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.

Enter latitude in decimal degrees
Enter longitude in decimal degrees; use negative values for Western Hemisphere
Choose the duration of interest (Currently only 24 hr data is available. More durations coming soon!)
Retrieval Type
Please choose if you would like to find the AEP for a precipitation accumulation or the precipitation accumulation that corresponds to an AEP
Precipitation (inches)
Enter precipitation amount
Annual Exceedance Probability
Enter AEP (See interpretive look-up table below)




Additional Information on Annual Exceedance Probabilities and Average Recurrence Intervals:

(scientific notation)
Percent chance
in any given year
Odds in any
given year
1.6321.00E-00100%1 in 1
2.3932.00E-0050%1 in 2
5.1815.00E-0020%1 in 5
100.9511.00E-0110%1 in 10
250.0402.50E-014%1 in 25
500.0205.00E-012%1 in 50
1000.0101.00E-021%1 in 100
2000.0052.00E-020.5%1 in 200
5000.0025.00E-020.2%1 in 500
1,000 0.0011.00E-030.1%1 in 1,000
10,000 0.00011.00E-040.01%1 in 10,000
100,000 0.000011.00E-050.001%1 in 100,000
1,000,000 0.0000011.00E-060.0001%1 in 1 million
10,000,000 0.00000011.00E-070.00001%1 in 10 million
100,000,000 0.000000011.00E-080.000001%1 in 100 million
1,000,000,000 0.0000000011.00E-090.0000001%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:

ari aep

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.