The Frugal Off Grid Water Calculator
This rainwater harvesting calculator helps estimate how much water you can collect from a roof or other catchment surface based on local rainfall and collection area.
It also includes tools for estimating pond or underground cistern capacity and calculating approximate liner size. I use calculations like these when planning water systems on my own off-grid homestead.
If you want to see one of the cheapest water-storage options I've used, this is the cistern writeup:
How to Build a Cheap Water Cistern for Rainwater Storage
See the Water System in Real Life
If you are new to rainwater harvesting, watch the video first so the numbers make sense in real life. Then come back here and run your own roof size and rainfall so you can plan with reality instead of hope.
These are simple water calculators I use when planning off-grid systems. They are meant to provide useful planning numbers before you spend money on storage, liners, pumps, or other infrastructure.
Rainwater Harvesting Calculator
Estimate how much rainwater you can collect from a roof based on catchment area and annual rainfall.
Note: This is a rough estimate. Actual collection depends on roof material, gutters, first-flush diversion, overflow, evaporation, and other losses.
Pond and Underground Cistern Capacity Calculator
Estimate how many gallons will fit in a simple rectangular pond or underground cistern based on length, width, and depth.
Note: This assumes a rectangular shape. Sloped pond sides and irregular excavations will change the actual capacity.
Pond and Cistern Liner Size Calculator
Estimate liner size for a pond, underground cistern, or similar lined excavation. This uses the length, width, and depth entered above.
Use the Calculator as Part of a Larger Water Plan
A calculator can estimate how much rain falls on a roof or how much water a cistern can hold. The larger question is how collection, storage, distribution, conservation, and daily use work together.
Explore Off-Grid Water Systems
If you're new to off-grid living and want the broader step-by-step path:
If you want the larger collection of guides, videos, diagrams, and resources: