How allowed downtime is calculated
An uptime SLA (service level agreement) promises that a service is up for a share of the time. The rest is the downtime it allows: allowed downtime = measured time × (1 − SLA). At 99.9% uptime, 0.1% of the time can be down. An average month of 30.44 days is 2,629,746 seconds, and 0.1% of that is 2,630 seconds, or 43m 50s. The same math applies to an internal SLO.
| SLA | Per day | Per month | Per year |
|---|---|---|---|
| 99% | 14m 24s | 7h 18m 17s | 3d 15h 39m 30s |
| 99.5% | 7m 12s | 3h 39m 8.7s | 1d 19h 49m 45s |
| 99.9% | 1m 26s | 43m 50s | 8h 45m 57s |
| 99.95% | 43s | 21m 55s | 4h 22m 58s |
| 99.99% | 8.6s | 4m 23s | 52m 36s |
| 99.999% | 0.86s | 26s | 5m 16s |
Each extra nine divides the downtime by 10, so going from 99.9% to 99.99% takes a month from about 44 minutes to about 4.
Why 43m 50s and not 43m 12s?
Calculators get different answers because they use different periods, not different math. This page uses the average Gregorian year of 365.2425 days, a month that is a twelfth of it (30.44 days) and a quarter that is a fourth of it (91.31 days), so 12 months add up to one year. The same SLA looks different elsewhere: the availability table in Google's SRE Book works out to a 30-day month and a 365-day year, so it shows 43.2 minutes a month and 8.76 hours a year for 99.9%. Twelve of those months make 360 days, so its month and year don't line up.
Neither is wrong, but your contract decides. Cloud SQL's SLA, for one, is worked out per calendar month. On that basis, a 99.9% target allows 40m 19s in a 28-day February and 44m 38s in a 31-day month. When the exact figure matters, set Month length above to the month you are checking.
Also check what counts as downtime. Cloud SQL's SLA ignores outages shorter than a minute, and Amazon S3's counts failed requests in 5-minute intervals, so the figures here are only a guide for it.
Downtime to uptime, compare and working hours
Downtime to uptime starts from an outage you already had. One downtime gives a different uptime for each period: 1h 20m of downtime leaves 94.4444% uptime over a day and 99.9848% over a year. Compare shows how much downtime you give up with a stricter SLA, such as moving from 99.9% to 99.95%, which halves the allowance to 21m 55s a month. Working hours is for an SLA measured only in agreed hours. A schedule of 8 hours, Monday to Friday, measures 40 hours a week instead of 168, so 99.9% allows 10m 26s a month. Downtime outside those hours doesn't count at all.
Pitfall: the allowance doesn't carry over
Allowed downtime is counted per period, so a quiet month doesn't save anything up for the next one. A single one-hour outage leaves 99.9886% uptime over a year, which looks fine, but 99.8631% over the average month. If your SLA is measured monthly, that one outage breaks a 99.9% target, even if the rest of the year is perfect. Check which period your SLA counts before you divide the year's figure by 12.
FAQ
How much downtime does 99.9% uptime allow?
43m 50s a month, using an average month of 30.44 days. That is also 8h 45m 57s a year, 10m 4.8s a week and 1m 26s a day. A 30-day month gives 43m 12s and a 31-day month gives 44m 38s.
How much downtime does 99.99% uptime allow?
4m 23s a month, using an average month of 30.44 days. That is also 52m 36s a year, 1m 0.5s a week and 8.6s a day, a tenth of what 99.9% allows.
Why do uptime calculators give different numbers for the same SLA?
The math is the same, and the length of the period is not. A month can be 30 days, 30.44 days or the calendar month, and a year can be 365 or 365.2425 days. That is why 99.9% appears as 43m 12s in some places and 43m 50s in others. This page uses a 30.44-day month and a 365.2425-day year, and you can change the month.
How do I convert downtime to an SLA percentage?
Divide the downtime by the length of the period, subtract that from 1, and multiply by 100. 1h 20m over an average month is 100 × (1 − 4,800 ÷ 2,629,746), which is 99.8175%. Over a year the same downtime is 99.9848%, so always say which period you mean.
What does three nines mean?
Three nines is 99.9%, four nines is 99.99% and five nines is 99.999%. 99.95% is often called three and a half nines, but that is only a name. It allows half the downtime of 99.9%, while a true halfway point would allow about a third. Counted exactly, 99.95% is about 3.3 nines.
Does an SLA count nights and weekends?
It depends on the contract. Many SLAs count every minute: Cloud SQL's, for one, uses the total minutes in the month. Some business systems are measured only in agreed service hours. Downtime outside those hours doesn't count, and the allowance is smaller: 99.9% of 8 hours a day, Monday to Friday, is 2m 24s a week instead of 10m 4.8s. The Working hours tab works that out.