Service Availability Calculator
Calculate service uptime percentage and allowed downtime budget for an SLA target.
About Service Availability Calculator
Availability is the fraction of a time period a service was up and functioning, expressed as a percentage. SLAs conventionally describe availability in terms of 'nines' — 99% ('two nines') permits about 3.65 days of downtime per year, while 99.999% ('five nines') permits only about 5.26 minutes per year — because each additional nine reduces allowed downtime by a factor of 10.
Formula
Availability percentage
availability = (total_time − downtime) / total_time × 100
- T
- total time in the period
- D
- total downtime in the period
Worked example
With the default values below, Service Availability Calculator returns Availability of 99.991438%. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Total Time Periodminimum 0.01 hours
- 8,760 hours
- Total Downtimeminimum 0 minutes
- 45 minutes
Results
- Availability
- 99.991438%
- Nines Rating
- 99.99% (four nines)
- Uptime
- 525,555.00 minutes
- Allowed Downtime for 99.9% (over this period)
- 525.60 minutes
- Allowed Downtime for 99.99% (over this period)
- 52.56 minutes
Step by step
- 1Values used
- 2Availability percentage
- 3Availability= 99.991438
- 4Nines Rating= 99.99% (four nines)
- 5Uptime= 525,555.00 minutes
- 6Allowed Downtime for 99.9% (over this period)= 525.60 minutes
- 7Allowed Downtime for 99.99% (over this period)= 52.56 minutes
Assumptions and limits
- The result is derived only from the 2 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax or fees or charges into account.
- Figures are rounded for display to 6 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How much downtime does 99.9% availability allow per year?
99.9% ('three nines') allows about 8.76 hours of downtime per year (0.1% of 8,760 hours) — roughly 43.8 minutes per month.
How much downtime does 99.99% availability allow per year?
99.99% ('four nines') allows about 52.6 minutes of downtime per year — an order of magnitude tighter than three nines, which is why each additional nine is significantly more expensive to engineer for.
Does availability account for planned maintenance?
That depends on the SLA's definition — some exclude scheduled maintenance windows from the downtime count, while stricter SLAs count all unavailability, planned or not. Be explicit about which convention you're using when reporting a number.