Exceptions to the rule
Different leagues have different levels of stringency as to regulations on matchday balls. However, outside of official matchdays, there are no real restrictions on the game ball. When inflating the soccer ball, follow the manufacturer’s instructions, usually printed next to the air nozzle.
Size 5 soccer balls are the standard for adult and professional play. Other sizes (Size 4, 3, 1) are available for youth players or soccer-like games such as futsal and beach soccer.
Soccer ball price tiers
When buying a soccer ball, it is important to note that there are different price tiers. Generally, you get what you pay for: the higher the price, the better quality of the ball. The increase in quality does not mean increased durability; in fact usually the opposite. Playing with matchday (the highest-tier) balls on anything but the grass will ruin them quickly. Otherwise, the higher-tier, more expensive balls are lighter, fly "truer," move quicker (both on the ground and in the air), give a more consistent touch, and are more predictable in how they roll and spin. To a seasoned player, the differences are noticeable.
What it takes to design a soccer ball
If you would like to go in-depth and find out more about the math behind the design of a soccer ball: check out our post on the design of the classic Adidas Telstar that changed the way the game was played with its unique stitching.
This Jabulani soccer ball that was designed and produced exclusively for the 2010 World Cup in South Africa, was a marvel of design at the time. It was mathematically the most "spherical" ball that Adidas has made up until that point.