Single Loss Expectancy (SLE)

Single Loss Expectancy (SLE) is the monetary loss expected from a single occurrence of a specific risk against a specific asset. It is calculated as asset value (AV) multiplied by exposure factor (EF): SLE = AV x EF. The exposure factor is the percentage of the asset’s value destroyed by one incident. A data centre worth 2,000,000 with a flood exposure factor of 25 per cent gives an SLE of 500,000.

The nuance sits in the exposure factor. EF is rarely 100 per cent: most incidents damage part of an asset rather than all of it, and the exam expects you to apply the stated percentage, not assume total loss. Asset value should also reflect the full value to the organisation (replacement cost, lost revenue, reputation), not just the purchase price. SLE says nothing about likelihood; it deliberately ignores how often the event occurs, which is the job of the Annualized Rate of Occurrence.

Exam relevance: if a scenario gives an asset value and a percentage of damage, multiply them: that is SLE. If the question then adds a frequency per year, it has moved on to Annualized Loss Expectancy, which is SLE x ARO. The confusable is exactly that pair: SLE is per incident, ALE is per year, and wrong answer options usually swap them. Both figures feed the cost-benefit case for whichever risk treatment is chosen.