Step 1
Measure the length and width of the filter in feet if the filter is square or rectangular. Measure the diameter if it is round. Only measure the area covered by filter material; do not include filter walls in your measurement.
Step 2
Calculate the surface area of the filter material. For a square or rectangular filter, multiply the width and length together to obtain the surface area in square feet. For a round filter, divide the diameter by two, square the result, and multiply by pi (3.14).
Step 3
Estimate the water's velocity through the filter. If your calculations are for a municipal or commercial water treatment plant, get an estimate of the velocity from a plant employee. Otherwise, estimate 0.015 to 0.15 gallons per minute per square foot for a slow sand filter and 2 to 3 gallons per minute per square foot for a rapid sand filter, according to Virginia Community Colleges. Use the size of the filter material to help you estimate; coarser sand results in a faster velocity.
Step 4
Multiply the surface area of the filter by the velocity of the water through the filter to obtain the volumetric flow rate in gallons per minute. For example, for a surface area of 100 square feet and a velocity of 0.1 gallons per minute per square foot, (100 ft^2)*(0.1 gpm/ft^2) = 10 gallons per minute.