Availability SLA calculator, yearly and monthly
How much downtime fits in 99.9%? What was the availability of a trunk that was down for an hour this month? The calculator answers both ways, with real years and months, and takes planned maintenance out.
What is the availability SLA formula?
Availability = (time in the period − planned maintenance − downtime) ÷ (time in the period − planned maintenance) × 100. The downtime a target allows is the time in the period, with maintenance already taken out, multiplied by (100% − target).
A year has 365 days, or 366 in a leap year, and a month has 28 to 31 days. That is why 99.9% is not always the same amount of time: in a 31-day month it allows 44 minutes and 38 seconds of downtime, and in February, 40 minutes and 19 seconds. The table below uses the same formula as the calculator.
| Period | 99% | 99.5% | 99.9% | 99.95% | 99.99% |
|---|---|---|---|---|---|
| 365-day year | 3 d 15 h 36 min | 1 d 19 h 48 min | 8 h 45 min 36 s | 4 h 22 min 48 s | 52 min 34 s |
| Leap year (366 days) | 3 d 15 h 50 min | 1 d 19 h 55 min | 8 h 47 min 2 s | 4 h 23 min 31 s | 52 min 42 s |
| 31-day month | 7 h 26 min 24 s | 3 h 43 min 12 s | 44 min 38 s | 22 min 19 s | 4 min 28 s |
| 30-day month | 7 h 12 min | 3 h 36 min | 43 min 12 s | 21 min 36 s | 4 min 19 s |
| Leap-year February (29 days) | 6 h 57 min 36 s | 3 h 28 min 48 s | 41 min 46 s | 20 min 53 s | 4 min 11 s |
| February (28 days) | 6 h 43 min 12 s | 3 h 21 min 36 s | 40 min 19 s | 20 min 10 s | 4 min 2 s |
| Week | 1 h 40 min 48 s | 50 min 24 s | 10 min 5 s | 5 min 2 s | 1 min |
| Day | 14 min 24 s | 7 min 12 s | 1 min 26 s | 43 s | 9 s |
How do you use the result?
Compare the downtime allowed with the downtime you measured. If the contract's target is yearly, add up the outages of the whole year: one bad month can still fit inside the yearly target. The guide Telephony SLA: yearly and monthly calculation explains what counts as downtime, how to treat maintenance and why to measure with real calls.
Monitor's SLA reports do this sum for every voice circuit, every month, with the 12-month figure and the month for reference.
Leave the SLA sums to whoever measures
Monitor calls every circuit all day and delivers the year's and the month's availability in a report in English or Portuguese.