mac to the screen and nothing comes back. So a monitor uses downstream bandwidth and none of the upstream.
An SSD works differently. Writing to it sends data down. Reading from it sends data up. Those two jobs sit in different pipes.
Put the two facts together and you get the problem people post about. Plug in a 5K display at 8-bit colour. It takes about 22 of your 40 downstream gigabits. Your SSD writes now have to fit in what is left, and the dock takes its own cut too. So write speed can drop by half. Your SSD reads go up the other pipe. The display never touches that pipe, so reads barely change.
There is a twist here, and it runs the other way. Set that same display to 10-bit colour and it wants about 28 Gbps. One DisplayPort 1.4 tunnel cannot carry that, so the Mac turns on compression. Compressed, the display takes about 9 Gbps instead of 22. So asking for better colour can leave your drive with more bandwidth, not less. That sounds backwards. It is still true, and it is why every suggestion this tool makes is run through the model before you see it.
Before any of the arithmetic matters, your Mac has a hard ceiling on external displays, and it lives in the chip. Every M1 and M2 base chip drives exactly one. Not one per port. One, total. A 14-port dock with three video outputs will still light up a single screen.
This is the most expensive misunderstanding on the subject, because the bandwidth maths says it should work. Two 4K screens are 25 Gbps of a 40 Gbps link. There is room. The display engine simply does not exist. No dock, cable, adapter, firmware or macOS update has ever changed it.
There is a second, separate limit on top: one Thunderbolt port carries a fixed number of displays through a dock or a daisy chain. It was two on every Mac up to the M4 generation. Apple raised it with the September 2026 machines — three per port on the M5 Pro, four on the M5 Max and M5 Ultra — which is why a dock that ran out of room on an M4 Pro may not on an M5 Pro. It only works when the port, the cable and the dock are all genuine Thunderbolt. A USB4 or plain USB-C hub gets one. macOS has never supported MST, so an MST hub advertising dual monitors will mirror the same picture onto both.
Pick your Mac at the top of the page and both limits get applied for you, along with a map of which port to use.
A DisplayLink dock renders the picture on your CPU, compresses it, and ships it as ordinary USB data. That is why it sidesteps the display-engine ceiling entirely. It is a real option and plenty of people are happy on it. It is not free:
Add a DisplayLink screen in the tool above and it models the bandwidth and states the costs alongside it.
There is a second problem sitting on top of the maths, and nobody can calculate this one for you. Real docks do not reach their theoretical numbers. Howard Oakley at the Eclectic Light Company measured this in December 2024. What he found:
The shape of it shows up in ordinary reviews too. Testing of the CalDigit Element Hub reported an SSD writing at under 800 MB/s with an LG 5K display attached to the same hub, while reads stayed above 2,500 MB/s. Moving the display to a port on the Mac restored the write speed immediately. Two directions, two completely different outcomes, one cable.
That last one is why this tool sometimes says depends on your dock. When the measurements disagree, a made up number helps nobody. A test you can run yourself is worth more.
Those figures were taken on 40 Gbps hubs, so the tool charges the same fixed overhead whatever generation you pick. Scaling it up with the link would quietly claim a Thunderbolt 5 measurement that nobody has taken.
Displays go downstream only. They also get served first. The link gives a display what it asks for before anything else gets a turn.
To work out what a display needs, multiply width by height by refresh rate by three times the bit depth. Then add about 5% for timing overhead. At 8-bit colour that gives you:
One DisplayPort 1.4 tunnel on Thunderbolt 3 or 4 carries 25.92 Gbps. That is why 5K 60Hz fits on one cable and 6K does not. A 6K panel uses compression at about 3 to 1. You will not see it. And it leaves much more of the link for everything else.
Storage uses both pipes. Writes go down. Reads come up.
The real limit is not the link. It is the PCIe tunnel inside it. On Thunderbolt 3 and 4 that tunnel carries about 24 Gbps, or roughly 3 GB/s in practice. This is why so many drives land on the same number no matter what they cost. Thunderbolt 5 raises the tunnel to about 63 Gbps.
Before you spend anything on that, move the drive to a spare Thunderbolt port on the Mac. Off the dock it stops sharing the downstream pipe with your displays, and it costs nothing. That is usually the whole fix.
A 10GbE port takes 10 Gbps in whichever direction you are moving data. The tool can reserve it at line rate, which answers "what if I push it hard", or treat it as idle, which is the honest picture most of the time. A quarter of a 40 Gbps link is a lot to hand over to a port that is doing nothing.
If you are pulling video off a NAS while writing to a dock SSD, those two are fighting each other. Several Macs have ethernet built in, which takes the whole question off the dock.
Keyboards, mice, webcams, audio gear and card readers are grouped into one small allowance here. Listing them one by one would suggest we know each number exactly. We do not.
