Rainwater Harvest
One millimetre of rain is one litre per square metre. Multiply roof area by rainfall for what fell, then take about 85% after splash and wetting losses.
Your numbers
Water collected
2,040L
Litres
2,400L
Weight
2.04t
Formula
Water collected = Roof area × Rainfall (mm) × 0.85
Use the horizontal plan area of the roof, not the sloped surface — rain falls vertically.
What to enter
| Input | Default | Accepted range |
|---|---|---|
| Roof area (m²)The roof’s horizontal plan area, seen from above. | 80 | 0 and up |
| Rainfall (mm) (mm)The depth of rain that fell; 1 mm is 1 L per square metre. | 30 | 0 and up |
Step by step
Quick reference table
Results when only Roof area (m²) changes and everything else stays put.
| Roof area (m²) | Water collected (L) | Litres (L) | Weight (t) |
|---|---|---|---|
| 40 | 1,020 | 1,200 | 1.02 |
| 60 | 1,530 | 1,800 | 1.53 |
| 80 | 2,040 | 2,400 | 2.04 |
| 120 | 3,060 | 3,600 | 3.06 |
| 160 | 4,080 | 4,800 | 4.08 |
What each result means
| Result | At default values |
|---|---|
| Water collected (L)The water you can actually collect after losses. | 2,040 |
| Litres (L)Capacity in litres; one litre is 1,000 cm³. | 2,400 |
| Weight (t)Weight in tonnes; one tonne is 1,000 kg. | 2.04 |
Common mistakes
Use the horizontal plan area of the roof, not the sloped surface — rain falls vertically.
Glossary
- Roof area
- The roof’s horizontal plan area, seen from above.
- Rainfall (mm)
- The depth of rain that fell; 1 mm is 1 L per square metre.
- Water collected
- The water you can actually collect after losses.
- Litres
- Capacity in litres; one litre is 1,000 cm³.
- Weight
- Weight in tonnes; one tonne is 1,000 kg.
Frequently asked questions
Q. How is Rainwater Harvest calculated?
Water collected = Roof area × Rainfall (mm) × 0.85 — One millimetre of rain is one litre per square metre. Multiply roof area by rainfall for what fell, then take about 85% after splash and wetting losses.
Q. Can you walk through an example?
With Roof area 80m², Rainfall (mm) 30mm, the answer is Water collected 2,040L.
Q. What do I need to enter?
Enter Roof area, Rainfall (mm). The result recalculates as you type, and an empty box counts as zero.
Q. How much does the answer move if I change a number?
Changing only Roof area (m²) moves the answer to Roof area (m²) 40 → Water collected (L) 1,020 and Roof area (m²) 160 → Water collected (L) 4,080. The table below lays out five steps.
Q. How are the numbers rounded?
Money is shown to the nearest whole unit, percentages to one decimal place and everything else to two. What you see is rounded; the calculation itself carries the unrounded value forward.
Q. Anything to watch out for?
Use the horizontal plan area of the roof, not the sloped surface — rain falls vertically.
Related calculators
Keep your length units consistent across inputs — mixing centimetres and metres throws results far off.