Upload speed
Why is my upload speed so slow?
Short answer: on cable internet, low upload is usually by design: the network reserves only a thin slice of its capacity for sending. If your upload is far below what your plan promises, the next most common culprit is a modem that isn't approved for your plan's upload tier, followed by Wi-Fi and background uploads.
1. Check what your plan actually promises
Look for the upload figure on your bill or plan page, not the headline number. Cable plans are often sold as 300, 500 or 1,000 Mbps download with 10 to 100 Mbps upload. Fiber plans are frequently symmetrical. If your plan says 20 Mbps up and you measure 18 Mbps, nothing is broken.
2. Why cable upload is so much smaller
Cable networks were built for TV, which needed almost no return traffic. In a well-known answer on Super User, a network engineer explains that North American cable systems traditionally used 5–42 MHz for upstream and 54 MHz–1 GHz for downstream, so the upstream band was "only about 4% the size" of the downstream one (Super User, 2020). Moving that split means replacing amplifiers and filters across the neighborhood, which is why upgraded upload tiers arrive area by area.
Fiber and fixed 5G don't share that history, which is why the same household can see 900/900 on fiber and 900/35 on cable.
3. The modem trap
When a cable provider upgrades an area for faster upload, older customer-owned modems may keep working but stay on the old, slower upload tier. A real example from the Xfinity community forum, May 2024: a customer on the 1 Gig plan, advertised at 1,000 down and 100 up, measured "about 930 download and 23 upload" with the PC plugged directly into a Motorola MB8600. The forum expert's reply: "The MB8600 is not approved for Gigabit service or for the new faster upload speeds" and pointed to the provider's list of compatible devices (Xfinity forum thread).
What to do: search your provider's support site for its compatible-devices list and check that your exact modem model is approved for your plan's upload speed, not just "approved for service".
4. Rule out your own network in five minutes
| Test | If upload jumps | If upload stays low |
|---|---|---|
| Pause cloud backup, photo sync and game uploads, then retest | Something in the house was using the upstream | Move on |
| Test on a cable plugged into the router | Wi-Fi is limiting you (often a weak 2.4 GHz link) | Move on |
| Test on a cable plugged straight into the modem | The router is the bottleneck (old hardware or a QoS setting) | The limit is the modem, the line or the plan |
Also check the latency numbers in our result. If latency under load shoots up only when you upload, your router is queueing too much outgoing data. Turning on a "smart queue" or SQM feature, and setting its upload limit slightly below your measured upload, usually fixes call stutter even though the speed number barely changes.
5. How much upload do you actually need?
| Activity | Upload | Source |
|---|---|---|
| Zoom 1080p group video call | 3.8 Mbps | Zoom |
| Microsoft Teams video meeting, recommended | 2.5 Mbps | Microsoft |
| Google Meet 1080p video | up to 3.6 Mbps |
Add those up for everyone who might be on a call at once, then leave room for backups. Use the download time calculator with your upload speed to see how long a large backup or video upload will take, since the maths is identical in reverse.
6. When to call your provider
Call once you have run the modem-direct test at least three times at different hours and the result is well below your plan's upload figure. Give them the times, the numbers and your modem model. If the modem isn't on their approved list for your tier, they will usually tell you that first.
Related
Video call requirements compared · Why results vary between tests · How we measure upload