One thing is worth knowing though. USB 3.x on Macs stops at 10 Gbps. Apple has never shipped a 20 Gbps USB port on a Mac. So a drive sold as 20 Gbps will run at half that.
Thunderbolt 5 has a feature that sounds like free speed. It is not. In normal mode it runs 80 Gbps each way. Turn on Bandwidth Boost and it moves the lanes around. You get 120 Gbps downstream. The return path drops to 40 Gbps. Three lanes go one way, one lane comes back.
Display data goes downstream, so a very demanding monitor does gain from this. SSD reads come upstream. If you have a fast drive and an ordinary display, Boost cuts your read headroom from 80 Gbps to 40. In return you get downstream room you were not using. Your setup gets slower in the direction you notice, to fix a problem you did not have.
The calculator above spots this and tells you. A simple rule: leave Bandwidth Boost off. Turn it on only when a display really needs more than 80 Gbps downstream. Today that means 6K at high refresh, two 6K screens, or 8K.
Daisy-chaining. A chain still runs through one port on the Mac. Three devices in a chain share the same bandwidth as three devices in three sockets on one dock. It tidies your cables. It adds nothing.
A second dock on the same port. Same reason.
An MST hub. macOS does not support MST. Both screens show the same picture. This is true on every Apple Silicon Mac including the fastest ones.
A Thunderbolt 5 dock on a Thunderbolt 4 Mac. The link runs at the speed of the slower end. You get 40 Gbps and a dock that cost more. The tool handles this once you pick your Mac.
Six setups people really run. Load one into the calculator, or just read the answer.
Won't work. And bandwidth is not the reason.
The most expensive mistake on this page. Two 4K screens cost about 25 of your 40 Gbps, so the dock has room to spare, and the second monitor still will not turn on. The M1 chip drives one external display. No dock lifts that. A DisplayLink dock is the only route, and it costs you 60 Hz, some CPU, and video streaming on every screen.
Works, with a compromise. The display uses compression. Your SSD is fine.
This one surprises people. At 10-bit the Studio Display wants about 28 Gbps. One DP 1.4 tunnel cannot carry that, so it runs compressed. Compressed, it only takes about 9 Gbps. That leaves your drive almost the whole downstream pipe. The compression is the trade here, not the speed.
Comfortable. Both displays fit, with room left over.
Two 4K 60Hz screens cost about 12.5 Gbps each. Add dock overhead and a few USB devices and you are still inside 40 Gbps. The M4 drives three externals, so the chip is not in the way either. Nothing is using the return path.
Works, once the third display gets its own port.
On this Mac one Thunderbolt port carries two displays, whether through a dock or a daisy chain. The third has to go somewhere else. Put it on its own port on the Mac and everything works, with the drive still running near full speed. Leave all three on the dock and the third screen never appears. The 2026 machines lifted that ceiling — three per port on the M5 Pro, four on the M5 Max and M5 Ultra — so the same desk behaves differently on newer silicon.
Comfortable. Until you add a fast drive as well.
A 10GbE port takes 10 Gbps each way when you push it hard. With a 4K display and some USB gear that still fits. But a quarter of the link is gone before you plug in any storage. Tell the tool the network is idle and you get it back.
Comfortable, and Boost is still costing you read speed. Turn it off.
Boost sounds like free speed. It moves lanes around to give you 120 Gbps downstream and drops the return path to 40. A 4K display never needed the extra downstream, and your SSD reads come back up the pipe you just halved. Turn it on only when a display genuinely needs more than 80 Gbps.
Because the display and your SSD writes go the same way down the same cable. Thunderbolt is two separate one way pipes, not one shared pool. DisplayPort only goes downstream, from the Mac to the screen. SSD writes go downstream too. So the two fight each other. A 5K 60Hz display takes about 22 Gbps of a 40 Gbps downstream pipe before your drive gets anything at all. Reads come back on the upstream pipe. The display never touches that pipe. So writes fall off a cliff while reads stay near full speed. This gives you a quick test. If reads are fine and writes are not, your display is the cause. The fix is usually free: move the drive to a spare port on the Mac.
Probably not, and this is the most useful test on the page. Bandwidth sharing is directional. A display eats downstream, so it hits writes and leaves reads alone. If reads and writes are both down, something else is going on. Check heat first. Most portable NVMe enclosures start throttling somewhere between 60 and 85 degrees, and once they do you can lose 30 to 50 percent of write speed until they cool off. A plastic enclosure runs 15 to 20 degrees hotter than an aluminium one under sustained load. After heat, check the obvious: a USB-C cable rated for charging only will negotiate down to USB 2.0 speeds and look identical, and a drive formatted exFAT instead of APFS is slower on macOS for reasons that have nothing to do with the dock. Run the drive straight off a Thunderbolt port on the Mac for a minute. If it is still slow there, the dock was never the cause.
