Uptime percentage is the most commonly used metric for describing service reliability. This article explains how PulseAPI calculates it and how to interpret the numbers.
How Uptime Percentage Is Calculated
Uptime percentage = (successful checks ÷ total checks) × 100, over a specified time period.
A check is successful if it:
- Receives a response from the endpoint
- The response's HTTP status code matches the Expected Status Code configured on the monitor
- The response is received within the configured Timeout
Any check that times out, returns the wrong status code, or encounters a connection error is counted as a failure.
Example:
- 1,440 checks in 24 hours (one every 60 seconds)
- 1,432 successful, 8 failed
- Uptime = (1,432 ÷ 1,440) × 100 = 99.44%
Time Periods
The uptime percentage displayed on monitor detail pages is calculated for different time periods:
- 24 hours — reflects recent performance
- 7 days — week-over-week view
- 30 days — monthly reliability metric
You can switch between time periods using the selector on the monitor detail page.
What Uptime Numbers Mean in Practice
| Uptime % |
Max Downtime per Year |
Notes |
| 99% |
~3.65 days |
Often called "two nines" |
| 99.5% |
~1.83 days |
|
| 99.9% |
~8.77 hours |
Common SLA target |
| 99.95% |
~4.38 hours |
|
| 99.99% |
~52.6 minutes |
"Four nines" — common for critical services |
| 99.999% |
~5.26 minutes |
"Five nines" — requires very high availability infrastructure |
Impact of Check Interval
Faster check intervals give you more data points, which means:
- More accurate uptime calculation (fewer "gaps" in data)
- Faster detection of downtime (and therefore shorter outages affect the percentage less)
With a 60-second interval, a 2-minute outage represents 2 failed checks out of however many total. With a 5-second interval, the same outage represents 24 failed checks — a more accurate reflection of the actual downtime.
Uptime and Alert Rules
You can create an alert rule that fires when uptime drops below a threshold over a time window (e.g., "alert if uptime falls below 99.9% over the last 24 hours"). This is useful for SLA-based alerting rather than per-check alerting.
See Alert Rule Conditions: Uptime Percentage.
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