Here are the step-by-step instructions on how to use the post hole calculator:

1. Enter Number of Post Holes: In the provided field, input the number of post holes you plan to dig for your project.
2. Select Post Shape: Choose the shape of your post from the "Post Shape" dropdown menu. The options are "Round" and "Square."
3. Enter Post Diameter/Width: If your post is round, enter the diameter of the post in inches. If your post is square, enter the width of one side of the post in inches.
4. Enter Post Hole Depth: Input the depth of your post hole in feet. This depth should be approximately 1/3 to 1/2 the above-ground post height plus an additional depth to account for the frost line and the gravel layer.
5. Enter Gravel Depth: Input the depth of the gravel layer you plan to put at the bottom of the hole in inches. This is to ensure proper drainage and help protect against frost heaves.
6. Enter Concrete Price: Input the price of the concrete in your area per cubic yard. This will help to estimate the total cost of concrete for your project.
7. Click on 'Calculate': Once all necessary information is provided, click the 'Calculate' button. The calculator will estimate the total volume of concrete in cubic yards, the total volume of gravel in cubic yards, the number of concrete bags required for different bag weights (50lb, 60lb, 80lb), and the total cost of the concrete needed.

Please note that these calculations are an estimate and actual amounts may vary based on exact hole sizes and other factors.

Also, remember that the diameter of the hole should be about three times the diameter of the post and the depth of the hole should be about ½ the total length of the pole. This is to ensure a sturdy and stable installation. The calculator will not take into account variations in depth or diameter across different post holes.