Calculator
Rainwater calculator
A moderate rain gives a surprising amount of water if you have somewhere to collect it. Enter the roof area and the amount of rainfall.
What counts is the horizontal projection, not the sloping roof surface: a steep roof sheds as much water as a flat one if it is the same size seen from above. Collected rainwater is not drinking water, so treat it as the water purification calculator shows.
What it is good for, and what it is not
This calculator comes from the PrepperPapa app, where it is free too. The calculation runs in your browser: we send nothing anywhere and store no data about it.
PrepperPapa app
The same in your pocket, without internet
These calculators are in the app too, free, and they work without a network as well. Alongside them: stock, emergency card, compass and knowledge base.
See what it can do