What is uptime?
Uptime is the percentage of time that a website, server, application, network, or other service is available and functioning during a defined period. It is commonly used to describe the reliability of online services and is often included in service level agreements (SLAs).
For example, a service with 99.9% uptime was available for 99.9% of the measurement period. The remaining 0.1% represents the maximum downtime allowed by the calculation.
Uptime is closely related to downtime: if a system is available for 99.9% of the time, it is unavailable for 0.1%. That small percentage can still represent a meaningful amount of time when measured over a month or a year.
How to calculate uptime and downtime
Uptime and downtime are complementary percentages. Together they add up to 100%.
Uptime % = (Total Time − Downtime) / Total Time × 100 Downtime = Total Time × (1 − Uptime / 100) If you are working with a measured outage and need to calculate its duration first, our Time Duration Calculator can help determine the elapsed time between two points.
For a percentage-only calculation, you can also use our Percentage Calculator to work with the underlying percentage relationships.
Example: calculating uptime from downtime
Suppose a website was unavailable for 43 minutes during a 30-day month. A 30-day month contains 43,200 minutes.
(43,200 − 43) / 43,200 × 100 ≈ 99.9005% Rounded to one decimal place, that is approximately 99.9% uptime.
The reverse calculation is also useful. If your SLA target is 99.9%, the calculator shows how much downtime that target permits during the selected period.
What does 99.9% uptime mean?
A 99.9% uptime target sounds extremely high, but the remaining 0.1% still represents more than eight hours of potential downtime over a 365-day year.
Using a 30-day month and a 365-day year, 99.9% uptime allows:
- 1 minute 26.4 seconds per day
- 10 minutes 4.8 seconds per week
- 43 minutes 12 seconds per 30-day month
- 8 hours 45 minutes 36 seconds per 365-day year
This is why an uptime percentage should always be considered together with its actual downtime allowance. A difference of only a few hundredths of a percentage point can represent hours of additional availability over a year.
The nines table
Availability targets are often described as "nines." The more nines in the target, the less downtime is permitted. Moving from 99% to 99.9% reduces the allowed downtime by a factor of ten; moving from 99.9% to 99.99% reduces it by another factor of ten.
The table below uses a fixed 30-day month and 365-day year so that the figures are easy to compare.
| Uptime | Per Day | Per Week | Per Month | Per Year |
|---|---|---|---|---|
| 99% (two nines) | 14m 24s | 1h 40m 48s | 7h 12m | 3d 15h 36m |
| 99.5% | 7m 12s | 50m 24s | 3h 36m | 1d 19h 48m |
| 99.9% (three nines) | 1m 26.4s | 10m 4.8s | 43m 12s | 8h 45m 36s |
| 99.95% | 43.2s | 5m 2.4s | 21m 36s | 4h 22m 48s |
| 99.99% (four nines) | 8.64s | 1m 0.48s | 4m 19.2s | 52m 33.6s |
| 99.999% (five nines) | 0.864s | 6.048s | 25.92s | 5m 15.36s |
| 99.9999% (six nines) | 0.0864s | 0.6048s | 2.592s | 31.536s |
99.9% vs 99.99% uptime
The difference between 99.9% and 99.99% may look small when written as a percentage, but it is substantial when converted into downtime.
| Uptime target | Monthly downtime | Yearly downtime |
|---|---|---|
| 99.9% | 43m 12s | 8h 45m 36s |
| 99.99% | 4m 19.2s | 52m 33.6s |
| 99.999% | 25.92s | 5m 15.36s |
In other words, improving from 99.9% to 99.99% reduces the yearly downtime allowance from almost nine hours to less than one hour. Improving again to 99.999% reduces it to only a few minutes.
What is an SLA uptime target?
An SLA, or service level agreement, is an agreement that defines the expected level of service between a provider and its customer. For online services, availability or uptime is often one of the measured service levels.
An SLA may specify an uptime target such as 99.9%, 99.99%, or another availability level. The agreement can also define how downtime is measured, which outages are excluded, and what happens if the provider fails to meet the target.
The percentage by itself does not tell the whole story. Two providers could advertise the same uptime target while using different definitions of downtime, maintenance exclusions, measurement locations, or reporting periods.
