Gravel is one of the more noticeable pieces of decor in many aquariums, and is important not only for capturing dirt and debris from the fish, but also for securing other decor like plants or aquarium furniture. For a 30-gallon tank, there is a formula you can use, along with a little common sense, to help determine exactly how much gravel you need.
Measure your tank's length and width. Multiply theses two numbers and then divide by 10. The resulting number is the amount, in pounds, of gravel you should use. This is useful since most gravel is sold by the pound in large plastic bags.
There are two types of 30-gallon tanks often available, a shallower version known as a "breeder" and the more standard version, which measures 36 inches long by 12 inches wide and 16 inches tall. This calculates to a base area of 432 square inches, which equates to approximately 43.2 lbs. of gravel. Being a few pounds off won't hurt the tank, so you can use 40 and be just fine.
Breeder tanks are shallower than standard tanks, which are used mostly for display purposes. Their dimensions measure 36 inches long by 18 inches wide and 12 inches high. After multiplying the length and width we're left with 648, meaning you should use about 60 lbs. of gravel to compensate for the larger base area.
When placing gravel in the aquarium, don't bunch it all in one place. Instead, be careful to evenly distribute it across the entire bottom, providing about 2 inches of depth.