Uptime & SLA Calculator

Every additional nine divides allowed downtime by ten while typically multiplying the engineering cost to achieve it. That is why the jump from 99.9% to 99.99% is where most teams stop: 43 minutes a month can absorb a careless deploy and a manual rollback, whereas 4 minutes requires automated failover, tested rollback and alerting that fires before customers notice. Treating the allowance as an error budget rather than a target changes the conversation productively — if you are comfortably inside budget you can afford to ship faster and take more risk, and if you have burned it, that is the signal to slow down and invest in reliability. Note also that dependencies compound: three services each at 99.9%, all required, give roughly 99.7% overall.

Allowed downtime is the period length multiplied by (100% − availability). A month is taken as 30.4375 days and a year as 365.25, so the monthly and yearly figures are consistent with each other rather than drifting.

This models availability as a simple percentage of wall-clock time, which is rarely how a real SLA is written. Providers typically exclude scheduled maintenance, measure per-service rather than end-to-end, define downtime by their own monitoring, and compensate with service credits rather than refunds. Partial degradation — slow but not down — usually does not count at all. Read the actual contract before relying on any figure here.

99.9% allows about 43.8 minutes of downtime a month, or 8.77 hours a year. Adding one more nine, to 99.99%, cuts that to roughly 4.4 minutes a month — a tenfold reduction.

1. Enter your availability target, such as 99.9.
2. Pick the period you report against.
3. Optionally enter downtime already incurred.
4. Compare against the reference table to see what an extra nine would cost.

• Promising an SLA higher than your own dependencies can support.
• Forgetting that services in series multiply, lowering combined availability.
• Assuming scheduled maintenance counts as downtime — most SLAs exclude it.
• Treating the error budget as a target never to spend.

The Uptime & SLA Calculator converts an availability percentage into the downtime it actually permits — per day, week, month, quarter and year. Enter your real downtime and it also reports the availability you achieved and how much error budget remains.

SREs, platform engineers, and anyone negotiating or reporting against a service level agreement.

• Shows every period at once, so the numbers are easy to compare.
• Tracks error budget against real downtime, not just the target.
• Includes a reference table for the common availability tiers.
• Makes the tenfold cost of each extra nine immediately obvious.

• Working out what a 99.9% SLA actually commits you to.
• Checking whether this month’s incidents breached the target.
• Deciding whether an extra nine is worth the engineering cost.
• Explaining error budgets to a non-technical stakeholder.