STEP 1.  MULTIPLY THE WIDTH TIMES THE LENGTH OF THE FILTER YOU NEED TO GET SQUARE INCHES OF YOUR FILTER.

STEP 2.  SELECT THE CORRECT SQUARE INCHES FROM THE DROP DOWN MENU.

STEP 3. ENTER THE COMPLETE DIMENSIONS OF YOUR FILTER (WIDTH, LENGTH, THICKNESS) IN THE BOX.

STEP 4. ENTER THE QUANTITY YOU NEED IN THE BOX.

STEP 5. CLICK "ADD TO CART".