What is the difference between SLA, SLO, and SLI?

An SLI (Service Level Indicator) is the metric being measured (e.g. 99.95% successful requests). An SLO (Service Level Objective) is the internal team target (e.g. maintaining 99.9% availability). An SLA (Service Level Agreement) is the external contract with financial or service penalties if breached.

SRE & Architecture Guide• 8 min read • Updated August 2026

How to Calculate API SLA, Downtime & Error Budgets

Service Level Agreements (SLAs) define contractual availability guarantees between API providers and consumers. This guide covers the mathematical formulas, downtime conversions, and error budgeting strategies needed to maintain high reliability.

The Standard Uptime SLA Formula

Availability (%) = ((Total Period Seconds - Downtime Seconds) / Total Period Seconds) × 100

For example, in a 30-day billing cycle (2,592,000 total seconds), if your API experiences 25 minutes of confirmed downtime (1,500 seconds):
Availability = ((2,592,000 - 1,500) / 2,592,000) × 100 = 99.942%

The Cost of Nines: 99.9% vs 99.99%

Every additional "nine" of availability reduces allowed downtime by a factor of 10:

99.0% (Two Nines): Up to 7 hours and 18 minutes of downtime per month. Acceptable for internal prototypes.
99.9% (Three Nines): Up to 43 minutes and 49 seconds of downtime per month. The standard for most commercial SaaS products.
99.95%: Up to 21 minutes and 54 seconds of downtime per month. High-reliability standard for enterprise APIs.
99.99% (Four Nines): Only 4 minutes and 22 seconds of downtime per month. Requires automated failover and multi-region redundancy.
99.999% (Five Nines): Less than 26 seconds of downtime per month. Required for mission-critical telecommunications and banking rails.

What is an Error Budget?

An error budget is the inverse of your SLA target. If your SLO is 99.9%, your error budget is 0.1% (43.8 minutes per month). SRE teams use error budgets to balance development velocity with system stability:

Error Budget = 100% - SLA Target Percentage If Error Budget Remaining > 0%: Ship new product features and deployments. If Error Budget Exhausted (0%): Freeze feature releases and focus 100% on reliability engineering.

Frequently Asked Questions

Everything you need to know about API uptime, SLA calculations, and monitoring with Uptara.

The formula is: SLA % = ((Total Monitored Time - Confirmed Outage Duration) / Total Monitored Time) * 100. Both variables must be measured in the exact same time unit (seconds or minutes).