A baseball is built from a small cork-and-rubber core wrapped in roughly 370 yards of wool and cotton yarn, covered in two pieces of cowhide leather, and stitched together by hand with 108 double stitches of red waxed thread. Every official Major League ball weighs about 5 ounces and measures 9 inches around. The construction has remained largely the same since the 1930s, and the process of making a single ball still takes about ten days.
The Core: Cork, Rubber, and the “Pill”
At the very center of every baseball sits a small sphere called the “pill.” It starts with a piece of composition cork, which is molded into a layer of rubber. This cushioned cork center was introduced in 1931 and is still in use today. The pill is manufactured at the Muscle Shoals Rubber Company in Batesville, Mississippi, where it’s built up in distinct layers: two hemispheric shells of black rubber enclose the cork, the seams between those shells are sealed with red rubber, and a final layer of red rubber surrounds the entire assembly.
Cork was first added to baseballs in 1910, replacing an all-rubber center. The change mattered because cork gives the ball a more controlled bounce. Too much rubber makes a ball overly lively off the bat; the cork layer tempers that energy, and the surrounding rubber shells add durability and consistency. This layered design is what gives a baseball its characteristic feel when it’s hit solidly.
Wool and Cotton Yarn Windings
Once the pill is formed, it gets wrapped in four layers of yarn. This is where most of the ball’s volume comes from. The first three layers are wool (about 85% wool, 15% other fibers), and the fourth is a poly-cotton blend. Each layer has a specific thickness and density.
- First layer: a 4-ply gray wool yarn, the thickest of the four windings
- Second layer: a 3-ply white wool yarn, slightly thinner
- Third layer: a 3-ply gray wool yarn, thinner still
- Fourth layer: a white poly-cotton blend that sits just under the leather cover
Wool is the key material here because of how it compresses. When a bat strikes a baseball, the ball deforms dramatically for a fraction of a second. Wool fibers compress tightly and spring back, which helps the ball hold its shape over repeated impacts. The total yarn winding adds up to roughly 370 yards of material packed tightly around the pill. The outermost poly-cotton layer provides a smoother surface for the leather cover to sit against.
The Leather Cover
The white exterior of a baseball is genuine cowhide leather, specifically from dairy cows. Dairy cows tend to have thinner hides and finer hair than beef cattle, which produces leather with a smoother, more uniform surface. The hides are processed at Tennessee Tanning, a tannery owned by Rawlings, the exclusive manufacturer of MLB baseballs.
The leather is aniline-tanned, which is the highest quality grade of leather tanning. Aniline tanning uses dye without any paint or synthetic coating on top, so the leather retains its natural texture and grain. This is why a fresh baseball feels slightly tacky and porous rather than slick. It also means every ball has subtle natural variations in its surface. Machines ensure a consistent thickness across the hides before two figure-eight-shaped pieces are cut and placed over the wound core.
108 Hand-Sewn Stitches
No machine has ever been able to replicate the stitching on a baseball. The two figure-eight leather panels are sewn together by hand using 88 inches of red waxed thread, producing exactly 108 double stitches. An experienced stitcher at the Rawlings factory in Costa Rica, where every MLB ball has been made for over 45 years, can finish one ball in about ten minutes.
The raised red stitches aren’t just decorative. They create drag as the ball moves through the air, which is what allows pitchers to throw curveballs, sliders, and other breaking pitches. The height and tightness of the stitches affect how much the ball grips the air when it spins. Even small changes in seam height can alter how a pitch moves, which is why MLB regulates stitching so carefully.
Why These Materials Matter for Performance
Every layer of a baseball is designed to balance two competing needs: the ball has to be lively enough to travel when hit, but controlled enough to behave consistently. Sports governing bodies regulate a property called the coefficient of restitution, essentially how bouncy the ball is. Baseballs are tested by firing them at 60 mph against a rigid wall and measuring how fast they rebound. The materials inside the ball are viscoelastic, meaning they lose some energy during every impact. That’s by design. A perfectly elastic ball would be nearly impossible to field, and a dead one would make the game unwatchable.
The cork center absorbs some energy, the wool windings compress and recover, and the leather cover provides grip and protection. When a spinning ball hits a bat or the ground, the centrifugal force from the spin actually stiffens the ball slightly, reducing how much it deforms. This is one reason fastballs tend to jump off the bat differently than slower pitches. The interplay between all these materials, none of them particularly exotic on their own, creates a ball that performs within a narrow, predictable range game after game.
From Raw Materials to the Field
The full production timeline for a single baseball is roughly ten days. The pill is made in Mississippi, the leather is tanned in Tennessee, and final assembly happens at the Rawlings factory in Turrialba, Costa Rica. The cork is molded and wrapped in rubber, shipped to Costa Rica, wound with yarn on specialized machines, then covered and stitched entirely by hand. After stitching, each ball is inspected, measured, and weighed to confirm it falls within MLB specifications: 5 ounces, 9 inches in circumference.
MLB goes through an enormous number of baseballs each season. Between batting practice, foul balls, scuffed balls tossed out of play, and home runs hit into the stands, a single game can use 100 or more balls. That volume is part of why the handmade process in Costa Rica is so remarkable. Every one of those balls was stitched by a person whose fingers are strong enough to pull waxed thread through leather hundreds of times a day.