Two things cap this and they are different. First, the chip. Every M1 and M2 base chip in a laptop or iMac drives exactly one external display, whatever you plug it into. Pro, Max and Ultra chips drive more. No dock, cable or macOS update changes that number. Second, the port. A Thunderbolt port carried at most two displays through a dock or daisy chain on every Mac up to the M4 generation; the 2026 machines raised it to three on the M5 Pro and four on the M5 Max and M5 Ultra. Either way it only works when the port, the cable and the dock are all genuine Thunderbolt. A USB4 or plain USB-C hub gets one, because macOS has never supported MST and a second screen on an MST hub just mirrors the first. Pick your Mac in the tool above and it applies both limits for you.
Yes, quite often. And this is measured, not theory. Howard Oakley at the Eclectic Light Company published tests in December 2024. He found that a TB4 hub caps all downstream throughput at around 3 GB/s, no matter what else is plugged in. He also found that some TB3 SSDs lose half their write speed to a TB4 hub with nothing else attached. A USB4 SSD that reaches 3.2 to 3.5 GB/s on a direct port fell to about 2.9 GB/s through a TB4 hub. Docks do not reach their own numbers. Plan for some loss even on an empty dock.
Yes, and it is the only way past the chip limit. It is also the one upgrade on this page with real costs attached. DisplayLink renders the picture on your CPU, compresses it, and sends it as ordinary USB data. That means 60Hz per stream whatever the panel is rated for, roughly 5 to 15 percent CPU overhead, and cursor lag you can feel. The one that catches people out is HDCP: with a DisplayLink display connected, Netflix, Apple TV+ and Disney+ are blocked on every screen, not just that one. Use a native Thunderbolt or HDMI output wherever you can, and keep DisplayLink for the screen you cannot get any other way.
No. A daisy chain still runs everything through one port on the Mac. Three devices in a chain share the same bandwidth as three devices in three sockets on one dock. Chaining tidies your cables. It does not add capacity. The only way to get more bandwidth is another port on the Mac, which is exactly what the desk map above is for.
Because Bandwidth Boost is a trade, not a gift. Thunderbolt 5 normally runs 80 Gbps in each direction. Boost mode moves the lanes around to give you 120 Gbps downstream. The return path drops to 40 Gbps. Display data goes downstream, so a very demanding monitor does gain from this. SSD reads come upstream. So say you have a fast drive and an ordinary display. Boost halves your read headroom. In return it hands you downstream room you were not using. Turn it on only when a display really needs more than 80 Gbps.
Multiply width by height by refresh rate by three times the bit depth. Then add about 5 percent for timing overhead. At 8-bit colour, 4K 60Hz needs about 12.5 Gbps. 5K 60Hz needs about 22 Gbps. 4K 120Hz needs about 25 Gbps. And 6K 60Hz needs about 31 Gbps. One DisplayPort 1.4 tunnel over Thunderbolt 3 or 4 carries 25.92 Gbps. That is why 5K 60Hz fits on one cable and 6K does not. A 6K display uses Display Stream Compression at about 3 to 1. You will not see it in normal use. It also means a 6K panel takes up far less of the link than its raw number suggests.
It depends on what is holding you back. If your drive sits at about 3 GB/s, that is the PCIe tunnel on Thunderbolt 3 and 4. It carries around 24 Gbps. Thunderbolt 5 raises that to about 63 Gbps, so the upgrade is real. If instead your writes only slow down when a monitor is plugged in, the problem is downstream sharing, and moving the drive to a spare port on the Mac fixes it for nothing. And if your Mac does not have Thunderbolt 5, a TB5 dock drops to 40 Gbps and buys you nothing. Check the Mac first.
Check which port first. Several Macs have USB-C ports that look the same but carry no PCIe at all. A Thunderbolt dock will not work on those. Mac Studio Max models do this on their front ports, including the 2026 M5 Max. So does every Mac mini from the M4 onwards, the M6 and M5 Pro included. So do the extra ports on four port M1 and M3 iMacs, and the MacBook Neo. If your Mac mixes Thunderbolt generations, use the fastest port. Pick your Mac above and the desk map marks the dead ports for you.
Only if it supplies enough. Compare the dock wattage against the adapter Apple ships with your Mac: 96W for most 14 inch MacBook Pros, 140W for the 16 inch, 30 to 70W for the Airs. A 60W dock will hold a 16 inch MacBook Pro steady at idle and lose ground while you are rendering. It is not dangerous, it is just slow. Pick your Mac and tell the tool what the dock supplies and it will grade it for you.
Tool
How many displays your chip can drive, and at what resolution on each port. The limit this tool applies, explained in full.
Open toolTool
Which port on your Mac will actually run a dock? Some USB-C ports carry no PCIe.
Open toolTool
Before the dock splits it, the cable has to carry it. Check any resolution against HDMI, DisplayPort and Thunderbolt.
Open tool