Enter an availability target such as 99.9% to see how much downtime it allows each day, week, month, quarter and year. You can also turn a real outage into an uptime percentage or combine the SLAs of several services.
Last updated:
What do you want to calculate?
%
e.g. 99.9 (three nines), 99.95, 99.99
One per line or comma separated, e.g. load balancer, app servers, database
How are they connected?
Measurement window
h
168 = every hour of the week
Enter your values to see the result.
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How to use the Uptime SLA Calculator
Choose a mode: uptime % to downtime, downtime to uptime %, or combine several SLAs.
Enter the target, for example 99.95, or the outage length and the period it happened in.
Set the measurement window: 24×7 or business hours only, plus the year and month length your contract uses.
Read the downtime budget for every period in the result cards and the table below them.
Copy or download the figures for your SLA document, status page or incident report.
Downtime allowed by common SLA levels
The table uses a 24×7 service, a 365.25-day year and an average month of 30.44 days. Pick other lengths in the calculator to match your contract.
Availability
Nines
Per day
Per week
Per month
Per year
99%
2
14m 24s
1h 40m 48s
7h 18m 18s
3d 15h 39m
99.5%
2.3
7m 12s
50m 24s
3h 39m 9s
1d 19h 49m
99.9%
3
1m 26.4s
10m 4.8s
43m 49.8s
8h 45m 58s
99.95%
3.3
43.2s
5m 2.4s
21m 54.9s
4h 22m 59s
99.99%
4
8.64s
1m 0.5s
4m 23s
52m 35.8s
99.999%
5
864 ms
6.05s
26.3s
5m 15.6s
The formulas
Allowed downtime = (1 − A) × period, where A is the availability as a fraction (99.9% = 0.999).
Availability from an outage = 1 − downtime ÷ period. A 2-hour outage in a 30-day month is 1 − 7,200 ÷ 2,592,000 = 99.72%.
Number of nines = −log10(1 − A). 99.95% is about 3.3 nines.
Series (all components required): A = A1 × A2 × … × An. Chaining services always lowers availability.
Parallel (redundant components): A = 1 − (1 − A1) × (1 − A2) × …, assuming failures are independent.
Tips for setting and tracking an SLA
Promise less than you measure. Teams usually set an internal objective (SLO) above the customer SLA so there is room to react before credits are owed.
Spend the error budget on purpose. If 99.9% gives you about 43 minutes a month, a single bad deploy can use all of it. Track how much is left before risky releases.
Define what "down" means. Full outage only, or also slow responses and elevated error rates? Partial outages are often counted pro rata.
Check your dependencies. A 99.99% app on a 99.9% database cannot deliver more than about 99.89% end to end.
Frequently asked questions
How much downtime does 99.9% uptime allow?
With an average month of 30.44 days, 99.9% allows about 43 minutes 50 seconds of downtime per month, 1 minute 26 seconds per day, 10 minutes 5 seconds per week and 8 hours 46 minutes per year (365.25-day year). With a 30-day month the monthly figure is exactly 43 minutes 12 seconds.
What is the formula for allowed downtime?
Allowed downtime = (1 − availability ÷ 100) × length of the period. For 99.95% over a 30-day month: 0.0005 × 2,592,000 seconds = 1,296 seconds, which is 21 minutes 36 seconds. The reverse is availability = (1 − downtime ÷ period) × 100.
What does 'five nines' mean?
Five nines is 99.999% availability, which allows roughly 5 minutes 16 seconds of downtime per year or about 26 seconds per month. Each extra nine cuts the downtime budget by a factor of ten, which is why each one is much harder and more expensive to reach.
How do I calculate the SLA of services that depend on each other?
If every component must be up (series), multiply their availabilities: 99.99% × 99.95% × 99.9% = 99.84%, which is lower than any single part. If components are redundant (parallel), multiply their unavailabilities and subtract from one: two independent 99.9% servers give 1 − 0.001² = 99.9999%.
Does planned maintenance count as downtime?
It depends on the contract. Many SLAs exclude announced maintenance windows, and some only measure business hours. Use the service hours option to model a 9×5 or 24×5 SLA, and leave maintenance out of the downtime you enter if your agreement excludes it.
Why do different SLA calculators show slightly different numbers?
They assume different period lengths. A year can be 365, 365.25 or 366 days and a month can be 28 to 31 days or an average of 30.42 or 30.44 days. This calculator lets you pick the same lengths your SLA uses so the numbers match your contract.
Is 100% uptime possible?
In practice no. Hardware fails, networks partition, deployments go wrong and dependencies have their own outages. That is why SLAs state a target below 100% and define an error budget: the downtime you can spend on incidents and risky changes while still meeting the promise.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.