Single Loss Expectancy (SLE) vs Annual Loss Expectancy (ALE)
Single Loss Expectancy (SLE) and Annual Loss Expectancy (ALE) are two of the core numbers in quantitative risk analysis, and they are often confused. SLE is the cost of a single occurrence of a risk — Asset Value × Exposure Factor. ALE is the expected cost over a year — SLE × Annualized Rate of Occurrence (ARO). In other words, SLE answers "how bad is one event?" and ALE answers "how much should we expect this risk to cost us per year?", with ARO (frequency) being the bridge between them. You use SLE to size a single incident and ALE to compare a risk against the annual cost of a control. This guide lays out both formulas and when each applies.
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.
Work both numbers for your own scenarios with the free ALE / SLE calculator →.
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.
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