Uptime Calculator
The downtime an uptime percentage or SLA allows per day, week, month and year, or the uptime a known outage gives, with the "nines" table.
Result…
How it's calculated
Downtime = (1 − uptime ÷ 100) × length of the period Uptime = (1 − downtime ÷ period) × 100% Nines = −log₁₀(1 − uptime ÷ 100)
An availability target such as 99.9% leaves 0.1% of the time for outages. Multiplying that share by the length of a day (86,400 s), week (604,800 s), month (a twelfth of 365.25 days, 2,629,800 s) or year (365.25 days, 31,557,600 s) gives the allowed downtime. Each extra nine cuts the downtime by ten. SLAs differ in the details: some use a 30-day month or a calendar month, some exclude planned maintenance, and some count only outages longer than a few minutes, so the contract decides what counts. The uptime of a chain of services that all must work is lower than any one of them.
Your numbers, step by step
About the Uptime calculator
This uptime calculator turns an availability target into the downtime it allows per day, week, month and year, or works the other way and turns a known outage into an uptime percentage. Engineers use it to read a service level agreement (SLA), to set an internal target, or to check whether last month's incidents broke the promise. Alongside the answer it gives the number of nines and a table from 90% to 99.9999%, so you can see how much each extra nine costs in allowed outage time. The steps show the multiplication with your numbers. A month is taken as a twelfth of an average year (30.44 days) and a year as 365.25 days, which the note states; your own SLA may count a 30-day or calendar month, or leave out planned maintenance.
How to use it
- Under “I know”, choose “Uptime %” or “Downtime”.
- For a target, enter the “Uptime”, such as 99.9 or 99.95.
- For an outage, enter the “Downtime”, pick “Downtime in” seconds, minutes or hours, and choose “Downtime per” day, week, month or year.
- Read the allowed downtime or the uptime, the nines and the table of common targets in the steps.
Worked example
A 99.95% target leaves 0.05% of the time for outages: 0.0005 × 2,629,800 s gives 21 min 55 s a month, 43.2 s a day, 5 min 2 s a week and 4 h 22 min 59 s a year, which is 3.3 nines. Going the other way, 20 minutes of downtime in a month is 1 − 1200 ÷ 2,629,800 ≈ 99.9544% uptime.
Updated
Questions people ask
How do I calculate downtime from an uptime percentage?
Subtract the uptime from 100% and multiply that share by the length of the period in seconds. For 99.9% over a 365.25-day year that is 0.001 × 31,557,600 s, which is 8 h 45 min 58 s.
How much downtime is five nines?
99.999% allows about 5 min 16 s a year, 26.3 s a month and under a second a day. Each extra nine divides the allowed downtime by ten.
What is the formula for uptime from downtime?
Uptime = (1 − downtime ÷ length of the period) × 100%. Both must be in the same unit, so the calculator converts everything to seconds. The percentage calculator handles other share calculations.
Why does my SLA give a slightly different number?
SLAs define the period differently: a 30-day month, a calendar month or a 365-day year all change the result a little. Many also exclude planned maintenance or only count outages above a few minutes, so the contract wording decides.
Is a higher uptime always worth it?
Each extra nine cuts the allowed downtime by ten, which usually takes more redundancy and monitoring, and the total uptime of services that depend on each other is lower than any one of them. The calculator shows what a target allows; whether it is worth the cost depends on the service.