Rainwater Harvesting Calculator

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In order to meet all your water needs in any emergency, we recommend installing a rainwater harvesting system. It is the easiest and most efficient way to collect rainwater on your land plot. The system includes only a few details: a roof of your house or shed, gutters, a downspout, and a rain barrel. In other words, you simply take advantage of rain falling on your roof and save the water for later when there will be a drought in your living area.

How much water should you expect to collect by using this system? There is a fast way to find this out! Use our rainwater harvesting calculator to predict the total amount of water collected depending on your roof area and the rainfall level. Keep reading this article to find out about the calculation in the smallest detail.

Why do you need to collect rainwater?

Before we talk about building a system for collecting rainwater and counting the amount of water needed, you need to understand whether or not you need this system.

If you live in a state where precipitation is unstable and, therefore, drought is a frequent occurrence (such as Texas), rainwater will be in high demand. Imagine that it is a hot summer, and for several weeks, the rainfall total is zero. Nevertheless, you maintain a vegetable garden and want to collect harvested vegetables, fruits, and grains in the near future. What to do in this situation? You will have to save rainwater in advance so that you always have water supplies for watering the cultivated plants.

In addition, your land plot may be located at a distance from the city and civilization in general, so you do not have access to a public water system. It is in this case that the collection of rainwater is especially necessary.

How to collect rainwater?

If you understand that you need rainwater supplies, you should definitely design a rainwater harvesting system. In fact, it is extremely simple.

If you live in an area where rainwater supply is in high demand, you can take advantage of gravity. All you need to do is create a system through which water will flow from the slope of your roof to a rain barrel or tank made of wood or plastic. You need to determine its size depending on the amount of water you want to keep there.

First off, you need to make sure that you have gutters installed. Thus, the water will first flow down there and then along the downspout, which is directed into the tank. This way, you can ensure stable watering of plants throughout all seasons, regardless of the weather.

Things to take into account when calculating the amount of rainwater collected

Before using the rainwater harvesting calculator, we recommend that you consider the factors that matter during the calculation. This allows you to estimate how much water you can keep in your tank, both by using a calculator and without utilizing any online resources.

  • Average annual rainfall

Average annual rainfall is a number that is calculated for each individual area, as the weather varies greatly depending on it. Find this indicator for your area to use when calculating. It can be specified in millimeters or inches.

  • Square footage of the land plot

Obviously, you also have to consider certain plant water needs that are unique for each homeowner. You can pay attention to how many sq ft is the area on which you grow plants. This indicator is not used in the formula when calculating, but you must take into account in advance how much water you require at all.

  • Square footage of the catchment area

A catchment area is an area of your roof in square feet. To calculate it, you simply need to multiply the length of the roof by its width. The good news is that you do not have to consider the design of your roof and the level of slope. In this case, it does not matter, so the calculations are as simple as possible.

  • Collection efficiency

In addition, you should keep in mind that the collection efficiency of asphalt or concrete roof is approximately 90%. All online calculators will usually show you the output that has been calculated, taking into account this number.

Ways to estimate the amount of rainwater collected

The first and best way to find out how much rainwater you should collect is by using our rainwater harvesting calculator. When using an online source, you will only need to fill in the input information and wait for an output. Therefore, you do not need to calculate the amount of plant water that can be collected, depending on the particular data filled in by each user yourself. You can also see the output in the form of a table or graph.

Another way to calculate the amount of rainwater is to use the formula. You just have to multiply the area of your roof in sq m by the precipitation level in mm to get the result in liters. To calculate the amount of water in gallons, multiply the roof area in sq ft by precipitation in inches and by 0.623. If you find it not so easy, read on!

Our Rainwater Harvesting Calculator to Boost Your Survival Chances

Our calculator can be instrumental in sustainable water management. It allows you to estimate how much rainwater you can collect for use in your household, garden, or farm. This can reduce reliance on municipal water supplies, save on water bills, and contribute to environmental conservation.

Here are the steps you need to take to use the calculator properly to get the most precise plan:

Step 1: Input the total area of your collection surface in square feet.

Step 2: Enter the average annual rainfall in inches for your area.

Step 3: Input an efficiency factor between 0 and 1 to account for losses due to evaporation and leakage.

Step 4: Click “Calculate” to get the estimated total amount of harvestable rainwater.

Use this calculator to estimate how much rainwater you can collect annually. Input your collection surface area, local annual rainfall, and an efficiency factor (between 0 and 1) to account for losses like evaporation and leakage.

Frequently Asked Questions — FAQ

How many gallons of rain per square foot is 1 inch?

According to the formula, for 1 inch of rainfall, you can capture o.623 gallons of rainwater on every square foot. You can also estimate this amount by using an online calculator. To do this, fill in the length and width of your roof and the annual rainfall level in your area.

How many inches of rain does it take to fill a 55-gallon rain barrel?

Well, to answer this question and determine this amount, we first need to know the length and width of your house’s roof. For instance, to fill in the rain barrel of this particular size, approximately 0.3 rainfall is needed when the area of a roof is 300 sq ft.

How do you calculate the size of a rainwater tank for harvesting?

To determine the size of the rain barrel, you also need to calculate the maximum rain harvesting capacity or amount of water that can be collected depending on the annual rainfall. Hence, you can use a rainwater harvesting calculator for this purpose.


A former USA Army sergeant and a highly educated survivalist and prepper with a degree and interest in Engineering and Electronics, Mike Millerson applies his extensive expertise in survivalism, homesteading, backpacking, hiking and hunting, spreading his deep knowledge about handling emergencies and prepping for them reasonably and effectively.




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