The Common Myth
"A '100-year flood' only happens once every 100 years. Since a severe storm happened last year, I am entirely safe for the next 99 years. I don't need flood insurance!"
Why it spreads: It sounds like a countdown timer. Our brains naturally want to treat cyclic labels as scheduled intervals.
The Plain Math Reality
A "100-year flood" actually means there is a 1% chance of occurring in any single year. Every single year, the clock resets. It is the atmospheric equivalent of rolling a 100-sided die on January 1st.
Rolling a '1' on your giant d100 this year doesn't prevent you from rolling a '1' next year. Nature has no memory of past rolls!
The "N-Year Event" Decoder
How does the year rating translate to actual yearly probability? Use the slider below to change the interval rating. Watch how the real annual chance shifts and, more importantly, how risk compounds over time.
The Arse-Simple Formula
Simply divide 100 by your Event Interval. E.g., 100 / 100 = 1%. That is your percentage chance of flooding each and every year.
Every year starts with a fresh dice roll with this exact probability.
Chance of experiencing at least one such event within a decade.
Over 1 in 4 chance! That is a massive probability over standard home ownership.
Notice it's NOT 100%? There is still a 36.6% chance a century passes without any event!
Roll the Century Weather Dice
See reality in action. Let’s play out a full century (100 years). Below is a grid representing 100 individual years. When you hit play, we will roll a dice every single year based on your selected risk rating ().
Frequently Asked Questions for Arses
Why do they call it a "100-Year Flood" if it's so misleading?
Blame historical engineers! Decades ago, hydrologists and civil designers coined "100-Year Event" as a standard risk rating for designing bridges and culverts. They understood the mathematics of annual exceedance probability (AEP), but they severely underestimated how the general public would interpret it. It is one of the worst public relations naming decisions in modern history.
Is it possible to have two 100-year floods in consecutive years?
Yes, absolutely. Since each year is a completely independent event (like flipping a coin), having a flood in 2024 has zero impact on whether it floods in 2025. In fact, many regions have experienced multiple "100-year events" within just a few years. It's rare, but entirely statistically normal.
How does climate change affect these N-Year rankings?
These ratings are calculated using historical rainfall and water flow data. If severe storms become more frequent, the baseline shifts. What used to be a "100-year flood" (1% annual chance) might be re-classified as a "30-year flood" (3.3% annual chance) because severe rainfall occurs more often than it did in the 20th century.
What does a "500-Year Event" mean then?
A "500-year event" simply means it has a 0.2% chance (1 divided by 500) of occurring in any given year. It is an extremely rare, apocalyptic-level event, but it is still not a zero-percent chance.