Your new deck will provide years of relaxation and entertainment, as long as it remains level and sturdy. Decks are usually freeform structures, supported by posts instead of concrete foundations. If you're constructing the deck on a beach or in another sandy spot, you'll support the structure of the deck by setting posts in the sand. Since sand shifts more than some types of soil, you'll need concrete to reinforce the posts and keep the deck from settling.
Dig the post holes at least 30 inches deep and 12 inches in diameter for each deck post. This is the standard depth and size to accommodate a 4-by-4-inch dimensional post, but if you're setting larger posts, increase the diameter of the hole to three times the diameter of the post width. For instance, if your post were 6-by-6-inches, your hole would be 18 inches in diameter.
Fill the hole with about 12 inches of wet concrete and then insert your post, tamping it up and down to settle it into the concrete. It's important to put a little bit of concrete in before you put the post in to prevent air voids under the post.
Level and align your post and have an assistant hold it upright while you shovel wet concrete into the hole around it.
Overfill the hole with concrete, creating an inverted cone shape. You can use the back of your shovel to do this or gloved hands. You will mold the concrete around the post about 3 inches high, sloping it downward toward the sides of the hole. This creates a natural drainage slope.
Plumb your post and brace it in place until the concrete sets up enough to hold it straight. It's very important to make sure the post is exactly where you want it now. Once the concrete sets, you won't be able to adjust it.
Set the rest of your deck posts in the same way.