# Average Exceedance Probability (AEP) Calculator

Have you ever wondered just how rare a certain rainfall event is in your area? On this page, you can try out our average exceedance probability (AEP) calculator, which takes a user-supplied precipitation value, at a given location for a certain duration and converts it into an equivalent AEP value/probability. Or you can provide an AEP and the calculator will compute the equivalent precipitation (in inches).

For more information about AEPs and how we use it here at MetStat, explore our AEP/ARI page and this blog post. **Also, refer to the table below for translating between AEPs and other common nomenclature of extreme events.**

Need to find the latitude and longitude for a given location? Look it up here.

**Additional Information on Annual Exceedance Probabilities and ****Average Recurrence Intervals****:**

ARI (years) | AEP | AEP (scientific notation) | Percent chance in any given year | Odds in any given year |
---|---|---|---|---|

1 | .632 | 1.00E-00 | 100% | 1 in 1 |

2 | .393 | 2.00E-00 | 50% | 1 in 2 |

5 | .181 | 5.00E-00 | 20% | 1 in 5 |

10 | 0.951 | 1.00E-01 | 10% | 1 in 10 |

25 | 0.040 | 2.50E-01 | 4% | 1 in 25 |

50 | 0.020 | 5.00E-01 | 2% | 1 in 50 |

100 | 0.010 | 1.00E-02 | 1% | 1 in 100 |

200 | 0.005 | 2.00E-02 | 0.5% | 1 in 200 |

500 | 0.002 | 5.00E-02 | 0.2% | 1 in 500 |

1,000 | 0.001 | 1.00E-03 | 0.1% | 1 in 1,000 |

10,000 | 0.0001 | 1.00E-04 | 0.01% | 1 in 10,000 |

100,000 | 0.00001 | 1.00E-05 | 0.001% | 1 in 100,000 |

1,000,000 | 0.000001 | 1.00E-06 | 0.0001% | 1 in 1 million |

10,000,000 | 0.0000001 | 1.00E-07 | 0.00001% | 1 in 10 million |

100,000,000 | 0.00000001 | 1.00E-08 | 0.000001% | 1 in 100 million |

1,000,000,000 | 0.000000001 | 1.00E-09 | 0.0000001% | 1 in 1 billion |

AEP is defined as the “probability that a given rainfall total accumulated over a given duration will be exceeded in any one year.” Keep in mind we can’t always use the simple relationship ARI = 1/AEP for frequent events; ARIs greater than 10 years are very closely approximated by 1/AEP (the reciprocal of the AEP). AEP is defined as the “probability that a given rainfall total accumulated over a given duration will be exceeded in any one year.” AEP relates to ARI as follows:

For a more information, see Laurenson, E.M. (1987). Back to Basics on Flood Frequency Analysis by E.M. Laurenson, Civil Engineering Transactions, 1987, pp. 47 to 53.