Packet Loss Explained: Causes, Effects and How to Test
Packet loss happens when data packets sent across a network never arrive. TCP hides it by retransmitting, which shows up as extra latency and lower speed; UDP applications such as calls and games cannot wait, so loss becomes glitches, freezes and rubber-banding. Even 1% loss is noticeable in real-time use, and sustained loss above 2 to 3% is a real problem to fix.
On this page
- What is a packet?
- How loss is measured
- Where packets get lost
- What packet loss does to TCP traffic
- What packet loss does to UDP and real-time traffic
- How much is too much?
- Loss, latency and jitter together
- How to detect packet loss
- How to fix packet loss
- Worked example
- Common mistakes and myths
- What to do next
Quick answer: packet loss is the share of data packets that never reach their destination. It is measured as a percentage. A healthy wired connection has 0%. Loss is bad for two reasons: for downloads and web pages, TCP must resend the missing packets, which adds delay and cuts speed; for calls, games and live video, there is no time to resend, so you hear or see gaps. Persistent loss above 1 to 2% deserves attention.
What is a packet?
Everything on the internet is cut into small units called packets, typically up to around 1,500 bytes on Ethernet. Each packet carries source and destination addresses and travels independently across routers. See how the internet works. Packets can arrive late, out of order, duplicated or not at all. The last case is packet loss.
How loss is measured
loss % = lost packets ÷ sent packets × 100. Send 100 packets, 3 do not come back, loss is 3%. Tools such as ping, mtr and game network overlays report it. Loss may be one-way: the request was received but the reply was lost, and from the sender's point of view that looks the same.
Where packets get lost
| Place | Typical cause | Pattern |
|---|---|---|
| Your Wi-Fi link | Interference, weak signal, too many devices | Random, worse far from the router |
| Your cable or port | Damaged or poorly crimped cable, bad connector | Constant or intermittent |
| Your router | Overloaded CPU, buffer overflow, faulty hardware | Worse under load |
| ISP access network | Congestion at peak hours, noisy line (DSL, cable), faulty node | Time-of-day pattern |
| Internet routers | Congested links, failing equipment, route changes | Affects some destinations only |
| Destination server | Overload, firewall rate limits, DDoS protection | Affects one host |
Most routers drop packets deliberately when their queue is full; this is how the internet signals congestion to senders. Loss is therefore a normal part of how TCP finds the available capacity, but excessive loss on a quiet network is a fault.
What packet loss does to TCP traffic
TCP guarantees delivery. The receiver acknowledges data; if the sender sees duplicate acknowledgements or no acknowledgement within a timer, it resends and also slows down, because it assumes congestion. Effects:
- Added delay: a lost packet is typically recovered after roughly one round trip (fast retransmit) or after a retransmission timeout, which can be hundreds of milliseconds.
- Lower throughput: TCP reduces its sending rate after loss. Even 1% loss can reduce speed on long-distance connections dramatically, because high round-trip time makes recovery slower.
- Latency spikes: the application sees a stall while the missing piece is recovered, which looks like jitter. See TCP and UDP explained.
What packet loss does to UDP and real-time traffic
Calls, games and live streams usually use UDP or protocols built on it, including QUIC, WebRTC and many game protocols. Missing packets are either concealed or dropped:
- Voice: clipped words, robotic sound, gaps. Codecs conceal around 1 to 5% random loss, but bursts of loss are much worse.
- Video calls: frozen or pixelated frames, lowered quality.
- Games: rubber-banding, players teleporting, shots that do not register. The game state is correct on the server but your client missed updates.
- Live streaming: buffering or quality drops.
See latency for gaming and video calls and VoIP.
