Uptime & SLA Downtime Calculator
Turn an uptime percentage into the downtime it really allows.
Enter an availability percentage to see the downtime it permits across every period, or switch to the other direction and convert observed downtime into the availability figure it represents.
Handy for reading a hosting SLA honestly: 99.9% sounds close to perfect, and allows nearly nine hours of outage a year.
How it works
Availability is simply uptime divided by total time. The arithmetic is trivial; the reason this calculator exists is that the numbers are wildly counter-intuitive. Each extra nine cuts the permitted downtime by a factor of ten, so the gap between 99% and 99.99% is not a rounding difference — it is three and a half days a year against fifty-two minutes.
The usual reference points, per year: 99% ("two nines") allows 3 days 15 hours. 99.9% allows 8 hours 45 minutes. 99.99% allows 52 minutes. 99.999% — "five nines", the figure telecoms quote — allows just 5 minutes and 15 seconds across an entire year, which is less than a single unplanned reboot.
What matters as much as the percentage is the measurement window. An SLA promising 99.9% monthly permits about 43 minutes a month, and a bad month cannot be offset by a good one. The same 99.9% measured annually permits 8¾ hours in a single stretch and still counts as met. Shared hosting is typically quoted monthly; read which one you are being offered.
Read the exclusions too. Almost every SLA excludes scheduled maintenance, outages caused by the customer's own code, DDoS attacks, and anything upstream of the provider's network. Those carve-outs are frequently where the real downtime lives. And note what the remedy is: nearly all hosting SLAs pay out as service credits proportional to the breach, not as compensation for lost revenue — so a day of downtime on a ₹500 plan is worth a few rupees of credit, whatever it cost your business.
Finally, be careful about compounding. If your site depends on a web host, a database service and a CDN, each at 99.9%, the realistic combined availability is the product of the three — about 99.7%, or roughly a full day a year. Adding dependencies lowers availability unless each one is genuinely redundant.
Frequently asked questions
How much downtime does 99.9% uptime allow?
About 8 hours 45 minutes a year, 43 minutes 28 seconds a month, or 1 minute 26 seconds a day. Whether that is acceptable depends entirely on whether it arrives as one long outage or as seconds spread across the year.
What does "five nines" mean?
99.999% availability — 5 minutes 15 seconds of downtime per year. It is a telecoms standard that demands full redundancy at every layer, and it is essentially never offered on shared hosting whatever the marketing page says.
Is uptime measured monthly or annually?
It depends on the contract, and the difference is large. A monthly window caps each individual month, so a bad month is a breach on its own. An annual window lets a single long outage be absorbed by the rest of the year. Check which applies before comparing two providers.
Does scheduled maintenance count as downtime?
Usually not. Most SLAs exclude maintenance announced in advance, along with customer-caused faults, DDoS attacks and upstream network failures. Those exclusions often matter more to your real availability than the headline percentage.
What happens if my host misses its SLA?
You typically receive service credits — a percentage of that month's fee — and you usually have to claim them within a stated window rather than being credited automatically. Consequential losses are almost always excluded, so the payout rarely reflects the business cost.
How do I combine the uptime of several services?
Multiply them, if each is a hard dependency. Three services at 99.9% give 99.7% combined, roughly a day a year. Redundancy inverts this: two independent components each at 99% give 99.99% availability provided either alone can serve traffic.
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