What this calculator does
See how much downtime an uptime promise really allows. A service level agreement (SLA) is a contract that usually promises availability as a percentage. Common targets are 99.9% uptime, called "three nines", or 99.99%, called "four nines". Those numbers look almost the same, but the downtime they allow is very different. This calculator turns any uptime percentage into the outage time it permits per day, week, month, quarter and year.
It also works the other way. Enter how long a service was down in a period, and it shows the availability that period achieved. That way you can check a month's outages against an SLA. A table of the common targets, from 90% to six nines, is included for quick comparison.
Where it's used
Why this math matters outside the classroom:
- Vendor comparison. An IT manager sees that moving from 99.9% to 99.95% uptime halves the allowed monthly downtime from about 44 to 22 minutes.
- Incident reviews. After a 45-minute outage, an operations team finds the month's availability fell to about 99.897%, just below a 99.9% target.
- Maintenance planning. Engineers budget a weekly maintenance window inside the 10 minutes a 99.9% target allows each week.
- Classroom percents. Students find 0.1% of the 86,400 seconds in a day to see why 99.9% still means 86.4 seconds of downtime.
- Status pages. Small businesses compare a provider's published uptime history with the downtime their own customers would notice.
Grade level and standards: Grade 6; Common Core State Standards for Mathematics 6.RP.A.3.c, 6.RP.A.3.d. © Copyright 2010. National Governors Association Center for Best Practices and Council of Chief State School Officers. All rights reserved.
How to use it
- Choose whether you're starting from an uptime percentage or from an amount of downtime.
- For a percentage, type the target, such as 99.95.
- For downtime, enter the amount with its unit and choose the period it happened in.
- Read the allowed downtime for each period, or the availability. Then compare targets in the table.
What the inputs mean
- Uptime (availability)
- The share of time a service is meant to be up. Each extra nine cuts the allowed downtime to a tenth.
- Downtime
- Total outage time in the chosen period. If there were several separate outages, add them up first.
- Per
- Months and quarters use average lengths. A month is a twelfth of a 365.25-day year (30.4375 days), and a quarter is a quarter of that year. A real 31-day month allows slightly more downtime than shown, and February slightly less.
Formula and method
downtime fraction = 1 − uptime % ÷ 100 allowed downtime = downtime fraction × length of the period uptime % = (1 − downtime ÷ length of the period) × 100 day = 86,400 s; week = 604,800 s; month = 2,629,800 s; year = 31,557,600 s
The "downtime fraction" is the share of time the service is allowed to be down: whatever the uptime percentage doesn't cover. Multiply it by the length of the period in seconds, and you get the allowance. The calculator then writes it in days, hours, minutes and seconds. It rounds to the nearest second, and amounts under a minute show hundredths of a second.
The year is 365.25 days, so leap years are averaged in. Months and quarters are fractions of that year. Some SLAs define a month as exactly 30 days, or use the actual calendar month. For those, multiply the per-day figure by the number of days.
Worked example
A hosting plan promises 99.9% uptime. How much downtime does that allow?
- Downtime fraction: 1 − 99.9 ÷ 100 = 0.001.
- Per month: 0.001 × 2,629,800 s = 2,629.8 s, which is 43 min 50 s.
- Per year: 0.001 × 31,557,600 s = 31,557.6 s, which is 8 h 45 min 58 s.
- Per day: 0.001 × 86,400 s = 86.4 s, about 1 min 26 s.
Result: 99.9% uptime allows about 43 min 50 s of downtime a month, or 8 h 45 min 58 s a year.
Understanding the result
Going from 99.9% to 99.99% cuts the monthly allowance from about 44 minutes to about 4 minutes. Reaching that usually takes backup systems that take over automatically (redundancy and failover), not just a faster response to alerts.
An SLA's percentage is a contract term. Before comparing figures, check how the provider measures it, what counts as downtime, and whether planned maintenance is left out.
Assumptions and limitations
- Availability is measured over continuous time, with every second counting equally.
- A month is 30.4375 days and a year is 365.25 days.
- Downtime is total outage time. A service that's slow but still up (partial degradation) isn't modeled.
- It leaves out contract details such as maintenance windows, how often uptime is measured, and service credits.
- It doesn't calculate the combined availability of several systems, whether they depend on each other (in series) or back each other up (redundancy).
- Results are rounded to the nearest second for display.
Frequently asked questions
How much downtime is 99.9% uptime?
About 1 min 26 s a day, 43 min 50 s a month, or 8 h 45 min 58 s a year, using a 365.25-day year. With a 365-day year, the yearly figure is 8 h 45 min 36 s.
What does five nines mean?
It means 99.999% availability. That allows about 26 seconds of downtime a month and a little over 5 minutes a year. It's often quoted as a target for telephone networks and other critical systems.
Why do different calculators give slightly different numbers?
They use different period lengths: a 30-day or 31-day month, or a 365-day or 365.25-day year. The differences are small, a few percent, and this page tells you the lengths it uses.
How do I check whether a month met the SLA?
Choose A downtime amount, enter the total outage time and pick Month. If the availability shown is at or above the SLA target, the month met it. The contract's own definitions still have the final say.
How this calculator is checked
Method: Definition of availability: uptime ÷ total time. Allowed downtime = (1 − availability) × period length. The calculation is covered by automated tests with known reference values and edge cases. Last updated 2026-10-18. Found a problem? Tell us.
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