A/B Testing SLA Availability Formula
Learn how monthly A/B testing availability, downtime allowance, excess outage time, and estimated service credits are calculated.
This calculation measures an A/B testing platform's availability over a billing month after removing downtime exclusions allowed by the SLA. It helps teams compare recorded incidents with the stated uptime target and estimate a capped credit under the entered terms.
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Actual Monthly Availability
Where:
Convert the billing period to minutes, subtract downtime that counts under the SLA, then divide the remaining available minutes by all minutes in the month.
Variables Explained
| Variable | What It Means | Unit |
|---|---|---|
| billingDays - Days in billing month | Number of calendar days in the billing period. | days |
| reportedDowntimeMinutes - Reported downtime | Total recorded minutes when the service was unavailable or materially impaired. | minutes |
| excludedMaintenanceMinutes - Excluded maintenance time | Recorded minutes excluded from downtime by the SLA. | minutes |
| targetAvailability - SLA availability target | Availability percentage promised for the billing period. | percent |
| monthlyServiceFee - Monthly service fee | Recurring monthly fee eligible for a credit calculation. | currency |
| creditPerExcessHour - Credit per excess downtime hour | Credit amount specified for each full or partial hour above the permitted allowance. | currency |
| maximumCreditRate - Maximum monthly credit cap | Maximum credit allowed as a percentage of the eligible monthly fee. | percent |
Step-by-Step Calculation
Calculate total monthly minutes
The billing-period length is converted from days to minutes.
totalMonthlyMinutes = billingDays * 24 * 60
Find chargeable downtime
Only downtime remaining after valid SLA exclusions counts against availability.
chargeableDowntimeMinutes = max(0, reportedDowntimeMinutes - excludedMaintenanceMinutes)
Calculate actual availability
Available minutes are divided by total billing-period minutes.
actualAvailability = ((totalMonthlyMinutes - chargeableDowntimeMinutes) / totalMonthlyMinutes) * 100
Calculate the SLA downtime allowance
This is the maximum chargeable downtime compatible with the target.
permittedDowntimeMinutes = totalMonthlyMinutes * (1 - targetAvailability / 100)
Find downtime above the allowance
Only downtime beyond the SLA allowance is treated as excess.
excessDowntimeMinutes = max(0, chargeableDowntimeMinutes - permittedDowntimeMinutes)
Estimate and cap the credit
The estimated hourly credit is limited to the agreement's maximum monthly credit.
serviceCredit = min((excessDowntimeMinutes / 60) * creditPerExcessHour, monthlyServiceFee * (maximumCreditRate / 100))
30-day month with a 99.9% availability target
Total monthly minutes
30 × 24 × 60
43,200 minutes
Chargeable downtime
max(0, 60 − 0)
60 minutes
Actual availability
((43,200 − 60) ÷ 43,200) × 100
99.861%
Permitted downtime
43,200 × (1 − 99.9 ÷ 100)
43.2 minutes
Excess downtime
max(0, 60 − 43.2)
16.8 minutes
Estimated service credit
min((16.8 ÷ 60) × 100, 1,000 × 25 ÷ 100)
$28.00
Final Result
Actual availability is 99.861%. Excess downtime is 16.8 minutes and the estimated service credit is $28.00, leaving an estimated fee of $972.00.
Assumptions
- ✓Availability is measured across every minute in the entered billing month.
- ✓Only the maintenance or outage time entered as excluded is removed from reported downtime.
- ✓Reported downtime is treated as a single total without overlapping-incident adjustments.
- ✓The per-hour credit is applied proportionally to excess downtime hours before the monthly cap is applied.
Limitations
- !A signed SLA may define availability, impairment, and exclusions differently from this estimate.
- !Some agreements round downtime, excess time, or credit amounts before calculating a result.
- !Contracts can use credit tiers, incident thresholds, claim deadlines, or service-specific fees not represented here.
- !An estimated credit does not establish entitlement or replace incident verification and contract review.
Common Mistakes to Avoid
Using a fixed 30-day allowance for a billing period that has 28, 29, or 31 days.
Subtracting maintenance that is not expressly excluded by the relevant SLA.
Entering all subscription charges instead of the fee that is eligible for service credits.
Treating target availability as an outage percentage rather than an uptime percentage.
Forgetting that a monthly credit cap can limit the calculated hourly credit.
Related Formulas
Frequently Asked Questions
What is the formula for monthly SLA availability?
Monthly availability equals available minutes divided by total minutes in the billing month, multiplied by 100. Available minutes are total minutes less chargeable downtime.
How is permitted downtime calculated from a 99.9% SLA?
Multiply total monthly minutes by 0.1%. In a 30-day month, 43,200 × 0.001 equals 43.2 minutes.
Does excluded maintenance affect actual availability?
Yes. The calculator subtracts entered excluded maintenance from reported downtime before calculating availability.
How is excess downtime calculated?
It is the positive difference between chargeable downtime and the downtime permitted by the target availability.
How is the service credit capped?
The calculator compares the estimated hourly credit with the monthly fee multiplied by the maximum credit percentage, then uses the lower amount.
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