Three nines over 30 days
99.9% availability across a 30-day period allows 43 minutes 12 seconds of downtime. The same percentage across a different period gives a different allowance.
Allowed downtime
43m 12s
2,592 seconds of downtime over 30 days
99.9% availability
29d 23h 16m 48s available time
All days are exactly 24 hours. Calendar months use their actual day count, including February in leap years. No local daylight saving adjustment is applied. Availability = (period − downtime) ÷ period × 100.
This is a time-based estimate. Contractual SLAs can exclude maintenance, use request success rates, or measure separate services. Read your provider’s definition before treating the result as an SLA allowance.
Calculations run locally and do not contact your services.
How to use the Uptime Calculator
- 01
Choose whether to calculate allowed downtime or availability from actual downtime.
- 02
Enter the percentage or downtime, then select a period. Use a specific calendar month to account for its real number of days.
- 03
Read the result in days, hours, minutes, and seconds, and copy or download the calculation with its assumptions.
Examples
Reverse an outage measurement
If a service is unavailable for 30 minutes during a 30-day measurement period, availability is about 99.930556%. Add separate outages before entering the total, and avoid counting overlapping incidents twice.
The availability formula
Time-based availability equals (measurement time minus downtime) divided by measurement time, multiplied by 100. The downtime allowance equals measurement time multiplied by (100 minus the target percentage), divided by 100. The calculation accepts 0% through 100%, including fractional percentages; it rounds duration displays to milliseconds.
Month and calendar assumptions
A default month is exactly 30 days. A specific calendar month uses its actual number of days, including February 29 in a leap year. You can select 365-day or 366-day years, a 7-day week, or a custom period. Every day is modeled as 24 hours, so local daylight saving changes do not shorten or lengthen a day here.
An estimate is not an SLA definition
A service contract may measure successful requests instead of elapsed time, exclude scheduled maintenance, define a minimum outage length, or use a different reporting window. A downtime budget is therefore only meaningful alongside the service’s own measurement rules. This calculator does not monitor a service or determine service credits.
What to know
- Time-based arithmetic only; no live availability monitoring.
- Days are fixed at 24 hours, without local daylight saving adjustments.
Common questions
How much downtime is 99.99% per month?
For exactly 30 days, 99.99% availability allows 4 minutes 19.2 seconds of downtime. Select a calendar month if your reporting month has a different length.
Does 100% guarantee the service will never fail?
No. Here 100% is a mathematical target that allows zero counted downtime within the chosen period. It is not a prediction or a reliability guarantee.
Can I calculate availability from several outages?
Yes. Add non-overlapping downtime intervals in a common unit and enter the total in reverse mode. The total must be between zero and the full period.