What is bufferbloat?
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Bufferbloat is extra delay that builds up when your internet connection is busy. Your router or modem holds outgoing and incoming data in a queue (a buffer). When the buffer is too large and fills up, every packet, including the small, urgent ones from games and calls, has to wait its turn. Latency that was 20 ms when the line was quiet can jump to several hundred milliseconds the moment someone starts an upload.
Signs you have bufferbloat
- Video calls freeze or go robotic when someone else in the house starts a download, a game update or a cloud backup.
- Online games lag at the same time every evening, or whenever a phone is backing up photos.
- Web pages feel slow to start loading during big transfers, even though your speed test looks fast.
If the speed test shows good download and upload but these problems happen anyway, bufferbloat is the usual culprit.
How HowFast measures it
- It measures latency while the connection is idle.
- During the download and upload rounds, it sends small latency probes to a separate server every 400 ms. Only probes that overlap a transfer of at least 250 ms count, so each one really was measured on a busy line.
- The increase is the median loaded latency minus the median idle latency, worked out separately for download and upload.
- The worse direction sets the overall grade.
At least five valid samples are needed in each set. On very fast connections the transfers can finish too quickly to load the line, and the result reads “Insufficient data” rather than guessing. Full details are on the methodology page.
What the grades mean
| Grade | Latency increase under load | What it means |
|---|---|---|
| A+ | 5 ms or less | Excellent queue management |
| A | Over 5 to 30 ms | Very good |
| B | Over 30 to 60 ms | Noticeable in competitive gaming |
| C | Over 60 to 200 ms | Calls and games degrade when the line is busy |
| D | Over 200 to 400 ms | Severe lag under load |
| F | Over 400 ms | Unusable for real-time apps while transferring |
Gaming is rated Good with an increase of up to 60 ms in the worse direction, and video calls Good with up to 150 ms while uploading. The grade is an indicator, not a laboratory classification: the test loads the line in bursts rather than continuously, so a very large buffer might not fill completely.
How to fix bufferbloat
- Turn on Smart Queue Management (SQM). Many routers have it under names like “Smart Queue”, “Adaptive QoS”, “SQM”, “fq_codel” or “CAKE”. It keeps queues short so urgent packets go first. Set its limits to slightly below your measured speeds (around 90–95%) and run the test again.
- Upgrade the router. Many provider-supplied and older routers have no queue management. A router or mesh system with SQM support usually fixes bufferbloat completely.
- Limit the heaviest uploads. Cloud backup apps often let you cap their upload speed or schedule them overnight.
- Use a cable for latency-sensitive devices. It doesn’t fix bufferbloat on the internet link, but it removes Wi-Fi queuing on top of it.
- Ask your provider. If you can’t control your router, ask whether a modem or router with queue management (AQM) is available.
Upgrading to a faster plan sometimes helps because the queue empties more quickly, but it is a costly and unreliable fix compared with SQM.
Frequently asked questions
Why is my upload grade worse than my download grade?
Upload links are usually slower, so their buffers take longer to drain, and many routers don’t manage the upload queue. This is why video calls, which upload constantly, are often the first thing to suffer.
Does bufferbloat affect streaming?
Barely. Streaming apps buffer several seconds of video ahead, so delay doesn’t show. Bufferbloat hurts real-time things: calls, games and the responsiveness of browsing.
Why did I get “Insufficient data”?
The test couldn’t collect enough latency probes while the line was busy, usually because a very fast connection finished each transfer in under a quarter of a second. Detailed mode transfers more data and often fixes this.