SLA Uptime & Downtime Calculator 🌎

How It's Calculated

The fundamental formula uses the complement of the uptime percentage applied to the total minutes in a given period:

Allowed Downtime = (1 − Uptime_Fraction) × Period_Minutes

Where:
  Uptime_Fraction = SLA% / 100
  Year  = 365 days × 24 h × 60 min = 525,600 minutes
  Month = 525,600 / 12             =  43,800 minutes  (30.44-day average)
  Week  = 7 × 24 × 60             =  10,080 minutes
  Day   = 24 × 60                 =   1,440 minutes

Example — 99.99% SLA:
  Annual  = (1 − 0.9999) × 525,600 = 0.0001 × 525,600 = 52.56 min
  Monthly = (1 − 0.9999) × 43,800  = 4.38 min
  Daily   = (1 − 0.9999) × 1,440   = 0.144 min ≈ 8.6 seconds

Note: some providers use a 30-day month (43,200 min) for contractual calculations. Always check the SLA document's definition of "month."

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Reference Table

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Typical Use Cases with Real Numbers

Case 1 — E-commerce checkout (99.99% SLA)
An online retailer processes $8,000/minute in sales. Their CDN provider offers a 99.99% SLA. Annual allowed downtime = 52.6 minutes. Worst-case annual revenue exposure = 52.6 × $8,000 = $420,800. The team decides to add a failover CDN to effectively achieve 99.999% combined uptime.

Case 2 — Hospital EMR system (99.9% SLA)
A regional hospital's electronic medical records vendor guarantees 99.9% uptime. That allows 8.77 hours/year of downtime. Regulatory guidance from HHS recommends contingency plans for any outage exceeding 8 hours—meaning this SLA sits right at a compliance boundary and likely requires a manual downtime procedure policy.

Case 3 — SaaS startup evaluating tiers
A startup chooses between a $200/mo shared plan (99.5% SLA = 1.83 days/year downtime) and a $800/mo dedicated plan (99.99% SLA = 52.6 min/year). Their app generates $500/hour in revenue. At 99.5%, expected annual revenue loss = 43.8 h × $500 = $21,900—far exceeding the $7,200/year price difference for the upgrade.

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Common Errors

1. Confusing uptime % with availability % — Some vendors quote "network uptime" (physical link up) separately from "service availability" (requests served successfully). A server can be "up" while a service is unreachable. Always clarify which metric the SLA covers.

2. Using a 30-day month instead of the calendar average — A 30-day month = 43,200 min; the calendar average is 43,800 min. This 1.4% difference can shift monthly downtime budgets by tens of seconds at five-nines levels, which matters in automated credit calculations.

3. Multiplying nines instead of adding them — Two independent systems each at 99.9% uptime do NOT yield 99.9% × 99.9% = 99.8%. In a series dependency (both must be up), combined availability = product of individual availabilities (99.8001%). In parallel/redundant setups, unavailability multiplies: 0.001 × 0.001 = 0.000001, yielding 99.9999%.

4. Ignoring planned maintenance windows — Many SLAs exclude scheduled maintenance from downtime calculations. A provider may guarantee 99.9% excluding a weekly 2-hour maintenance window, which could effectively mean 99.2% real availability. Always check exclusion clauses.

5. Treating SLA credits as full compensation — Standard cloud SLA credits (e.g., AWS offers 10–30% monthly fee credit) rarely cover actual revenue loss. At 99.9% SLA, AWS's maximum credit is 30% of the monthly bill—not 30% of your business revenue lost during the outage.

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