Downtime Calculator: Add Up Outages for an SLA Month
An SLA claim needs one number per service: how many minutes it was down in the month. Status pages, tickets and chat threads give you times in different time zones, incidents that overlap and outages that only hit some users. Log them here and the calculator converts, clips, merges and adds them up. If you are counting the Microsoft 365 outage that began on 12 November 2026, read what Microsoft has said in our Microsoft 365 outage briefing.
1. The SLA month
November 2026 has 30 days = 43,200 minutes. The month runs from 2026-11-01 00:00 UTC to 2026-12-01 00:00 UTC: boundaries are in UTC, so check how your provider defines its month.
2. The incident log
One row per incident window. Enter times as your status page or ticket shows them and pick that time zone; daylight saving is handled by your browser's time zone database. Use the same service name on every row for a service so its windows are merged.
Example, not Microsoft's figures: made-up times to show how the tool works. Two overlapping Exchange Online incidents in November 2026, one full and one degraded. Edit a row or press “Clear log” to enter your own.
- Incident 1
In UTC: 2026-11-12 14:30 UTC to 2026-11-12 17:15 UTC · 2 h 45 min inside November 2026
- Incident 2
In UTC: 2026-11-12 16:00 UTC to 2026-11-12 19:45 UTC · 3 h 45 min inside November 2026
Exchange Online
2 incidents logged · 1 merged windowAdding the rows up without merging would give 390 min: 75 min overlap and are counted once.
Downtime: users unable to send or receive email with Outlook Web Access.
Work out the credit amount in the SLA Credit Calculator
Merged windows (UTC)
- 2026-11-12 14:30 UTC to 2026-11-12 19:45 UTC (5 h 15 min)
How the numbers are worked out
- Every start and end is converted to UTC with your browser's time zone database, so daylight saving is applied for the date you entered. A time that happened twice (clocks went back) uses the first; a time that never happened (clocks went forward) is moved forward, and the row says so.
- Each incident is clipped to the calendar month in UTC. An outage that starts on the last evening of the month counts only up to midnight UTC.
- For each service, overlapping windows are merged so the same minute is never counted twice. Downtime (merged) is every minute with any impact.
- User-minute weighted counts each minute by the share of users affected at that minute (a full outage = 100%). Where partial incidents overlap, the larger share is used, not the sum, because they may hit the same people.
- Monthly uptime = (minutes in the month − weighted minutes) ÷ minutes in the month × 100. In November 2026 (43,200 minutes), 99.9% allows 43.2 min, 99% allows 432 min and 95% allows 2,160 min.
Microsoft 365 tiers used
For Exchange Online, Microsoft Teams, SharePoint Online and OneDrive for work or school, Microsoft's Service Level Agreement for Microsoft Online Services (WW), November 1, 2026 sets: below 99.9% = 25%, below 99% = 50%, below 95% = 100% of the monthly fees for that service, with uptime measured in user-minutes per calendar month. Each service has its own definition of downtime, so a service that was slow but usable may not count. Microsoft 365 Copilot is measured on failed prompts (below 99.5% = 25%, below 99% = 50%, below 95% = 100%), so a time log cannot place it in a tier. Other services get totals and uptime but no tier: check that provider's own SLA.
This is an estimate, not advice. Credits are not automatic: you submit a claim to Microsoft support, and for Microsoft 365 it must reach Microsoft by the end of the month after the Incident. Microsoft decides, typically within 45 days, and its outage notices do not confirm a credit is owed. Read 2026-11-13.
The incident log with UTC times, the per-service totals and the formula, ready to attach to an SLA claim or a post-incident review.
TL;DRWhat this calculator does
4 points
- Log each incident: service, start and end, the time zone the times are in, full outage or degraded (with the % of users affected) and an optional incident ID.
- Every time is converted to UTC with your browser's time zone database (daylight saving included), clipped to the calendar month you pick, and overlapping windows for the same service are merged so no minute counts twice.
- Per service you get total downtime minutes, user-minute weighted downtime for partial impact, monthly uptime % and, for Exchange Online, Teams, SharePoint Online and OneDrive, the tier in Microsoft's SLA (November 1, 2026 edition). It is an estimate, not advice.
- Free to use, signed out too. Everything runs in your browser and nothing is sent to us. Markdown and CSV exports of the log and totals are available with a paid plan.
Frequently asked questions
How do I calculate downtime for an SLA month?
Convert every incident to one time zone (UTC is safest), cut each one at the start and end of the calendar month, merge windows that overlap for the same service, and add up the minutes. Monthly uptime is then (minutes in the month minus downtime minutes) divided by minutes in the month, times 100. This calculator does each step and shows the merged windows so you can check them.
How many minutes of downtime does 99.9% uptime allow per month?
In a 30-day month (43,200 minutes) 99.9% allows 43.2 minutes, 99% allows 432 minutes (7.2 hours) and 95% allows 2,160 minutes (36 hours). A 31-day month allows a little more and February a little less: 99.9% of a 28-day February is 40.32 minutes. The calculator uses the real length of the month you pick.
Should overlapping incidents be counted twice?
No. If two incidents for the same service overlap, the minutes they share happened once, so they count once. The calculator merges them and shows how many minutes a simple sum would have double counted. Different services are kept separate, because each has its own service level.
How do I count a partial outage that hit only some users?
Weight it by the share of users affected. Microsoft measures Exchange Online, Teams, SharePoint Online and OneDrive uptime in user-minutes, so 60 minutes affecting 40% of users is 24 downtime minutes in that measure. Where two partial incidents overlap, the calculator uses the larger share rather than adding them, because they may affect the same people.
Which Microsoft 365 SLA tier applies to my outage?
For Exchange Online, Microsoft Teams, SharePoint Online and OneDrive for work or school, the Service Level Agreement for Microsoft Online Services (November 1, 2026 edition) gives 25% of that service's monthly fees below 99.9% uptime, 50% below 99% and 100% below 95%. Microsoft 365 Copilot is measured on failed prompts instead of time. Credits need a claim that reaches Microsoft by the end of the month after the Incident, and Microsoft decides.
How does the calculator handle time zones and daylight saving?
You pick the time zone the times were recorded in (UTC, US Eastern, Central or Pacific, UK, Central Europe, India, Japan or Sydney). The browser's built-in time zone database converts them to UTC for that exact date, so a clock change in the middle of an outage is handled. A time that happened twice when clocks went back uses the first occurrence, and the row tells you.
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