A/B Testing Uptime (Monthly) Formula
Learn how monthly downtime is estimated from control and variant uptime percentages.
This calculation converts each version's uptime percentage into estimated unavailable minutes for a selected calendar month. It helps quantify how a small availability difference can affect downtime in an A/B test.
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Estimated Downtime Saved
Where:
Find the total minutes in the month, then multiply them by the uptime difference expressed as a decimal. A positive result means the variant has less estimated downtime.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| D - Days in month | Number of calendar days used for the comparison. | days |
| U_c - Control uptime | Observed or expected uptime percentage for the control version. | percent |
| U_v - Variant uptime | Observed or expected uptime percentage for the variant version. | percent |
| M - Minutes in month | Total minutes in the selected month. | minutes |
| S - Estimated downtime saved | Control downtime minus variant downtime. | minutes |
Step-by-Step Calculation
Calculate minutes in the month
Convert the selected number of days into the full number of minutes available in that month.
monthlyMinutes = daysInMonth * 24 * 60
Convert control uptime to downtime rate
The portion remaining after uptime is the estimated unavailable portion.
controlDowntimeRate = 1 - controlUptime / 100
Estimate control downtime
Apply the control's downtime rate to all minutes in the month.
controlDowntimeMinutes = monthlyMinutes * controlDowntimeRate
Estimate variant downtime
Apply the variant's downtime rate to the same monthly period.
variantDowntimeMinutes = monthlyMinutes * (1 - variantUptime / 100)
Compare the two estimates
A positive result indicates fewer estimated downtime minutes for the variant.
downtimeDifferenceMinutes = controlDowntimeMinutes - variantDowntimeMinutes
Comparing 99.9% and 99.95% uptime over 30 days
Monthly minutes
30 * 24 * 60
43,200 minutes
Control downtime rate
1 - 99.9 / 100
0.001
Control downtime
43,200 * 0.001
43.20 minutes
Variant downtime
43,200 * (1 - 99.95 / 100)
21.60 minutes
Downtime saved
43.20 - 21.60
21.60 minutes
Final Result
The variant saves an estimated 21.60 minutes of downtime and has a 0.050 percentage-point uptime advantage.
Assumptions
- ✓Each uptime percentage represents availability across the full selected month.
- ✓Downtime is treated as proportionally distributed across all monthly minutes.
- ✓Both versions are evaluated over comparable periods and under comparable monitoring coverage.
- ✓The calculation measures availability only.
Limitations
- !A monthly uptime percentage does not show the timing or duration pattern of incidents.
- !Results can differ by region, device, traffic segment, and monitoring method.
- !The estimate does not measure latency, failed requests, degraded functionality, or user impact.
- !A numerical difference alone does not establish that the variant is more reliable in every context.
Common Mistakes to Avoid
Entering 0.1 as an uptime value when the field expects 0.1%, rather than entering 99.9%.
Confusing percentage points with percentage change.
Using 30 days for a month that has 28, 29, or 31 days.
Treating a positive downtime difference as proof of a statistically reliable test result.
Comparing uptime figures collected with different definitions of availability.
Related Formulas
Frequently Asked Questions
How do you calculate monthly downtime from uptime?
Multiply the total minutes in the month by one minus uptime divided by 100.
How much downtime does 99.9% uptime allow in 30 days?
A 30-day month has 43,200 minutes, and 0.1% of that is 43.20 minutes.
What does a positive downtime difference mean?
It means the variant has fewer estimated downtime minutes than the control.
Why does a 0.05 percentage-point change matter?
Near 100% uptime, small percentage-point changes apply to every minute in a month and can translate into meaningful downtime differences.
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