Ever hear the term “100-Year Flood Plain” or “50-Year Flood Plain” and wasn’t sure what it meant? It means that when flood maps are produced for a certain area, a place determined to be in a 100-year flood plain is expected to be flooded once every one hundred years. A 50 year flood plain means that the area is expected to be flooded once every 50 years.
If you want to find out the probability that a particular area will be flooded in any particular year, simply take that “year” number and divide it by 100. What’s the percentage chance of being flooded in any particular year in a 100-Year Flood Plain? 1%. How about 50 Year Flood Plain? 2%.
And what if there’s no chance (never say never!), then you’re just Not In A Flood Plain. 
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And to get the change of a flood over Y years where X is the chance in any one year (so X = 0.01 for a 100 year plain), the equation is:
1 - (1 - X)^Y
So over the life of a 30 year mortgage, it’s:
1 - 0.99^30 = 0.26
Or a 26% chance that you’ll experience at least one flood during that time.
Math is fun.
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