Learn about how probabilities work and relate to flood risks in addition to how risk accumulates over time.

A probability is a measure of how likely something is to happen. It is often expressed as a percentage, with 100% meaning something will definitely happen, and 0% meaning something will definitely not happen.

 Probability of an event = # of ways it can happen total number of outcomes

For example, flipping a coin has two possible outcomes: heads or tails. There is a 50% chance a flipped coin will land on heads, and a 50% chance it will land on tails.

Annual and cumulative probabilities

An annual probability is the chance of something happening at least once within a specific, singular year. Flood Factor includes three annual probabilities: the current year, 15 years in the future, and 30 years in the future.

A cumulative probability is the chance of something happening at least once over the course of multiple years. Flood Factor includes two cumulative probabilities: the next 15 years and the next 30 years.

For example, imagine each coin flip represented a year, and you wanted to know the chance of the coin landing on heads once within two flips. That would be its two-year cumulative probability. To calculate this, you have to find all possible outcomes and identify how many outcomes include the coin landing on heads. In this case, there is a 50% annual chance of the coin landing on heads, and a 75% cumulative chance of the coin landing on heads at least once within two “years” (coin flips).

Probabilities on Flood Factor

Flood Factor uses data from the First Street Foundation Flood Model to determine a location’s annual and cumulative likelihood of flooding.

The model considers inputs such as a location’s flood history and its geographic information such as elevation, climate, proximity to water, and adaptation measures, to identify its likelihood of flooding in the current year, an annual probability. The model then incorporates projected environmental changes to forecast its risk of flooding in future years, which are also annual probabilities. Using these annual probabilities, the model is then able to calculate a location’s chance of flooding over multiple years, known as its cumulative probability.

Using the example above, if a home had a 50% annual chance of flooding, it would have a 75% chance of flooding at least once within 2 years, a 96.9% chance of flooding within 5 years, and a 99.9% chance of flooding within 10 years.

Multiple probabilities

To provide a complete flood risk analysis, Flood Factor calculates multiple annual  probabilities, 0.2%, 1%, 10%, 20%, 50%, which consider different depth scenarios. Lower probabilities represent deeper floods that are less likely to occur, but more likely to reach a building and cause greater damage.

In an earlier example, there was a 50% annual chance that a flood of some depth would occur and reach the home. But perhaps this flood is very shallow, and while it can damage the yard, it does not pose a significant threat to the home. That same home might have a 10% chance of experiencing a more extreme flood that is less likely to occur but more likely to cause damage.

Defining a low or high probability

Although the probabilities on Flood Factor may seem small or unlikely, having any chance of flooding is significant, especially given how risk accumulates over time. A home that only has a 1% chance of flooding this year, for example, has a 26% chance of flooding over 30 years– the average life of a mortgage.

 Annual Probability About as likely as Over 5 years Over 15 years Over 30 years 50% (1/2) coin landing on heads 96.9% 100% 100% 20% (1/5) food poisoning in a year 67.2% 96.5% 99.9% 10% (1/10) being born prematurely 41% 79.4% 95.8% 1% (1/100) being audited by the IRS 4.9% 14% 26% 0.2% (1/500) being born with 11 fingers 1% 3% 5.8%

See how other annual probabilities accumulate over time on weather.gov.