What Is Bufferbloat? How to Test and Fix It

Bufferbloat is the extra delay that builds up when your router or modem holds too much data in a queue while the line is busy. Your speed can look fine and your idle ping can be low, yet a video call freezes the moment someone in the house starts a big download. The quickest way to see it is a speed test that measures latency under load, which times round trips while the line is full and grades the rise.

What causes bufferbloat?

Bufferbloat happens when a device on the path has a large buffer and no rule for keeping it short. When more data arrives than the connection can send, the extra packets wait in that buffer. A bigger buffer avoids dropped packets, but every packet behind the queue now waits too, including the small ones from a game or a call that only needed a few milliseconds.

The queue almost always forms at the slowest point between you and the internet. On most home connections that is the upload side of the modem or router, because upload is usually much slower than download. That is why a cloud backup or a video upload often hurts calls more than a download does.

How do you test for bufferbloat?

Measure your latency with nothing running, then measure it again while the connection is saturated, and compare the two. Our speed test does both automatically: it times 20 round trips on an idle line, then keeps timing round trips every quarter of a second while it downloads and uploads at full speed.

1. Use a wired connection if you can, so Wi-Fi does not add its own delay to the result.
2. Pause other downloads, streams and backups on the network.
3. Run the test and look at three numbers: idle latency, latency while downloading and latency while uploading.
4. Read the bufferbloat grade. It is based on the larger of the two rises.

A plain ping test cannot show bufferbloat on its own, because it measures an idle line. The problem only appears when something fills the connection.

What does the bufferbloat grade mean?

The grade is the increase in latency under load, on the scale used by the Waveform bufferbloat test, which itself is a version of the old DSLReports rubric. It grades the rise only, not your speed and not your idle ping.

A result can differ between download and upload. On a typical cable or DSL line the upload grade is worse, and on a well-tuned fiber line both stay at A or A+.

How do you fix bufferbloat?

The fix is a router that manages its queue, a feature usually called Smart Queue Management (SQM). Instead of letting the buffer fill, SQM keeps queues short and gives each flow of traffic its own turn, so a call is not stuck behind a download. The two common algorithms are FQ-CoDel, described in RFC 8290, and CAKE, which is built into Linux and OpenWrt.

1. Look for SQM in your router. Depending on the brand it may be called SQM, Smart Queue, Adaptive QoS or bandwidth control. Turn it on.
2. Set the speeds slightly below your real line speed. SQM only works when the router, not the modem, is the bottleneck. Run a speed test, then enter about 90% of the measured download and upload. Test again and adjust.
3. Retest. A working setup usually moves the grade to A or A+, at the cost of a few percent of peak speed.
4. If your router has no SQM, consider one that supports it, or a router that can run OpenWrt. Some providers also manage queues on their side, so it is worth asking yours.

Older QoS features that only prioritize by device or by app help less, because the queue still fills. Buying a faster plan reduces how often you hit the limit, but the delay comes back the moment anything fills the new, bigger line.

Bufferbloat or something else?

High latency that stays high on an idle line is not bufferbloat. It usually means distance to the server, a weak Wi-Fi signal or a congested route. Our guides on what ping is and how to lower ping cover those cases. If the latency only jumps when the line is full, it is bufferbloat, and SQM is the fix.

Frequently asked questions

Is bufferbloat bad for gaming?

Yes, but only while the line is busy. With a C grade or worse, a game feels fine until someone starts a download or a console update, then your ping jumps by tens or hundreds of milliseconds. Fixing it with SQM keeps gaming ping close to idle even during transfers.

Does a faster internet plan fix bufferbloat?

Not by itself. A faster plan fills up less often, but when it does fill, the same oversized buffer causes the same delay. The fix is queue management on the router.

Can Wi-Fi cause bufferbloat?

Yes. If your Wi-Fi is slower than your internet line, the queue forms in the Wi-Fi access point instead of the modem. Testing once on a cable and once on Wi-Fi shows which one is responsible.

Sources and further reading

• RFC 8290: The Flow Queue CoDel Packet Scheduler and Active Queue Management Algorithm (IETF)
• Waveform: Bufferbloat test and grading scale