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A/B Testing Uptime (Monthly) Calculator

Compare the estimated monthly downtime of two versions using their uptime percentages and the number of days in the month.

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Overview

This A/B testing uptime calculator estimates and compares monthly downtime for a control and a variant. Enter each version's uptime percentage and the number of days in the month to see the estimated downtime difference in minutes.

How it works

The calculator converts the selected number of days into total monthly minutes. It then multiplies those minutes by each version's downtime rate, calculated as 100% minus uptime. Subtracting the variant's estimated downtime from the control's estimated downtime shows the expected minutes saved or added by the variant. A positive downtime difference means the variant is more available.

How to use this calculator

  1. 1Enter the uptime percentage for your control version.
  2. 2Enter the uptime percentage for your variant.
  3. 3Choose the number of days in the month being evaluated.
  4. 4Review the estimated downtime for each version and the difference between them.

Example Calculation

Control uptime

100%

Variant uptime

100%

Days in month

30

Estimated downtime saved

21.60 minutes

Over a 30-day month, the 99.9% control has about 43.20 minutes of estimated downtime and the 99.95% variant has about 21.60 minutes. The variant saves an estimated 21.60 minutes of downtime.

Frequently asked questions

What does an uptime percentage mean?

Uptime is the share of a period when a service is available. For example, 99.9% uptime means 0.1% of the measured period is unavailable.

How much downtime is 99.9% uptime in a 30-day month?

A 30-day month contains 43,200 minutes. At 99.9% uptime, 0.1% is downtime, which is about 43.2 minutes.

Why can a small uptime change make a large difference?

Uptime percentages are measured against all minutes in the month. Near 100%, even a change of a few hundredths of a percentage point can represent many minutes of availability.

What does a positive downtime difference mean?

A positive result means the variant has fewer estimated downtime minutes than the control. A negative result means the variant has more estimated downtime.

Does this calculator prove that one A/B test version is better?

No. It compares the uptime values entered. Consider sample size, monitoring coverage, error rates, latency, and user outcomes alongside availability.

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Assumptions and warnings

Assumptions

  • Uptime percentages represent availability over the full selected calendar month.
  • Downtime is spread proportionally across all minutes in the month.
  • The calculation compares availability only and does not measure latency, errors, traffic volume, or user impact.
  • Results are estimates based on the uptime values entered.

Warnings

  • Availability figures can vary by traffic segment, region, monitoring method, and incident severity.
  • Use sufficient A/B test data before making reliability or release decisions.