Packet loss test

A connection can lose packets and still report a perfect download speed.

Packet loss is the share of data that never arrives and has to be sent again. Small amounts are normal on any network, but the effect is wildly disproportionate: 1-2% loss is enough to stall video, drop words out of calls and make games rubber-band, while a speed test that retransmits quietly in the background still reports a healthy number.

This test counts probes that never came back, as a percentage of probes actually sent. It is deliberately measured on small requests rather than during a bulk transfer, so what you see is the network dropping data — not your own download saturating the link and causing drops as a side effect.

Why a little loss causes a lot of damage

TCP, which carries most web traffic, treats loss as a congestion signal: it retransmits the missing data and then deliberately slows down. So loss costs you the round trip needed to notice and resend, plus a throughput reduction afterwards. Real-time traffic has it worse. Voice and video usually run over UDP with no retransmission at all, because a packet that arrives late is useless — so a lost packet is simply a gap you hear.

Common causes

On WiFi: weak signal, interference, and a band so congested that transmissions collide. On the wire: a failing cable, a dying port, or an overloaded link somewhere upstream. Inside your home: a router whose queues are full, which drops what it cannot hold. Persistent loss on a wired connection to the router almost always points at hardware or at the provider, and is worth reporting with evidence from repeated tests.

Reading the result

0% is what a healthy connection reports and what you should expect most of the time. Anything sustained above 1% will be noticeable in calls and games. Above 5%, ordinary browsing starts to feel broken. Loss that appears only while something else is downloading is a queue problem rather than a line fault — the bufferbloat test measures that case directly.

What a browser can and cannot see

Be aware of what this figure is. A web page cannot watch individual packets: TCP quietly resends anything that goes missing, so by the time data reaches the browser the loss has already been repaired and hidden. What this test measures is the share of its latency probes that never came back at all within a deadline. That is a genuine signal — a connection dropping whole requests is in worse shape than one merely resending packets — but it is a proxy, and it will read 0% on a line that is quietly retransmitting. A dedicated tool running outside the browser, or your router's own statistics, can see the real per-packet figure.

Frequently asked questions

Related tests

• Ping testLatency decides how responsive your connection feels. Speed does not.
• Jitter testJitter is why a call breaks up on a connection whose speed test looks perfect.
• Bufferbloat testThis is the measurement that explains why your call falls apart the moment someone else starts a download.

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