Weather and Garden: Wind Chill, Heat Index and Watering
The numbers behind the weather and a thriving garden — how wind chill and heat index work, what dew point tells you, when each becomes dangerous, and how to water and harvest rain.
Rainwater Harvesting Calculator
Verified formula Updated Jun 2026 Private — runs on your device
Verified formula
Private
Water harvested
4,250
For general information only — not financial, tax, legal or medical advice. Verify before you rely on it.
How to use the Rainwater Harvesting Calculator
The Rainwater Harvesting Calculator works out your water harvested in an instant. Enter roof catchment area, rainfall and runoff coefficient and the result updates as you type — it is free, needs no sign-up, and runs entirely in your browser so your figures stay private.
- Enter the roof catchment area.
- Enter the rainfall.
- Enter the runoff coefficient.
- Read off your water harvested — the calculator updates automatically, with no button to press.
Formula
The Rainwater Harvesting Calculator uses the formula:
Water harvested = Roof catchment area × Rainfall × Runoff coefficient
Worked example
For example, with roof catchment area of 100 m², rainfall of 50 mm and runoff coefficient of 0.85, the water harvested is 4,250.
| Roof catchment area | 100 m² |
|---|---|
| Rainfall | 50 mm |
| Runoff coefficient | 0.85 |
| Water harvested | 4,250 |
|---|
Results are estimates for educational use, not professional advice.
Key terms explained
- Area
- The amount of two-dimensional space a shape covers, measured in square units.
Frequently asked questions
Multiply roof area (m²) by rainfall (mm) and a runoff factor. 100 m² with 50 mm of rain at 0.85 runoff collects about 4,250 litres.
It is the fraction of rain that reaches your tank after losses to splashing, leaks and absorption. Smooth roofs are about 0.8–0.9.
Rain falls on the horizontal area the roof covers, so use the plan (footprint) area, not the sloped surface area.
Size it to your typical rainfall events and usage. A single heavy storm can fill a large tank quickly, so plan for overflow.
The Rainwater Harvesting Calculator uses the formula: Water harvested = Roof catchment area × Rainfall × Runoff coefficient. For example, with roof catchment area of 100 m², rainfall of 50 mm and runoff coefficient of 0.85, the water harvested is 4,250.
Enter the roof catchment area. Enter the rainfall. Enter the runoff coefficient. Read off your water harvested — the calculator updates automatically, with no button to press.
Related guides
Rainwater Harvesting Table: Water harvested by Roof catchment area
Reference table of water harvested for Rainwater Harvesting across a range of roof catchment area values — exact, engine-computed figures you can read off at a glance.
How to Calculate Rainwater Harvesting: Formula, Steps & Examples
Learn how to calculate Rainwater Harvesting — the formula explained step by step, with worked examples and a free calculator to check your answer.
Dew Point Table: Dew point (approx.) by Air temperature
Reference table of dew point (approx.) for Dew Point across a range of air temperature values — exact, engine-computed figures you can read off at a glance.