Rain Probability Calculator
Calculate the probability of rain during any outdoor event from duration and local daily rain chance.
Uses the complement rule for cumulative risk.
Rain probability for events helps you plan outdoor activities by calculating the cumulative chance of rain over multiple hours or days.
The math: If there is a 30% chance of rain on any given day, many people assume a 2-day event has a 60% chance of rain. That is incorrect. The correct calculation uses complementary probability:
Formula: P(rain at least once) = 1 - (1 - P_daily)^n
Where:
- P_daily = probability of rain on a single day (as a decimal)
- n = number of days (or time periods)
- P(rain at least once) = cumulative probability
Example: If there is a 30% chance of rain each day during a 3-day camping trip:
- P(no rain all 3 days) = (1 - 0.30)³ = 0.70³ = 0.343
- P(rain at least once) = 1 - 0.343 = 0.657 = 65.7%
So there is nearly a two-thirds chance of seeing rain at some point during the trip, more than twice what the 30% daily forecast suggests to most people reading it.
For hourly planning: A daily forecast covers a window of roughly 12 hours in which rain realistically falls. If you are outside for 6 of them, you are exposed to about half the day’s risk, so the calculator spreads the daily probability across that window and takes your share of it.
Two limits worth knowing. Rain is not spread evenly through the day, so an afternoon event in a region with afternoon storms carries more than its share. And your exposure caps at the window itself: standing outside for 24 hours cannot be riskier than the day being rainy at all, which is a ceiling the arithmetic has to be told about.
How to use this for event planning:
- Under 25%: Low risk, proceed without rain backup
- 25–50%: Moderate risk, have a backup plan ready
- 50–75%: High risk, strongly consider indoor alternative
- Over 75%: Very high risk, plan for rain or reschedule
Tip: Check the forecast the day before and morning of your event. Weather predictions become significantly more accurate within 24–48 hours. A 30% chance today might become 10% or 80% by tomorrow.
This calculator assumes each day’s weather is independent, which is a simplification. In reality, weather patterns often persist for multiple days, meaning consecutive rainy days are more common than pure probability suggests.
How we build and check this calculator
This calculator runs entirely in your browser, so the numbers you enter stay on your device. The math behind it is written by hand and tested against worked examples and standard references before the page goes live.
SuperGlobalCalculator is independently built and maintained. See how we build and verify our calculators.