What Is Buffer Bloat?

What causes buffer bloat?

Every network device — router, modem, cable modem — has transmit buffers: small queues that hold packets before sending them. The problem arises when a router has an oversized buffer and the connection becomes saturated.

Here's the sequence:

1. You start downloading a large file
2. Your router fills its transmit buffer with file download packets
3. A game packet arrives and joins the queue behind hundreds of file packets
4. The game packet waits until every file packet ahead of it is sent first
5. Your game sees 200–400 ms of ping instead of 15 ms — rubber-banding, lag spikes

The problem is worse on slower connections where the buffer takes longer to drain, and on ISP-provided routers which often have the largest buffers.

Why speed tests miss buffer bloat

A standard speed test fills your connection with bulk transfer traffic — exactly the condition that causes buffer bloat. But because the test only measures throughput, not latency, it reports your full download speed as a positive result. Your latency during that same test may have spiked to 400 ms — but the speed test doesn't show you that unless it specifically measures it.

Our Advanced Speed Test includes a buffer bloat measurement that tests latency under load alongside throughput.

How to test for buffer bloat

1. Run our Ping Test to get your idle baseline ping (e.g. 12 ms)
2. Start a large download (a game on Steam, a system update) to fully saturate your connection
3. Run the ping test again while the download is active
4. Compare the two results — if latency jumped from 12 ms to 150 ms or more during the download, you have significant buffer bloat

A well-configured connection should show less than 20 ms of additional latency under full load. Consumer routers with default settings often show 200–600 ms.

Buffer bloat grades

Latency increase under load	Grade	Experience
Under 20 ms	A	Gaming and calls unaffected by simultaneous downloads.
20–50 ms	B	Minor impact. Most users won't notice.
50–200 ms	C	Noticeable lag spikes during downloads. Games and calls disrupted.
Above 200 ms	D/F	Severe. Gaming unplayable, calls break up whenever anything else uses the connection.

How to fix buffer bloat

Fix 1: Enable QoS (Quick fix)

Quality of Service (QoS) prioritises latency-sensitive traffic (gaming, VoIP) over bulk transfers. Log into your router admin panel (192.168.1.1 or 192.168.0.1), find QoS settings, and enable gaming or VoIP priority. This doesn't eliminate buffer bloat but ensures your important traffic jumps the queue.

Fix 2: Enable SQM / FQ-CoDel (Best fix)

Smart Queue Management (SQM) with the FQ-CoDel algorithm is the most effective buffer bloat solution. It actively manages the queue depth to keep latency low even under full load. Routers running OpenWrt firmware support this natively. Some consumer routers (TP-Link, Asus with Merlin firmware, Netgear with Tomato) also support it. Enable it in the QoS or Traffic Management section of your router.

Fix 3: Cap your speed slightly below your maximum

If your router supports speed limiting (bandwidth control), set your download cap to ~95% of your actual maximum speed. This prevents the buffer from filling completely, leaving headroom for latency-sensitive packets. It's a partial fix but often reduces the worst spikes significantly.

Fix 4: Replace your router

ISP-provided routers are the worst offenders. They're built to a budget with large buffers and no QoS. A router running OpenWrt or one with dedicated QoS hardware (many Wi-Fi 6 routers) will dramatically reduce buffer bloat. Look for routers specifically marketed as low-latency or with "SQM" support.

Fix 5: Schedule large downloads for when you aren't gaming or calling

The fastest pragmatic fix — use your download scheduler (Steam, PlayStation, Xbox, Windows Update) to run updates overnight rather than while you're actively using the connection. This avoids the problem entirely without changing hardware.