What does 40% chance of rain actually mean?
In short: a 40% chance of rain means that on four days in ten like this one, rain falls where you are during that hour. So there is a 60% chance it stays dry. It says nothing about how much rain, or for how long.
- It is the chance at your spot, not the share of the map that gets wet. That is how the Met Office defines it for UK forecasts.
- The Met Office counts rain from 0.1mm in the period; Weather for Dads marks its own forecasts at 0.2mm, when the pavement goes properly dark.
- Dry forecasts are the safer bet. In our marking from June to October 2026, a dry call stayed dry about 19 hours in 20; rain calls came true about half the time.
What does chance of rain mean?
The chance of rain is the likelihood that rain falls at your location during that period, usually one hour. Imagine today's weather playing out ten times over. If rain reached your garden on four of them, that is a 40% chance, and on the other six it stayed dry.
"Rain" has a bar behind it. The Met Office counts 0.1mm or more, which is a few drops more than nothing. When we mark forecasts at Weather for Dads we use 0.2mm, roughly the amount that turns the pavement properly dark.
What does 40% chance of rain not mean?
A 40% chance of rain does not describe the area, the duration or the amount of rain. Each of those is a common misreading, and one of them circulates widely online.
Not rain over 40% of the area
A 40% chance does not mean 40% of your town will get wet. The Met Office is clear that its percentage is the chance at any one place, such as your garden. The confusion comes from the United States, where the National Weather Service multiplies its confidence by the share of the area it expects to be hit, so an American 50% can mean "certain, over half of you".
Not rain for 40% of the time
An hourly 40% does not mean 24 minutes of rain. If rain comes, it might be a two-minute shower or the whole hour, because the chance and the duration are separate questions.
Not how much rain
An 80% chance of 0.1mm is a damp five minutes, while a 40% chance of 6mm will soak you to the skin. The percentage only answers "will it rain?". Weather for Dads shows the second question, "how wet will I get?", as one mark for a shower, two for wet and three for a soaking.
Not the share of water that falls out of the cloud
A popular claim says the number is the share of a cloud's water that falls as rain. No weather service has ever defined it that way.
Where does the percentage come from?
The percentage comes from running the forecast many times over, each run starting from slightly different conditions. Nobody can measure the whole atmosphere perfectly, so forecasters test how much small differences matter. If 40 of 100 runs bring rain to your spot that hour, that is roughly a 40% chance, which forecasters then adjust using how those runs have performed in the past.
What do 10%, 30%, 50% and 70% chance of rain mean in practice?
Below 30% you can usually carry on, and above 60% you should plan for rain. The table is how we read it.
| Chance | What it means | What to do |
|---|---|---|
| 10% | Dry nine times in ten | Washing out, mow the lawn |
| 30% | Probably dry, a shower is possible | Go ahead, keep an eye on the sky |
| 40% to 50% | A real toss-up | Check how heavy it would be before deciding |
| 70% | Rain more likely than not | Coat on, washing in |
| 90% | It is going to rain | Plan B |
The middle of the range is where the heaviness matters most. A 40% chance of a light shower is not worth changing plans for; a 40% chance of a downpour might be.
Does chance of rain mean the same on a weather app?
Yes, a weather app's chance of rain means the same thing, but the period it covers can differ. Some apps show the chance for each hour and others for the whole day, and 40% across a day is a much drier day than 40% in every hour of it. Apps also draw the line for "rain" in different places, which is one reason two of them can show different numbers for the same afternoon.
Do the percentages actually come true?
A well-calibrated forecast's 30% comes true on about 30% of the occasions it is given. Few forecasters publish that check about themselves. We mark every forecast we show against what happened, on the accuracy page, and we compared five forecasters in which UK weather forecast is most accurate.
So should you take a coat?
Take a coat above about 60%, carry on below 30%, and in between check how heavy the rain would be. Whatever the number, a dry forecast is more reliable than a rainy one: a shower can miss you by a street, while a dry day tends to stay dry. If the details of the screen are new to you, how to read Weather for Dads walks through each part.
Sources
Weather services: Met Office, what our forecasts mean.
Science bodies: Royal Meteorological Society, what does 30% chance of rain mean?
Our own figures: Weather for Dads accuracy page, forecasts marked 12 June to 10 October 2026 against Met Office observing stations.
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