Calculate exactly how much downtime your SLA allows.
Enter your SLA percentage and time period to see the maximum allowed downtime in days, hours, minutes, and seconds. Use the error budget tracker to monitor how much of your downtime allowance has been consumed.
Daily
1m 26s
Weekly
10m 5s
Monthly
43m 12s
Quarterly
2h 9m 36s
Yearly
8h 45m 36s
Enter your actual downtime to see how much of your error budget has been consumed.
| SLA Level | Monthly Downtime | Yearly Downtime |
|---|---|---|
| 99% | 7h 12m | 3d 15h 36m |
| 99.5% | 3h 36m | 1d 19h 48m |
| 99.9% | 43m 12s | 8h 45m 36s |
| 99.95% | 21m 36s | 4h 22m 48s |
| 99.99% | 4m 19s | 52m 34s |
| 99.999% | 26s | 5m 15s |
A Service Level Agreement (SLA) is a contractual commitment between a service provider and their customers that guarantees a minimum level of uptime. For ISPs, the SLA defines how much downtime is acceptable over a given period — typically measured monthly. The ITU-T E.800 recommendation defines availability as "the ability of an item to be in a state to perform a required function at a given instant of time."
Missing an SLA target usually results in service credits, contract penalties, or customer churn. According to the Uptime Institute's 2024 Annual Outage Analysis, 55% of operators experienced an outage in the past three years, and the average cost of a significant outage exceeded $100,000. Understanding exactly how much downtime your SLA allows — in concrete hours, minutes, and seconds — is essential for planning your monitoring strategy, scheduling maintenance windows, and sizing your incident response team.
The formula is straightforward:
Allowed Downtime = Total Minutes in Period × (1 − SLA% ÷ 100)
For example, a 99.9% monthly SLA with a 30-day month (43,200 total minutes) allows 43,200 × 0.001 = 43.2 minutes of downtime per month. That is less than 45 minutes to handle all unplanned outages, failed deployments, and infrastructure issues.
Different customer segments warrant different SLA commitments. Offering a higher SLA tier means less room for downtime, requiring better monitoring, more redundancy, and faster incident response.
| SLA | Monthly Downtime | Typical ISP Use Case |
|---|---|---|
| 99% | 7h 12m | Internal tools, back-office systems |
| 99.5% | 3h 36m | Basic residential broadband |
| 99.9% | 43m 12s | Standard broadband, small business |
| 99.95% | 21m 36s | Premium residential, managed WiFi |
| 99.99% | 4m 19s | Business fiber, enterprise leased lines |
| 99.999% | 26s | Carrier-grade, critical infrastructure |
An error budget is your total allowance of downtime for a given SLA period. It represents the gap between perfect uptime (100%) and your SLA commitment. The concept was popularized by Google's Site Reliability Engineering (SRE) methodology, which treats reliability as a feature with a measurable budget. Use the budget tracker above to see how much of your allowance has been consumed.
When your error budget is healthy, your team can deploy network changes, firmware upgrades, and new configurations with confidence. When the budget is running low, all efforts should shift to stability — deferring non-critical changes and prioritizing incident prevention.
Real-time monitoring with smart alert correlation and fast polling intervals helps ISPs protect their error budgets by detecting and resolving issues before they accumulate into SLA-threatening downtime.
Quick-reference formulas for availability, reliability metrics, and service credit calculations.
Availability (%) = (Total Time − Downtime) / Total Time × 100Example: A service was down for 45 minutes in a 30-day month (43,200 total minutes):
(43,200 − 45) / 43,200 × 100 = 99.896%
This falls below a 99.9% SLA, triggering a service credit.
MTBF = Total Operational Time / Number of FailuresMean Time Between Failures — how often outages occur
MTTR = Total Repair Time / Number of RepairsMean Time To Repair — how quickly you recover. Calculate yours →
MTTA = Total Acknowledgement Time / Number of IncidentsMean Time To Acknowledge — how fast your team responds
Service Credit = Monthly Fee × Credit Percentage for SLA BreachTypical ISP credit tiers:
Availability = MTBF / (MTBF + MTTR)Example: An OLT fails once every 90 days (MTBF = 129,600 min) and takes 45 minutes to restore (MTTR = 45 min):
129,600 / (129,600 + 45) = 99.965%
Reducing MTTR from 45 to 15 minutes improves availability to 99.988%. Faster detection through real-time alerting is the most effective way to lower MTTR.
Common questions about SLA uptime, downtime calculations, and error budgets for ISPs.
NetSense NMS monitors your FTTH network 24/7 with 30-second polling, smart alert correlation, and proactive fault detection. Catch issues before they consume your error budget.
Learn more: Alerting & Escalation · OLT Monitoring · Compare NetSense