Single Loss Expectancy (SLE): Formula, vs ALE
TL;DR · the short version
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 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.
Frequently asked questions
What is single loss expectancy?
Single Loss Expectancy (SLE) is the expected cost of a single occurrence of a risk. It is calculated as Asset Value × Exposure Factor, where the Exposure Factor is the fraction of the asset lost in one event — so a $500,000 asset with a 0.4 exposure factor has an SLE of $200,000. SLE answers "how bad is one event?"; multiply it by the Annualized Rate of Occurrence (ARO) to get Annual Loss Expectancy (ALE), the expected cost per year.
What is the difference between SLE and ALE?
SLE (Single Loss Expectancy) is the cost of one occurrence of a risk. ALE (Annual Loss Expectancy) is the expected cost of that risk over a whole year. They are linked by the Annualized Rate of Occurrence (ARO): ALE = SLE × ARO. SLE describes the severity of a single event; ALE annualizes it by frequency so you can compare it to yearly control costs.
What are the formulas?
SLE = Asset Value × Exposure Factor, where Exposure Factor is the fraction of the asset lost in one event. ALE = SLE × ARO, where ARO is the expected number of occurrences per year. So ALE = (Asset Value × Exposure Factor) × ARO.
When do I use SLE vs ALE?
Use SLE when you need the impact of a single incident — for example sizing worst-case damage, or an insurance limit for one event. Use ALE when you need to compare a risk against the annual cost of a safeguard, or to rank risks by expected yearly cost. SLE is about severity; ALE is about severity times frequency.
Why does ARO matter so much?
ARO is the frequency that converts a single-event cost (SLE) into an annual expectation (ALE), so it can completely change the picture: a high-severity event that almost never happens can have a low ALE, while a moderate event that happens often can have a high ALE. Because ARO is also the hardest input to estimate for rare events, it is where most of the uncertainty in an ALE lives.