Enter the number of floods and the total period of time into the calculator to determine the flood probability. This calculator can also evaluate any of the variables given the others are known.

Flood Probability Formula

The following formula is used to calculate the flood probability. 

FP = (NF / TP) * 100

Variables:

  • FP is the flood probability (%)
  • NF is the number of floods that have occurred in the area in the past
  • TP is the total period of time being considered (usually in years)

To calculate the flood probability, divide the number of floods that have occurred in the area in the past by the total period of time being considered. Multiply the result by 100 to convert it into a percentage. This will give you the probability of a flood occurring in any given year, based on historical data.

What is a Flood Probability?

Flood probability refers to the statistical likelihood of a flood occurring in a specific area within a certain time period. It is usually expressed as a percentage or a ratio, such as a “1 in 100” chance, indicating a 1% chance of a flood occurring in any given year. This calculation is based on historical flood data, geographical and environmental factors, and can be used to inform flood risk assessments, urban planning, and insurance policies.

How to Calculate Flood Probability?

The following steps outline how to calculate the Flood Probability.

  1. First, determine the number of floods that have occurred in the area in the past (NF).
  2. Next, determine the total period of time being considered (TP), usually in years.
  3. Next, use the formula FP = (NF / TP) * 100 to calculate the Flood Probability.
  4. Finally, calculate the Flood Probability by inserting the values of NF and TP into the formula.
  5. After calculating the result, check your answer with the calculator above.

Example Problem:

Use the following variables as an example problem to test your knowledge.

Number of floods (NF) = 5

Total period of time (TP) = 10 years