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Tech & ITSLA (Service Level Agreement)
By Sitraka Forler · Lecturer, Durham Business SchoolUpdated 13 September 2026 About this site
A contract promising a level of service, such as uptime, usually with penalties if missed.
An SLA is the promise made to customers, usually backed by service credits (money off a future bill) if it is broken. Engineers aim for a stricter internal service level objective (SLO), tracked with a service level indicator (SLI) such as the share of successful requests. Always check the period: 99.9% uptime allows 43.2 minutes of downtime in a 30-day month, but 8.76 hours over a 365-day year.
Example
minutes_in_month = 30 * 24 * 60 # 43,200 minutes
for target in (0.999, 0.9995, 0.9999):
allowed = minutes_in_month * (1 - target)
print(f"{target:.2%} uptime: {allowed:.2f} min of downtime allowed")
# 99.90% uptime: 43.20 min of downtime allowed
# 99.95% uptime: 21.60 min of downtime allowed
# 99.99% uptime: 4.32 min of downtime allowed