SLA uptime calculator
Free · Runs in your browser · Updated 26 September 2026
SLA uptime calculator
| Period | Allowed downtime |
|---|---|
| Day | 1 min 26 s |
| Week | 10 min 5 s |
| Month (average) | 43 min 48 s |
| Month (30 days) | 43 min 12 s |
| Quarter | 2 h 11 min 24 s |
| Year (365 days) | 8 h 45 min 36 s |
99.9% allows 8 h 45 min 36 s of downtime a year.
Reference
The nines, in minutes.
The downtime each common uptime level allows. A year is 365 days and the month here is 30 days, the usual choice in SLAs.
| Uptime | Per day | Per week | Per month (30 days) | Per year |
|---|---|---|---|---|
| 90% | 2 h 24 min | 16 h 48 min | 3 d | 36 d 12 h |
| 95% | 1 h 12 min | 8 h 24 min | 1 d 12 h | 18 d 6 h |
| 99% | 14 min 24 s | 1 h 40 min 48 s | 7 h 12 min | 3 d 15 h 36 min |
| 99.5% | 7 min 12 s | 50 min 24 s | 3 h 36 min | 1 d 19 h 48 min |
| 99.9% | 1 min 26 s | 10 min 5 s | 43 min 12 s | 8 h 45 min 36 s |
| 99.95% | 43.2 s | 5 min 2 s | 21 min 36 s | 4 h 22 min 48 s |
| 99.99% | 8.6 s | 1 min | 4 min 19 s | 52 min 34 s |
| 99.999% | 864 ms | 6 s | 25.9 s | 5 min 15 s |
| 99.9999% | 86.4 ms | 604.8 ms | 2.6 s | 31.5 s |
How it is calculated
Downtime is the leftover fraction of the period.
Allowed downtime
For 99.9% over a 365-day year: (1 − 0.999) × 31,536,000 seconds = 31,536 seconds, which is 8 hours 45 minutes 36 seconds. Working backwards from an outage, uptime = (1 − downtime ÷ period) × 100.
- Day is 86,400 seconds and week is 604,800 seconds.
- The average month is a year divided by 12, which is 30.42 days. The 30-day month is 30 days. The quarter is a year divided by 4.
- A leap year has one more day; the calculator uses 365 days throughout.
Chains of services
Dependencies lower the total; redundancy raises it.
All must work (series)
Three services at 99.99%, 99.95% and 99.9% together give 0.9999 × 0.9995 × 0.999 = 99.840065%. The chain is worse than its weakest link, because each service can fail on its own.
Any one is enough (parallel)
Two independent copies at 99% give 1 − 0.01 × 0.01 = 99.99%. The word that matters is independent: two copies in one data centre, on one power feed or behind one load balancer do not fail independently, so the real figure is lower.
FAQ
Questions people ask first.
- How many minutes of downtime does 99.9% uptime allow?
- 43 minutes 48 seconds in an average month (a year divided by 12), 43 minutes 12 seconds in a 30-day month, and 8 hours 45 minutes 36 seconds in a 365-day year.
- Why does my provider quote a different number for the same percentage?
- They are measuring over a different period. A 30-day month, a calendar month of 28 to 31 days and an average month of 30.42 days each give a slightly different allowance. The calculator shows both the average month and the 30-day month so you can match your contract.
- What is five nines?
- 99.999% uptime, which allows about 5 minutes 15 seconds of downtime in a 365-day year, or about 0.9 seconds a day.
- How do I work out the uptime of several services together?
- If every service must work for a request to succeed, multiply their uptimes: 99.9% times 99.9% is 99.8001%. If any one of several redundant copies is enough, multiply their downtimes instead: two 99% services give 1 − 0.01 × 0.01 = 99.99%, assuming they fail independently.
- Does uptime cover planned maintenance?
- That depends on the agreement. Many SLAs exclude scheduled maintenance from the calculation, and some count only certain kinds of failure. Read how your provider defines downtime, the window it is measured over and what remedy you get, because the percentage alone does not say.
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