# Single Loss Expectancy (SLE) vs Annual Loss Expectancy (ALE)

> SLE is the cost of one security incident; ALE is the expected cost per year. How the two relate through ARO, the formulas, and when to use each.

Source: https://playciso.com/blog/single-loss-expectancy-vs-ale · Published: 2026-09-26 · Publisher: PlayCISO (https://playciso.com)

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SLE and ALE are constantly mixed up, and the difference is simple once you see it: **SLE is the cost of one event; ALE is the cost per year.** The bridge between them is how often the event happens. Get that relationship straight and quantitative risk analysis stops being intimidating. ## The two formulas - SLE = Asset Value × Exposure Factor. The cost of a single occurrence, where the Exposure Factor is the fraction of the asset lost in one event. - ALE = SLE × ARO. The expected cost per year, where ARO (Annualized Rate of Occurrence) is how many times per year you expect it. So ALE = (Asset Value × Exposure Factor) × ARO. SLE answers "how bad is one?"; ALE answers "how much per year should we plan for?" ## A quick example A $500,000 asset with a 0.4 Exposure Factor has an **SLE of $200,000** — that is the cost of one incident. If it happens once every five years (**ARO 0.2**), the **ALE is $40,000/year**. Same event, two different questions answered. ## When to use each - Use SLE to size a single incident — worst-case damage, a per-event insurance limit, or the "how bad is one breach?" conversation. - Use ALE to compare a risk against the annual cost of a control, or to rank risks by expected yearly cost. ## Why ARO carries the uncertainty ARO is the frequency that turns SLE into ALE, and it can flip the conclusion: a catastrophic-but-rare event can have a modest ALE, while a moderate-but-frequent one can dominate your annual expected loss. It is also the hardest input to estimate for rare events, so it is where most of an ALE's uncertainty lives — which is why ALE should guide decisions alongside scenario analysis, not replace it. For the full ALE walkthrough see [Annual Loss Expectancy explained](/blog/annual-loss-expectancy-explained). Work both numbers for your own scenarios with the free [ALE / SLE calculator →](/tools/fair-ale). ## Frequently asked questions **Difference between SLE and ALE?** SLE is the cost of one event; ALE is the expected cost per year. ALE = SLE × ARO. **Formulas?** SLE = Asset Value × Exposure Factor; ALE = SLE × ARO. **When to use each?** SLE for single-incident severity; ALE to compare against annual control costs and rank risks. **Why does ARO matter?** It converts single-event cost to an annual expectation and holds most of the estimate's uncertainty.