A baseball is made of three main components: a small cork-and-rubber core called the “pill,” multiple layers of tightly wound wool and polyester yarn, and a cowhide leather cover held together by 108 hand-sewn stitches. The finished ball weighs between 5 and 5.25 ounces and measures 9 to 9.25 inches around. Each layer serves a specific purpose, from controlling how the ball bounces to how it feels off the bat.
The Core: Cork, Rubber, and the “Pill”
At the very center of every baseball sits a small sphere of composition cork, roughly the size of a marble. This cork is molded inside a layer of rubber, forming what manufacturers call the “pill.” Two hemispheric shells of black rubber encase the cork first, and the seams where those shells meet are sealed with a cushion of red rubber. A final layer of red rubber then surrounds the entire assembly, creating a compact, springy core about the size of a bouncy ball.
The pill is engineered to balance two competing needs. It has to be lively enough that a well-hit ball can travel over 400 feet, but dead enough that the game doesn’t turn into a home run derby. If you dropped a baseball from a set height onto a hard surface, it would only bounce back to about 30% of that height. That’s far less bouncy than a tennis ball or golf ball, and the cork-and-rubber pill is largely responsible for that controlled energy return.
Wool and Yarn Windings
Surrounding the pill are four distinct layers of yarn wound tightly in concentric circles. Wool is the primary material here, and it’s chosen for a practical reason: wool fibers compress under pressure and spring back to their original shape, which helps the ball hold its form through hundreds of impacts per game.
The first winding is four-ply gray wool yarn and is by far the thickest layer. Once wrapped around the pill, it brings the circumference of the unfinished ball to about 7.75 inches, roughly 85% of the way to the final size. The second layer is three-ply white wool yarn, followed by a third layer of three-ply gray wool yarn. Each successive layer is thinner than the one before it. A final winding of white polyester-and-cotton finishing yarn goes on last, creating a smooth, firm surface for the leather cover to grip.
The yarn is wound under tension using machines that keep it uniformly tight. Inconsistent winding would create soft spots or lopsidedness, either of which would make the ball behave unpredictably off the bat or on a bounce.
The Cowhide Cover
The outer shell of a Major League baseball is made from two figure-eight-shaped pieces of full-grain cowhide leather. The hides come from dairy cows and are processed at a dedicated tannery in Tennessee. The leather receives what’s known as an aniline tanning treatment, which is the highest quality grade. Unlike cheaper leathers that get a thick coating of paint or finish to mask flaws, aniline-tanned leather shows the actual surface of the hide. That means every piece has to be nearly perfect, with no scars, blemishes, or inconsistencies in thickness.
Major League Baseball holds its supplier to tight specifications. The hides are run through machines that ensure a consistent thickness across every piece, and only the highest-grade leather makes it into game balls. Lower grades get sorted into practice balls or minor league use. The white color you see on a new baseball is the leather itself, not a painted-on coating.
108 Stitches, All by Hand
The two leather pieces are stitched together with red waxed thread in a pattern of 108 double stitches. Every one of those stitches is sewn by hand. The red color serves a functional purpose: it helps batters, fielders, and umpires see the ball’s spin and movement during play.
The raised seams created by the stitching aren’t just structural. They’re aerodynamically critical. Pitchers grip the seams to throw curveballs, sliders, and cutters, and the way air flows over those raised threads determines how much a pitch breaks. A four-seam fastball, where four rows of stitches rotate into oncoming air per revolution, behaves very differently from a two-seam fastball. The height and tightness of the stitching directly affect the game.
How Humidity Changes the Ball
Because a baseball contains so much organic material (wool, cork, leather), it’s surprisingly sensitive to moisture. Testing at the University of Wisconsin showed that the ball’s bounciness, measured as its coefficient of restitution, drops from about 0.55 in dry conditions to roughly 0.50 at 100% humidity. That may sound like a small shift, but it translates to meaningful differences in how far a batted ball travels. A waterlogged ball absorbs more energy on impact rather than transferring it back to flight.
This is why the Colorado Rockies store their game balls in a climate-controlled humidor at Coors Field. Denver’s thin, dry air already makes balls fly farther than at sea level, and keeping the balls at a controlled humidity helps bring performance closer to the league average. MLB requires game balls to meet a coefficient of restitution of 0.546, plus or minus 0.032, when tested against a block of ash wood.
From Raw Materials to Game Ball
The manufacturing process builds outward from the center. The cork-and-rubber pill is formed first, then placed into a winding machine that applies each yarn layer under precise tension. After winding, a thin coat of adhesive is applied to the yarn surface so the leather cover will bond cleanly. The two cowhide pieces are then moistened to make them pliable, positioned over the ball, and hand-stitched. After stitching, the ball is placed in a rolling machine that flattens the seams to a uniform height, then inspected and weighed.
The official rules allow a weight range of 5 to 5.25 ounces and a circumference of 9 to 9.25 inches. Balls that fall outside those ranges are rejected. For Major League play, Rawlings manufactures every game ball to these specifications, producing roughly 900,000 baseballs per year for use across the league.