What Is Bufferbloat? How to Test and Fix It
Quick answer
Bufferbloat is extra delay caused by your router or modem queuing too much data when the connection is busy. Calls stutter and games lag while someone downloads. Test it with a bufferbloat test; fix it with Smart Queue Management (SQM) or adaptive QoS on your router.
What's happening
When more data arrives than your line can send, the router stores it in a buffer. Big buffers keep throughput high, but every new packet — your game input, your voice on a call — has to wait behind the queue. The result: ping that's 15 ms when the line is quiet jumps to 300 ms during a download.
Picture a supermarket with one till. With a short queue, someone buying a single item is through in seconds. If the queue is twenty full trolleys long, that single item waits just as long as everyone else. Your video call is the person with one item; the cloud backup is the trolleys. Bufferbloat is the queue being allowed to grow far too long.
What it feels like
- Games are smooth until someone starts a download, then lag spikes and rubber-banding begin.
- Video calls go robotic or freeze when a phone starts backing up photos.
- Web pages take a long time to start loading while a big upload runs.
- The speed test shows a good download number, yet everything feels sluggish when the house is busy.
The giveaway is timing: the problem comes and goes with other activity in the home, not with the time of day.
How to test it
Run the bufferbloat test. It measures ping while the line is idle, then again during the download and upload, and grades the extra delay:
| Grade | Extra delay under load |
|---|---|
| A | Under 30 ms |
| B | 30–60 ms |
| C | 60–200 ms |
| D | 200–400 ms |
| F | 400 ms or more |
Grades A and B are good for gaming and calls. C is noticeable in fast games. D and F mean real-time apps will struggle whenever the connection is busy. Look at the "loaded ↓" and "loaded ↑" numbers separately — they often tell different stories.
How to fix it
- Turn on SQM (sometimes called "Smart Queue", "adaptive QoS" or "Fair Queue") in your router settings. These features keep queues short and share the line fairly between devices and apps.
- Set it slightly below your real speed — around 90–95 % of your measured download and upload — so the router, not the modem, controls the queue.
- Re-test. A grade jump from D or F to A or B is common.
- No SQM option? A newer router with SQM often helps more than a faster plan. Many mesh systems and gaming routers include it.
Upload is usually worse
Most bufferbloat shows up during uploads, because upload speeds are lower and queues fill faster. Pausing cloud backups during calls helps immediately. See why upload is slower than download.
Quick workarounds without changing settings
- Schedule backups, photo sync and game updates for overnight.
- Limit upload speed in cloud apps — many have a bandwidth setting.
- Use a cable for gaming and calls, so at least Wi-Fi isn't adding its own delay. See Wi-Fi vs Ethernet.
- Ask others in the house to pause big downloads during important calls.
Why routers have such big buffers
Buffers exist for a good reason: they absorb short bursts so data isn't thrown away. Memory became cheap, and many routers and modems were built with very large buffers to maximise throughput in benchmarks. Nobody measured how long packets waited inside. Modern queue management fixes this by keeping the queue short and sending small, time-sensitive packets first — without losing much throughput at all.
Why normal speed tests miss it
Most tests report ping on an idle line. Bufferbloat only appears under load, so you need a test that measures latency during the transfer — which ours does for both directions. Related reading: what is ping, what is jitter and how to lower ping.
Bufferbloat on mobile and 5G
Mobile networks and 5G home routers can show bufferbloat too, and you often can't change their settings. Using a router with SQM behind the 5G modem helps when the modem allows it.