How to Calculate Gallons to Swales

Calculate the surface area of the land that will drain into the swale by measuring the width and length of a rectangular lot in feet and multiplying the two numbers together. Use a measuring wheel on the ground or find the length and width from a scale map. For example, if the width is 150 feet and the length is 200 feet, multiply 150 times 200 to get 30,000 square feet. If you know the size of a large lot in acres, multiply the number of acres by 43,560 to calculate the square feet.

Multiply the square feet by 1/12 to calculate the cubic feet of water from a 1-inch rainstorm. If typical storms in your area drop more or less water, substitute the typical amount, expressed in feet. For example, 30,000 square feet times 1/12 equals 2,500 cubic feet of water from a 1-inch rainstowm.

Multiply the result by 7.48 to calculate the number of gallons of water a typical rainstorm will drop on the area that the swale will be draining. For example, 2,500 cubic feet times 7.48 equals 18,700 gallons of water from a 1-inch rainstorm on an area 150 feet by 200 feet.

Measure the length of the swale in feet using a measuring wheel or a scale map. Measure along the curves, if any, rather than straight from one end to the other.

Measure the width of the swale in feet, 1 to 2 inches above the bottom, and multiply the width by the length. For example, if the swale is 150 feet long and 3 feet wide, multiply 150 by 3 to get 450.

Multiply the result by 4/12 if you want the swale to hold water 4 inches deep after a typical storm. You may want only 1 inch of water after a typical storm and if so, multiply by 1/12 instead. For example, multiply 450 from the previous result by 4/12 if you want the swale to hold 4 inches of water, to get 150.

Multiply the result by 7.48. The answer is the number of gallons your swale will hold with the water at the depth you chose. If the number of gallons draining into the swale after a typical storm is significantly larger than the capacity of the swale, you'll need to add a second swale or widen or lengthen the current one. For example, 150 multiplied by 7.48 is 1,122 gallons. Since this is much less than the 18,700 gallons which would fall on the area in the example during a typical storm, you'd need to lengthen the swale, widen it, or add another one.