Uptime & SLA Calculator
How much downtime each uptime percentage actually permits — per year, month, week and day — and what a chain of dependent services adds up to.
Downtime allowed is simply the complement of the uptime percentage applied to the period: 99.
How the uptime & sla calculator works
Downtime allowed is simply the complement of the uptime percentage applied to the period: 99.9% of a 365.25-day year leaves 8 hours 46 minutes. Each additional nine divides the allowance by ten, which is why the step from 99.9% to 99.99% is so much harder than it looks. When services depend on each other, availabilities multiply — three components at 99.9% each give 99.7% overall, because any one of them failing takes the whole chain down.
Formula: downtime = (1 − uptime%) × period; chained availability = a₁ × a₂ × … × aₙ
Worked examples
| Inputs | Downtime allowed per year | Note |
|---|---|---|
| 99.9% — three nines | 8 h 46 min | 8 h 46 min a year |
| 99.99% — four nines | 52 min 36 s | 52 minutes a year |
| Three services at 99.9% each | 8 h 46 min | 99.7% overall — about 26 hours a year |
FAQFrequently asked questions
What does 99.9% uptime mean?
Just under 9 hours of downtime a year, or about 43 minutes a month. It sounds strict until you realise a single long incident uses the whole budget.
How much harder is each extra nine?
Ten times. Going from 99.9% to 99.99% cuts the yearly allowance from nearly 9 hours to 52 minutes, which usually means removing every single point of failure.
Why is my system less available than its parts?
Because dependencies multiply. Five services at 99.9% give 99.5% overall — any one failing takes everything down.
Does planned maintenance count?
It depends on the contract. Many SLAs exclude scheduled maintenance windows, which is worth checking before comparing two providers.
What uptime should I promise?
Less than you can achieve. Promise 99.9% and deliver 99.99%; the reverse costs credits and trust.
Where these figures come from
- IEC 80000-13 — Information science and technology (quantities and units) — the decimal (kB, MB) versus binary (KiB, MiB) prefixes used throughout
- RFC 4632 — Classless Inter-domain Routing (CIDR) — the address-plan arithmetic behind the subnet calculator
- RFC 1918 — Address Allocation for Private Internets — the private ranges the subnet calculator recognises
- NIST SP 800-63B — Digital Identity Guidelines, Authentication — length over composition rules; the basis of the password guidance here
- NIST SP 800-57 Part 1 — Recommendation for Key Management — key-strength comparisons used by the key-space calculator
- National Cyber Security Centre — password guidance — UK national guidance on password policy
Last checked: September 2026. Units follow the SI decimal convention (IEC 80000-13 defines the binary alternatives); network and security figures cite the defining standard.