Rainy Day Percent Calculator
Turn rainy days and total days into rain percent, dry days, and dry percent.
What it does
Rain frequency reads cleaner as a percent than as scattered memories. Nine rainy days in a 30 day month is 30% wet and 70% dry, which plans outdoor work better than it rained a lot does.
The tool above takes rainy days and total days. It returns rain percent, dry days, and dry percent, so months, seasons, and trips compare on one scale.
This math fits counted days with a fixed rain threshold. It does not fit rainfall amounts, storm intensity, or forecast probabilities, which answer different questions.
Climate data programs frame the same count: daily observations archived into normals that show how often rain falls. Forecast pages then give the chance for single days ahead.
Fix the threshold before counting. A trace versus a tenth of an inch changes rainy totals for the same month.
How to use
- Count days meeting your rain threshold.
- Enter total days in the period.
- Read rain percent first for the frequency line.
- Read dry days for the workable count.
- Read dry percent for the complement check.
- Compare months with matching thresholds only.
Formula
- rainy
- Days meeting the rain threshold.
- total
- All days in the counted period.
- dry
- Days below the threshold.
Rainy cannot top total days. Zero rainy gives 0% rain with all days dry. Threshold choice drives every comparison.
Worked example
Example: 9 rainy days of 30
Rainy days 9, total days 30.
- Rain: 9 / 30 x 100 = 30.0 %.
- Dry: 30 - 9 = 21 dry days.
- Dry percent: 100 - 30 = 70.0 %.
- Tool shows rain 30.0 %, dry 21 dry days, dry 70.0 %.
- Read: about one day in three needed rain gear.
Reference table
| Rainy days | Rain percent | Dry days |
|---|---|---|
| 5 | 16.7 % | 25 dry days |
| 9 | 30.0 % | 21 dry days |
| 15 | 50.0 % | 15 dry days |
Limits
- Frequency only. Amounts and intensity need gauge data.
- Threshold choice changes every result.
- Ignores streaks that matter more than shares.
- Forecast chance differs from counted frequency.
Common mistakes
- Mixing trace days with measurable days.
- Comparing months with different thresholds.
- Reading forecast percent as counted frequency.
- Counting travel days outside the period.
- Rounding days before dividing.
Try it yourself
Quick checks. Answers sit tucked under each question.
0 rainy of 31 days?
Rain 0.0 %, 31 dry days, dry 100.0 %.
12 rainy of 30 days?
Rain 40.0 %, 18 dry days, dry 60.0 %.
7 rainy of 14 days?
Rain 50.0 %, 7 dry days, dry 50.0 %.
Questions
How do I figure rain frequency?
Rainy days divided by total days times 100. 9 of 30 is 30%.
What counts as a rainy day?
Your fixed threshold, such as 0.01 inch at a gauge. Keep it constant.
What is dry percent?
100 minus rain percent. 30% rain leaves 70% dry.
Is this a forecast?
No. It summarizes counted days, not the chance for one day ahead.
Why do sources differ?
Thresholds and periods differ. Match both before comparing.
Where do long records live?
Climate archives publish daily observations and normals by station.