Rainfall Calculator

This rainfall calculator will help you quickly calculate the volume of rainfall over a given catchment area depending on the amount of rain that has fallen. You can also use it to determine the area of land needed to collect a certain volume of rainfall. You will also learn how to calculate rainfall volume, why it might come in handy, and the science behind why it rains.

Does the water cycle ring any bells for you? Are you now having flashbacks (or maybe nightmares) of your grade school science class? If you don't remember, here's a (kind of) basic refresher on the 5 stages of the water cycle.

• Step 1: The first stage of the water cycle is evaporation. Water at the surface absorbs heat from the sun and turns into water vapor.

• Step 2: The next stage is condensation. When this water vapor reaches high altitudes where the temperature is low, the water vapor changes into tiny particles of ice or water droplets that come close together and form fog (near the ground) or clouds (across the atmosphere). Learn more about the clouds and their temperature by checking out our cloud base calculator

• Step 3: This is the stage we've all been waiting for, folks. Are you still with me? Precipitation! When the wind or temperature changes, the water droplets combine to form bigger droplets. Eventually, the air can't hold any more water, and it precipitates. The water droplets will fall as rain, or if the temperature is below 0 degrees Celsius (or 32 degrees Fahrenheit, for our American reader), they will fall as snow.

• Step 4: Precipitation also leads to runoff, the process where water runs over the Earth's surface (kind of like Usain Bolt).

• Step 5: Last, but certainly not least, infiltration (which sounds more like a spy game than a water cycle stage, to be honest). Water moves deep into the soil, where it seeps down, creating pure and potable water.

Calculating rainfall volume sounds like a headache, but it's actually pretty simple. All you need to know is the following:

• The size of the catchment area, or area of land; and
• The depth of rain that falls onto the catchment area

You then multiply these two values, and tada - you get the volume of rainfall.

Maybe you want to know how much water falls on your garden or your plot of land. Perhaps you want to find out how much runoff you can expect based on the land area. Wouldn't it also be neat if you could calculate how much water has contributed to a particular pond or lake over a year? For example, your backyard covers 50 m² and the average rainfall in your area yesterday was 5 cm. You can figure out exactly how much water fell on your property using the rainfall calculator!

This rainwater calculator is based on the rain catchment formula:

where:

• $V_\text{rainfall}$ - rainfall water volume;
• $d_\text{rainfall}$ - rainfall depth; and
• $A_\text{catchment}$ - catchment area.

and the catchment area in the rain catchment formula can be calculated as follows:

where:

• $l_\text{catchment}$ - catchment length; and
• $w_\text{catchment}$ - catchment width.

So to use this rainfall calculator:

1. Enter the catchment area value. You can also click on Advanced mode and input the length and width to have the catchment area automatically calculated for you.
2. Enter the rainfall depth.
3. Alternatively, if you already have the rainfall water volume and want to figure out how big the catchment area is, you can enter the rainfall volume and rainfall depth to calculate the catchment area.

Don't worry about unit conversions. We've got that covered for you!