Calculate your rain harvesting potential

Tell us where you want to harvest rain water and we'll look up the nearest weather data.

Metric Imperial

Catchment area

Monthly indoor demand

Tank size

The roof area is size of the catchment area you plan to use to collect rainwater. This is usually more or less the footprint of your house or whatever roof area that has a gutter system.

This is the amount of water you expect to use each month. You may intend to use your rainwater for something specific like a pool or garden irrigation so try your best to estimate your demand for that specific intention. If you hope to meet all domestic water needs, you can use the average water use per person per day in the United States, which is 80-100 gallons, or 363-454 liters (2400-3000 gallons or 10890-13620 liters per month per person).

This is the size of the tank you plan to use to store rainwater. If you live in a region with prolonged dry seasons, your tank may turn out to be one of the most limiting factors in your rainwater harvesting system. Capturing enough rain to last multiple months will be near impossible without an exceedingly large tank.

The roof area is size of the catchment area you plan to use to collect rainwater. This is usually more or less the footprint of your house or whatever roof area that has a gutter system.

Rain capture projections are merely rough estimates. The future is hard to predict. Learn more about how we calculate rainwater harvest projections.

How we calculate rain capture projections.

It's pretty straightforward to calculate the amount of rainwater you can capture given the size of your roof area and the amount of rainfall in your area. However, given the irregularity of rainfall, it's a bit more complicated to calculate how much rainwater you might be able to capture & use over the course of a year. By breaking down the rainfall data into monthly averages and taking your storage capacity into account, we try to calculate how much rainwater you can expect to capture each month, how much of your water demand you can meet with that captured rainwater, and how much supplemental water you may need to meet any remaining water demand.

To make the projection more conservative, we assume 20% of the rainwater your roof captures will be lost to evaporation, overflow, leaks, and other losses. We also assume that each year will rain 10% less than the previous year to account for climate change. Climate change obviously won't have a linear effect on rainfall, but this is a simple way to account for the fact that rainfall is likely to decrease over time.

We obtain rainfall data from Open Meteo's API using the past 10 years of data. We only use the last 10 years of data for two main reasons: 1) to minimize expensive API calls, and 2) due to accelerating climate change, we don't believe data from more than 10 years ago is necessarily relevant to future expectations.

The calculation result will display the minimum, maximum, and average rainfall per month over the past 10 years for your reference, but the projection formula only uses the monthly average to calculate the amount of rainwater you're expected to capture each month.

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