How to Calculate Acre-Feet

Water in lakes can be measured in acre-feet.

An acre-foot represents the amount of water it would take to cover an acre of land 12 inches deep. The term is commonly used in irrigation and water resource management to allocate water resources and to calculate the volume of water in reservoirs and other bodies of water. Because acre-feet are measures of volume, you will need to know the surface area and the average depth of a body of water to calculate the number of acre-feet it contains.

Things You'll Need

• Calculator

Instructions

1. Determine the surface area of the reservoir. If the reservoir is roughly rectangular, measure the length and the width in feet. Multiply the length times the width to find the surface area in square feet. For circular bodies of water, calculate the surface area in feet with the formula "radius x radius x 3.14." If you only know the circumference (distance around the edge) of the water body, you can find the radius by dividing the circumference by 6.18.

2. Find the average depth of the body of water. Take measurements in feet uniformly throughout the water body, and average the results to find the average depth.

3. Multiply the surface area by the average depth to find the number of cubic feet in the reservoir.

4. Divide the number of cubic feet in the reservoir by the number of cubic feet in an acre-foot (43,560) to find the number of acre-feet in the reservoir. For example, if the reservoir contains 100,000 cubic feet of water, divide 100,000 by 43,560 to get the result of 2.3 acre-feet.

Tips & Warnings

• One acre-foot equals 43,560 cubic feet, 325,851 gallons, 1,233,482 liters or 1,233 cubic meters.

References

• Southern Regional Aquaculture Center: Calculating Area and Volume of Ponds and Tanks
• LakeLawnandPon.com: Determining The Size of Your Pond

Resources

• Vertex: Surface Area and Volume Calculator