How to Use This Tool
Type an uptime percentage — the number your hosting provider, cloud vendor, or internal SLA promises, such as 99.9% or 99.99% — or click one of the quick-select chips for a common tier. The three cards above instantly convert that percentage into concrete allowed downtime for a single day, a 30-day month, and a 365-day year, so you can see exactly what a promise like "three nines" actually means in practice rather than just comparing abstract percentages.
The reference table below the cards lists several common SLA tiers side by side, which is useful when you're comparing quotes from different vendors that each advertise a slightly different number of nines.
Why "One More Nine" Matters So Much
Uptime percentages compress an enormous range of real-world downtime into a small set of digits, and each additional "nine" shrinks the allowed downtime by roughly a factor of ten. Moving from 99% to 99.9% cuts allowed yearly downtime from about 3.65 days down to under 9 hours; moving again to 99.99% brings that down to under an hour a year. That's why SLA tiers above 99.9% typically come with a much higher price tag — achieving them usually requires redundant infrastructure across multiple availability zones or regions, automated failover, and round-the-clock monitoring.
- 99% ("two nines") — about 3.65 days of downtime a year. Acceptable for internal tools or low-stakes side projects.
- 99.9% ("three nines") — about 8.75 hours a year. A common baseline for production business applications.
- 99.99% ("four nines") — about 52 minutes a year. Typical for services where downtime has a direct revenue or safety impact.
- 99.999% ("five nines") — about 5 minutes a year. Reserved for critical infrastructure like telecom switching or core financial systems.
When you're evaluating a vendor's SLA, it's worth checking not just the percentage but what counts as "downtime" in their contract, and what remedy — if any — you're entitled to if they miss it. Pairing an uptime target with a realistic cost estimate for the redundancy it requires will usually explain why the jump from three nines to four nines costs so much more than the numbers alone suggest.