Uptime and bandwidth
🏠The everyday picture
A shop's opening hours and the width of its door. A shop that's often unexpectedly closed loses customers. A narrow door makes a queue on busy days.
📘What it really is
- Uptime is the share of time a service is working. Hosts promise it in an SLA (Service Level Agreement):
| Uptime | Allowed downtime per year | Per month |
|---|---|---|
| 99% | about 3.7 days | about 7 hours |
| 99.9% | about 8.8 hours | about 44 minutes |
| 99.99% | about 53 minutes | about 4 minutes |
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Bandwidth is how much data can flow at once (like the width of the door). Data transfer is the total data sent in a month. Big images and videos use far more of both than text.
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Your own app's uptime also depends on you: a broken update or an expired domain takes a site down however good the host is.
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Monitoring tools check your site every few minutes and alert you when it's down (Level 6).
🖱️Try it
Find your host's uptime promise (often on its plans or terms page). Convert it into hours of allowed downtime per year using the table.
✅Quick check
A host promises 99.9% uptime. Your site was down for 2 hours last month. Did they break the promise?
Show the answer
Probably yes: 99.9% allows about 44 minutes a month. Check their SLA for how they compensate.
Your homepage has a 25 MB video that plays automatically. What does it do to bandwidth?
Show the answer
Every visitor downloads 25 MB, using a lot of data transfer and slowing the page for people on weak connections. Compress it, or load it only when someone presses play.