Runoff Volume Calculator

Are you looking to determine the volume of water runoff from a specific area after rainfall? Use the runoff volume calculator. This tool can provide you total runoff volume in gallons by calculating your inputs total runoff area and total inches of rainfall.

Runoff Volume Calculator

Enter any 2 values to calculate the missing variable

Runoff Volume Calculation tool

What is Runoff Volume?

Runoff volume is the amount of water that flows off a surface (like a roof or pavement) during and after a rain event. This volume is influenced by the total area of the surface and the amount of rainfall.

How to Use the Calculator

1. Basic Calculator

  1. Enter the total runoff area (RA) in square feet (ft²).
  2. Enter the total inches of rainfall (RF) in inches (in).
  3. Leave the Runoff Volume (RV) field empty if you want to calculate it.
  4. Press "Calculate" to find the missing value.

Input Example:

  • Total Runoff Area (ft²): 1000
  • Total Inches of Rainfall (in): 2

2. Advanced Calculator

  1. Enter multiple runoff areas separated by commas.
  2. Enter the corresponding rainfall per area separated by commas.
  3. Press "Calculate" to find the total runoff volume.

Input Example:

  • Multiple Runoff Areas (ft²): 100,200,300
  • Rainfall per Area (in): 1,2,3

How To Calculate Runoff Volume

The formula to calculate the runoff volume (RV) is:

RV=RA×RF12×7.481\text{RV} = \frac{\text{RA} \times \text{RF}}{12 \times 7.481}


RV  is the Runoff Volume (gallons)
RA  is the Total Runoff Area (ft²)
RF  is the Total Inches of Rainfall (in)

Calculation Example

1. Basic Calculation

Let's calculate the runoff volume for an area of 500 ft² with 3 inches of rainfall.

Total Runoff Area (ft²) 500
Total Inches of Rainfall (in) 3
Runoff Volume (gallons) 16.69

Using the formula:

RV=500×312×7.48116.69 gallons\text{RV} = \frac{500 \times 3}{12 \times 7.481} \approx 16.69 \text{ gallons}

2. Advanced Calculation

Let's say we have three areas with total mass 100 ft², 200 ft², and 300 ft², with corresponding rainfall of 1 inch, 2 inches, and 3 inches.

Multiple Runoff Areas (ft²) 100, 200, 300
Rainfall per Area (in) 1, 2, 3
Total Runoff Volume (gallons) 15.08

Using the formula for each area and summing up:

RV1=100×112×7.4811.12 gallons\text{RV1} = \frac{100 \times 1}{12 \times 7.481} \approx 1.12 \text{ gallons}
RV2=200×212×7.4814.48 gallons\text{RV2} = \frac{200 \times 2}{12 \times 7.481} \approx 4.48 \text{ gallons} \text{RV3} = \frac{300 \times 3}{12 \times 7.481} \approx 9.48 \text{ gallons}


Runoff Volume = 1.12 + 4.48 + 9.48 = 15.08 gallons


1. What units should I use for the inputs?

Use square feet (ft²) for area and inches (in) for rainfall.

2. Can I calculate the runoff area if I know the volume and rainfall?

Yes, enter the volume and rainfall, then leave the area field empty.

3. Is the calculator accurate for all surfaces?

The calculator is accurate for impermeable surfaces. For permeable surfaces, results may vary.

I hope you found the Runoff Volume Calculator is a useful tool for calculating the runoff volume from a specific area based on rainfall. Please don't forget to send you feedback.


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