What counts as downtime?
There is no universal definition that applies to every service. The exact definition depends on the monitoring system or SLA being used.
Depending on the agreement, downtime may include a complete service outage, failed requests, or an inability to reach a service from specified monitoring locations. Scheduled maintenance, partial outages, degraded performance, or third-party failures may be treated differently.
For that reason, this calculator should be viewed as an arithmetic tool: enter the uptime target or the downtime that you have measured, and it calculates the corresponding percentage or time allowance. It does not determine whether a particular incident legally or contractually qualifies as downtime.
Why uptime calculations can differ
If two uptime calculators produce slightly different results, the difference is often caused by the length of the period being assumed.
This calculator uses a 30-day month and a 365-day year. This makes the results consistent and easy to compare across uptime targets.
A calculation based on an actual calendar month will vary because months have different numbers of days. Likewise, a yearly calculation can differ in a leap year.
For exact operational or contractual reporting, use the period and measurement rules specified by your SLA rather than assuming that every provider uses the same convention.
Uptime vs availability
The terms uptime and availability are often used interchangeably, but they can describe slightly different concepts depending on the context.
Uptime generally focuses on whether a system is running or available, while availability can incorporate a more specific definition of whether a service is usable according to agreed performance criteria. An SLA may therefore define availability more precisely than simply asking whether a server is powered on.
Uptime for websites and hosting
Website owners commonly encounter uptime targets when choosing web hosting, cloud infrastructure, monitoring services, or other infrastructure providers. A higher availability target generally means that less downtime is acceptable, but the percentage should not be the only factor used to evaluate a provider.
Website reliability also depends on infrastructure, application code, databases, DNS, networking, third-party services, deployment practices, and monitoring. Traffic capacity is a separate concern: if you are estimating the network resources required to serve a website, use our Website / Hosting Bandwidth Calculator.
Similarly, if you need to estimate how long a file transfer will take over a particular connection, our Download Time Calculator and Upload Time Calculator calculate transfer duration rather than service availability.
How to use this uptime calculator
- Enter your target uptime percentage, such as 99.9%, to calculate the maximum downtime for different periods.
- Use the preset buttons for common uptime targets such as 99%, 99.9%, 99.99%, and 99.999%.
- Switch to Downtime → Uptime % if you already know how long a service was unavailable.
- Select the measurement period and enter the actual downtime in days, hours, minutes, and seconds.
- Compare the resulting uptime percentage with your target or SLA.
Frequently asked questions
How much downtime does 99.9% uptime allow?
Using a 365-day year, 99.9% uptime allows 8 hours 45 minutes 36 seconds of downtime per year. Using a 30-day month, it allows 43 minutes 12 seconds per month, or 1 minute 26.4 seconds per day.
How much downtime does 99.99% uptime allow?
Using a 30-day month and 365-day year, 99.99% uptime allows about 4 minutes 19.2 seconds of downtime per month and 52 minutes 33.6 seconds per year.
What does five nines uptime mean?
Five nines means 99.999% uptime. Using a 365-day year, that corresponds to about 5 minutes 15.36 seconds of downtime per year.
How do you calculate uptime percentage?
Uptime percentage is calculated as (Total Time − Downtime) / Total Time × 100. For example, if a service is down for 43 minutes during a 30-day month, the resulting uptime is approximately 99.9005%.
Why do different uptime calculators give slightly different answers?
The most common reason is the length of the period used in the calculation. This calculator uses a fixed 30-day month and 365-day year. Calculations based on calendar months, leap years, or an average month length can produce slightly different downtime allowances.
What counts as downtime?
That depends on the specific SLA or monitoring definition. Complete outages, failed requests, scheduled maintenance, partial outages, and degraded performance may be treated differently. This calculator only performs the arithmetic after you know the downtime that should be counted.
Is 99.9% uptime good?
99.9% uptime is a common availability target, but whether it is sufficient depends on the service and its business requirements. A 99.9% target still permits about 8 hours 45 minutes 36 seconds of downtime in a 365-day year.
Does scheduled maintenance count as downtime?
Not necessarily. Whether scheduled maintenance counts toward downtime depends on the terms of the SLA or the rules used by the monitoring service. Always check the specific agreement before comparing uptime figures.