How much is too much?
| Loss | Browsing / downloads | Calls | Gaming |
|---|---|---|---|
| 0% | Perfect | Perfect | Perfect |
| Under 0.5% | Fine | Usually fine | Occasional hiccup |
| 1% | Slightly slower on long routes | Occasional glitches | Noticeable in fast games |
| 2 to 3% | Visibly slower | Frequent glitches | Frustrating |
| 5% or more | Very slow, stalls | Barely usable | Unplayable |
These are rules of thumb, not standards; protocols and applications differ.
Loss, latency and jitter together
These three problems often appear together because they share causes (congestion, poor wireless, bad hardware). A saturated link first raises latency (queues grow), then jitter (queues vary), then loss (queues overflow). That order is why a rising ping graph often precedes dropped packets. See jitter explained and bufferbloat explained.
How to detect packet loss
- Ping a stable address for a few minutes and look at the loss percentage (for example
ping -c 100 8.8.8.8on Linux/macOS orping -n 100 8.8.8.8on Windows). - Run mtr or pathping to see at which hop loss begins. Loss that appears at one hop but not later is often just that router rate-limiting replies and is harmless; loss that starts at a hop and continues to the end is real.
- Compare wired and wireless. If loss disappears with a cable, the cause is your Wi-Fi.
- Watch the no-reply rate in Monitor for several destinations: if many show intermittent no-replies at the same time, the problem is local. See the Monitor guide.
- Check at different times to detect peak-hour congestion.
How to fix packet loss
- Replace or reseat Ethernet cables; try another port.
- Move closer to the router, use 5 GHz or 6 GHz, change channels, or use a mesh system or powerline/MoCA adapters.
- Reduce load: pause large uploads and downloads. Enable smart queue management (SQM, fq_codel or CAKE) on your router if available.
- Restart the router and modem; update firmware.
- Test with the modem directly connected to a computer to isolate the router.
- If loss remains, give your ISP a pattern with times, a cable test and test reports. See talking to your ISP.
Worked example
You play online and see rubber-banding in the evenings. A 200-packet ping to your game server shows 4% loss at 21:00 but 0% at 10:00. With a cable plugged in, evening loss drops to 0.5%. Conclusion: most of the loss is on Wi-Fi; moving the console to a cable fixes most of it. The remaining 0.5% may be ISP evening congestion; document it.
Common mistakes and myths
- "Loss at one traceroute hop proves a fault." Routers often deprioritise replies to probes; only loss that persists to the destination counts.
- "A fast speed test means no packet loss." Short tests may not expose it, and speed tests do not report it.
- "Latency tools show loss." Only some do. A reply/no-reply count is not the same as packet-level loss.
- "Loss is always my ISP's fault." Home Wi-Fi is the most common source.
What to do next
Read troubleshoot high latency for a structured process, how to reduce latency for fixes, or open Monitor and watch your no-reply rate over time.
Frequently asked questions
What is packet loss?
Packet loss is when one or more data packets travelling across a network fail to reach their destination. Data is sent as many small packets; if some are dropped by a congested router, a bad cable or a noisy wireless link, they must be resent or are simply missing.
How much packet loss is acceptable?
Ideally 0%. Under about 0.1% is usually unnoticeable. Around 1% can cause glitches in calls and games. Sustained loss of 2 to 3% or more is a problem for real-time use and noticeably slows downloads. The tolerance depends on the application.
Does packet loss increase latency?
For TCP traffic, yes. A lost packet is detected only after a timeout or duplicate acknowledgements, then resent, which adds tens to hundreds of milliseconds to that data. UDP does not resend, so latency stays flat but data is missing.
Can GlobeLatency measure packet loss?
Not directly. It records whether each request got a reply. Repeated no-replies for a destination in Monitor suggest intermittent loss or blocking, but it is a rough indicator and not a packet-level loss percentage. Tools like ping and mtr measure packet loss more directly.
What causes packet loss on Wi-Fi?
Interference, distance, walls and too many devices on the same channel cause corrupted frames that must be retried. Some are recovered by the Wi-Fi layer (adding latency and jitter) and some are lost completely.