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Uptime vs Availability: What's the Difference?
Uptime and availability are related but different metrics. Uptime measures whether your service is running. Availability measures whether it's actually usable by customers.
Quick Definitions
Uptime
The percentage of time your service is running and responding to requests. A binary metric: it's either up or down.
Formula: (Total time - Downtime) / Total time × 100
Availability
The percentage of time your service is both running AND performing acceptably. Considers performance and errors, not just "is it up?"
Formula: Successful requests / Total requests × 100
The Key Difference
A service can have 100% uptime but poor availability. How?
- Server is running but responding slowly — 10-second response times are technically "up"
- Returning errors — 500 errors mean the server is responding, but users can't use it
- Partial functionality — Login works but checkout is broken
Real-World Example
Your API served 1 million requests today:
- • Server was running 24/7 → 100% uptime
- • 50,000 requests returned 500 errors
- • 100,000 requests timed out
- • 850,000 requests succeeded
Availability: 850,000 / 1,000,000 = 85%
The server never went down, but 15% of users had a failed experience.
The Nines
Both metrics are often expressed in "nines":
Which Should You Track?
Both. They measure different things:
- Track uptime to know when your infrastructure fails completely
- Track availability to know how users actually experience your service
For SLAs with customers, availability is usually the more meaningful metric. Customers don't care if your server is "technically running" if they can't complete their tasks.
How UptimeSignal Helps
UptimeSignal monitors uptime by checking if your endpoints respond with the expected status codes. We track response times too, so you can set thresholds for what counts as "too slow." This gives you visibility into both uptime and availability.