Tech calculator
Uptime Calculator
See the downtime budget behind an availability target, measured per day, month, and year.
Average monthly downtime
43m 50s
How uptime percentages translate to downtime duration
Uptime is expressed as a percentage between 0 and 100 representing the fraction of time a service is available. The calculator accepts an uptime target (e.g., 99.9 percent for "three nines") and converts it to actual downtime. Downtime is calculated by inverting the uptime: if uptime is 99.9 percent, downtime is 0.1 percent (100 − 99.9). This downtime percentage is then multiplied by a period length to find allowed downtime duration.
The calculator uses a months calculation based on the average month length of 365.25 ÷ 12 = 30.4375 days. This accounts for the fact that some months have 28, 29, 30, or 31 days, averaging to 30.4375 over a full year. The main output shows average monthly downtime in days, hours, minutes, and seconds. Detail rows show per-day downtime and annual downtime, all formatted with days, hours, minutes, and seconds as appropriate.
Worked example: 99.9 percent uptime target
Starting with 99.9 percent uptime: downtime fraction = 1 − (99.9 ÷ 100) = 1 − 0.999 = 0.001. Monthly downtime in seconds: 30.4375 × 86,400 × 0.001 = 2,629,800 seconds × 0.001 = 2,629.8 seconds. Converting: 2,629.8 ÷ 86,400 = 0.0304 days, which is approximately 43 minutes and 50 seconds (2,629.8 − 2,580 = 49.8 seconds).
Daily downtime: 86,400 × 0.001 = 86.4 seconds, or 1 minute and 26.4 seconds. Annual downtime: 365.25 × 86,400 × 0.001 = 31,557,600 × 0.001 = 31,557.6 seconds, which converts to 8 hours, 45 minutes, and 57.6 seconds. A single service outage exceeding 50 minutes per month breaks the 99.9 percent target.
Uptime tiers and their practical implications
90 percent uptime allows about 73 hours of downtime in an average month, acceptable only for non-critical internal tools. 99 percent (two nines) allows 7 hours 18 minutes per month, typical for systems that tolerate scheduled maintenance windows. 99.5 percent allows 3 hours 39 minutes. 99.9 percent (three nines) allows 43 minutes 50 seconds per month, which requires automated failover and careful coordination of updates. 99.99 percent (four nines) allows only 4 minutes 23 seconds per month, close to impossible without redundancy at multiple layers. 99.999 percent (five nines) allows about 26 seconds of downtime per month; only the most critical services like DNS and payment systems target this.
These percentages describe uptime, not mean time between failures (MTBF). A service that is down for one 10-minute stretch each month runs at 99.977 percent. A service that is down 1 minute every day loses about 30 minutes per month and lands near 99.93 percent. Real availability depends on failure rate and recovery speed combined. High availability architectures use redundancy, automated failover, and geographic distribution to meet ambitious uptime targets.
Real-world availability targets and budgets
- 95 percent uptime: about 36.5 hours of downtime per month. Acceptable for hobby projects and non-commercial services where users expect occasional outages.
- 99 percent uptime: 7 hours 18 minutes of downtime per month. A common floor for small web services. Monitoring and automated restarts usually cover it without geographic redundancy.
- 99.5 percent uptime: 3 hours 39 minutes of downtime per month. A workable target for e-commerce and SaaS on a single region with fast detection and recovery.
- 99.9 percent uptime (three nines): 43 minutes 50 seconds of downtime per month. Enterprise standard. Demands automated failover, load balancing, and careful maintenance windows.
- 99.99 percent uptime (four nines): 4 minutes 23 seconds of downtime per month. Requires redundant infrastructure in multiple datacenters, automatic failover, and sophisticated monitoring.
- 99.999 percent uptime (five nines): about 26 seconds of downtime per month. Only achievable with distributed systems, continuous deployment, and graceful degradation.
Frequently asked questions
What counts as downtime for an uptime SLA?
SLA definitions vary, but typically downtime is the period when a user cannot access the service at all or receives errors. Brief spikes in latency or partial feature outages often do not count. Scheduled maintenance windows may be excluded from SLAs. Check your service agreement for the exact definition.
If I achieve 99 percent uptime one month, can I miss my SLA the next month?
It depends on how the SLA rolls. Many SLAs measure over a 12-month rolling window, so a poor month is partially offset by better months. Some SLAs reset monthly, so each month is independent. Others measure quarterly or annually. Check the agreement.
Does this calculator account for planned maintenance?
No. Planned maintenance windows are often excluded from uptime calculations by SLA agreement. If an agreement excludes 8 hours per month of maintenance, the downtime budget is 43 minutes for unplanned outages, not total. Account for maintenance separately.
What is the difference between uptime and availability?
Uptime measures whether the system is accessible. Availability accounts for whether it is also performing its function correctly (no errors, adequate speed). A system might be up but so slow it is unusable, reducing availability below uptime.
Can I add percentages if I have redundant systems?
Percentages do not add, but independent redundancy does compound. Two independent systems at 99 percent each are both down only 0.01 × 0.01 = 0.0001 of the time, so the pair reaches about 99.99 percent. The caveat is independence: shared power, networking, or deployment pipelines create correlated failures that erase most of that gain.
How is uptime measured in practice?
Monitoring services run synthetic tests from multiple locations, pinging the service or making API calls and recording latency and errors. Uptime is calculated as (successful requests ÷ total requests) × 100. Some providers also measure availability from the user-facing application rather than just backend servers.