Accuracy of "Data Sent/sec" in Activity Monitor on MacOS Catalina I have a 2018 MacBook Pro i7 running MacOS Catalina 10.15.3. I'm uploading a 4.88 GB video to YouTube, using the YouTube Studio web interface in Google Chrome 79. My MacBook Pro is connected to my 5 Ghz SSID on my TP-Link EAP245 (Generation 1) access point, which is connected via gigabit ethernet to my Ubiquiti EdgeRouter X, which connects to my Netgear CM1000 cable modem. I don't have any VLANs or special routing rules configured.

According to the Network tab of Activity Monitor, I'm uploading data at roughly 10 megabytes per second (MB/sec). However, my Internet service is only supposed to support 35 megabits per second (Mb/sec), which comes out to about 3.5 megabytes per second.



I'm reasonably confident that there is no other major data transfer occurring in parallel with my YouTube video upload.

I can use the Python-based glances utility to observe that only a single network interface (my en0 Wi-Fi adapter) is pushing out approximately ~80 Mb/sec, which roughly matches the 10 MB/sec. that I'm seeing in Activity Monitor. When I'm not uploading a video, the idle utilization of the en0 network interface is only a few KB/sec.

My MacBook's en0 network interface is using the default MTU of 1500, according to ifconfig. All my other network devices use the default MTU.



In addition, according to my router's traffic monitor, I'm actually pushing out 44 Mbps (35 Mbps slightly over-provisioned).



Question: How can I explain the discrepancy between a 3.5 MB/sec. Internet service, versus the ~10 MB/sec being indicated by Activity Monitor in MacOS?
The main reason is that Activity Monitor shows the sum of data sent/received on all network interfaces, whereas your router only shows the data actually routed through it.

This means that there is traffic that your router does not see, and thus does not count into its statistics. This could for example local area network traffic directly between your MacBook Pro and other devices on the local area network reachable without routing. Another very common example is the loopback network interface, found on all Macs, which offers communication internally within the same computer (i.e. between different processes/programs running on your Mac).

This means basically that you cannot equate a number from the sum in Activity Monitor and any sort of "my internet connection is used at this speed".

If you want to figure out what exactly is the cause of the discrepancy - i.e. even if you say that you're "reasonably confident" that there's no other data transfers, there actually must be other data transfers - you can identify them by measuring or sniffing the traffic on all your network interfaces seperately. An easy to use tool for this could be "Wireshark", which will allow you to sniff, dump and do statistics on data transfers on all network interfaces, including WiFi, cabled ethernet, VPN virtual interfaces and loopback interfaces.

The only explanations that can fit your observations, if you're correct that there are no other data transfers, are that either (1) Activity Monitor is flawed and does not display the correct values, or (2) your data transfer comes with an extreme amount of overhead. Both explanations seem very unlikely.