What does 'chance of rain' mean on a BOM forecast?
The percentage is more precise than it looks — and it isn't telling you what you think it is.
You open the Bureau of Meteorology app, see "70% chance of rain", and hesitate over the washing. But that percentage is more precise than most people realise — and it isn't telling you what you probably think it is.
What exactly does the percentage measure?
On a Bureau of Meteorology (BOM) forecast, the "chance of any rain" is a probability of precipitation: the likelihood that at least a measurable amount of rain will fall at your specific location during the forecast period. A 70% chance means that, given the atmospheric conditions the Bureau is forecasting, roughly seven times out of ten you would record rain at that spot. It is a statement of confidence, not of coverage, duration or intensity.
Crucially, the figure is a point probability. It refers to the chance of rain reaching your location, not the chance that it rains somewhere in the general area. That distinction matters: a scattered-shower day can carry a modest percentage precisely because any single point is unlikely to be under a shower at the critical moment, even though the region as a whole will clearly get wet.
Why 0.2 mm? What counts as "rain"
The Bureau's threshold for "rain" is at least 0.2 millimetres in the forecast period. That is the smallest amount a standard rain gauge reliably records, so it is the natural cut-off for "did it rain or not". A 90% chance of rain that delivers 0.3 mm of drizzle has still verified as a hit, even though it wouldn't trouble your umbrella. This is why a high percentage sometimes feels like an overreaction: the probability was about occurrence, not significance.
Chance of rain versus how much rain
BOM forecasts usually give you two separate pieces of information, and conflating them is the most common mistake. The percentage answers "will it rain?". A separate figure — the possible rainfall range, such as "5 to 15 mm" — answers "how much?". These are calculated differently and should be read together.
The Bureau's convention for that amount range is specific: the lower figure is the total that has about a 50% chance of being reached or exceeded, and the higher figure has roughly a 25% chance. So "5 to 15 mm" means a coin-flip on getting at least 5 mm, and only a one-in-four shot at 15 mm or more. A day can show a high chance of any rain but a low amount range (persistent drizzle), or a lower chance but a large possible amount (an isolated but heavy storm).
| Percentage | Plain-English reading | What to do with the washing |
|---|---|---|
| 0–10% | Rain very unlikely | Hang it out |
| 20–30% | Slight chance — keep an eye out | Probably fine, stay alert |
| 40–60% | Rain genuinely possible either way | Undercover or be ready to dash |
| 70–80% | Rain likely | Leave it in |
| 90–100% | Rain almost certain | Definitely in |
The Bureau pairs its percentages with words such as "slight chance", "possible", "likely" and "very likely"; the table above is a practical reading of those bands, not a substitute for the forecast text.
Does 70% mean it rains 70% of the day?
No. A 70% chance does not mean rain for 70% of the daylight hours, nor that 70% of your suburb gets wet. It is a single probability for the event "measurable rain at this location today". Ten minutes of rain and eight hours of rain both count as the same "yes". If you want a sense of when, the Bureau's hourly graphs in MetEye and the app break the day down; the headline percentage is a summary, not a timeline.
How the Bureau works out the number
Modern rain probabilities are built on numerical weather prediction. Rather than one deterministic run, forecasters lean heavily on ensemble forecasts — the same model run many times with slightly perturbed starting conditions. If 70 of 100 ensemble members produce measurable rain at your grid point, that is the raw basis for a 70% chance. Human forecasters then apply judgement, local knowledge and statistical calibration on top. The percentage is therefore a distilled measure of how many plausible versions of the day end up wet where you are.
Where model disagreement fits in
Because the chance of rain reflects spread across possible outcomes, it is really a confidence signal in disguise. That is exactly the idea behind checkweather.io's AU edition: instead of trusting one app's number, it lines up the major global models — the European ECMWF, America's GFS and Germany's ICON — and turns their agreement or disagreement into a plain-English confidence score, alongside a link to the Bureau's own outlook. When all three soak your Saturday, a "60% chance" is a confident 60%. When they split, that same number is genuinely a coin toss, and you should check again closer to the day.