A tennis ball is made of a hollow rubber core filled with pressurized air, covered in a fuzzy felt made from wool and nylon. That simple two-part structure, core plus felt, has remained largely unchanged for decades, though the specific materials and engineering behind each layer are more interesting than they first appear.
The Rubber Core
The core of a tennis ball starts as two separate half-shells of vulcanized rubber, each shaped like a small bowl. Vulcanization is the process of adding sulfur to rubber and heating it, which transforms soft, sticky raw rubber into something firm and elastic. The rubber itself comes from either natural or synthetic sources. Natural rubber is harvested as latex from the Hevea brasiliensis tree, the same source used for most rubber products worldwide. Synthetic rubber, derived from petroleum, is also common in mass-produced balls.
To assemble the core, a rubber latex adhesive is applied to the rim of each half-shell. Small tablets of sodium chloride and sodium nitrite are placed inside one half along with a measured amount of water. The two halves are then pressed together and heated. The chemical reaction between the tablets and water generates gas inside the sealed core, inflating it to the desired internal pressure. The result is a hollow, bouncy sphere roughly the size of a small orange.
Pressurized vs. Pressureless Balls
Most tennis balls you’ll find in a store are pressurized. They carry an internal pressure of about 10 to 13 psi above atmospheric pressure, which gives them their characteristic lively bounce. That extra pressure is also why new balls come sealed in pressurized cans: the can’s internal pressure (about 12 to 15 psi above atmosphere) matches the ball’s, preventing the gas from slowly leaking out through the rubber before you open the container. Once you crack the seal, the balls begin losing pressure over a few weeks.
Pressureless balls take a different approach. Their internal air pressure is roughly equal to the atmosphere around them, so their bounce comes entirely from the rubber itself rather than trapped gas. These cores use a specialized synthetic rubber called high-cis polybutadiene, which has exceptional impact resilience, meaning it snaps back into shape faster and more completely after being compressed. Pressureless balls feel slightly heavier and stiffer when new, but they don’t go flat over time, making them popular for ball machines and casual practice.
The Felt Covering
The fuzzy outer layer is a woven felt made from a blend of wool and nylon fibers, typically in a ratio of about 60 to 70 percent wool and 30 to 40 percent nylon by weight. Wool provides the soft, fibrous texture that grabs the court surface and the strings of a racket, creating spin. Nylon adds durability so the felt doesn’t shred after a few games.
The felt is cut into two dumbbell-shaped pieces (sometimes called “dog bones” for their shape). These pieces are dipped in the same type of rubber latex adhesive used on the core, then pressed onto the outside of the ball so they wrap around it with a distinctive seam line. That seam, a slightly raised curving path where the two felt pieces meet, affects aerodynamics. It creates turbulence as the ball moves through air, which is part of what makes spin shots curve.
Extra Duty vs. Regular Duty Felt
Not all felt is identical. Tennis balls come in two main felt types designed for different court surfaces. Extra duty balls have a thicker felt woven a bit looser around the core. This construction holds up better on hard courts, where the rough surface would quickly shear away a thinner covering. Regular duty balls use a thinner, more tightly woven felt that resists absorbing moisture and dirt, making them better suited for clay and indoor courts where fluffing can become a problem.
If you play on hard courts with regular duty balls, you’ll notice the felt puffs up quickly and the ball starts feeling sluggish. Use extra duty balls on clay, and they’ll pick up too much surface material. Matching the ball to your court surface makes a noticeable difference in how long a can lasts.
Why Tennis Balls Are Yellow
Tennis balls weren’t always that familiar neon color. Historically, they were either black or white depending on the court’s background color. In 1972, the International Tennis Federation introduced yellow balls into the rules after research showed they were significantly more visible to television viewers. The specific shade is often called “optic yellow,” a fluorescent tone that stands out against both green courts and blue skies on screen. Today, white balls are still technically legal under ITF rules, but yellow dominates at virtually every level of the sport.
High Altitude Balls
At higher elevations, where air pressure drops, a standard pressurized ball bounces higher and flies faster because there’s less air resistance slowing it down. To compensate, manufacturers produce high altitude balls with diameters roughly 6 percent larger than standard balls. The bigger size increases air resistance enough to bring the flight and bounce back in line with what players experience at sea level. If you play above about 4,000 feet, these balls keep the game from feeling unnaturally fast.
What’s Inside the Can
The gas inside both the ball and the sealed can is ordinary air, just filtered and dried to reduce moisture. Manufacturers don’t use pure nitrogen or any special inert gas. Drying the air helps prevent mold growth and corrosion inside the can during storage. The can itself is pressurized to roughly 12 to 15 psi above atmospheric pressure, creating an equilibrium that keeps the balls from losing their internal pressure while sitting on a store shelf. That satisfying hiss when you open a new can is simply the pressurized dry air escaping.