How a Baseball Is Made
What is inside a baseball, from the cork and rubber core to the wool yarn, the cowhide cover and the 108 hand-sewn stitches, plus the rule book limits, the science of the bounce and the differences between pro and youth balls.
A major-league baseball is a sphere built from five distinct materials: a cork core, layers of rubber, wool and cotton yarn wound around the core, a cowhide cover, and red thread stitched by hand through 108 double stitches. It weighs a little over five ounces and is a little under three inches across. Every ball used in the majors goes through the same sequence of steps, and the rule book sets limits on the weight and size that those steps must produce.
The ball is the one piece of equipment that every pitch, every hit and every defensive play depends on. A slightly livelier ball produces more home runs, a slightly heavier one produces fewer, and a ball with a scuffed or wet cover behaves differently from a new one. That sensitivity is why the construction matters, and why scientists, league officials and fans pay attention to changes in how balls are made.
This article follows a baseball from the inside out. It covers what the rules require, what each layer does, how the cover is stitched, why balls are rubbed with mud before games, what makes one ball bounce more than another, and how the balls used by young players differ from the pro version. For the equipment that meets the ball, see how a wooden bat is made and the evolution of the glove.
What the rule book requires
The Official Baseball Rules describe the ball in a few words. It is a sphere formed by yarn wound around a small core of cork, rubber or similar material, covered by two pieces of white leather tightly stitched together. It must weigh not less than five ounces and not more than five and one-quarter ounces, and it must measure not less than nine inches and not more than nine and one-quarter inches around.
Those limits are narrow, and a few conversions show how narrow. The geometry of a sphere lets us work out the other dimensions from the circumference: the diameter is the circumference divided by pi, and the volume is four-thirds pi times the cube of the radius.
| Quantity | Lower limit | Upper limit |
|---|---|---|
| Weight | 5 ounces (about 142 grams) | 5.25 ounces (about 149 grams) |
| Circumference | 9 inches | 9.25 inches |
| Diameter (calculated) | about 2.87 inches (7.3 cm) | about 2.94 inches (7.5 cm) |
| Volume (calculated) | about 12.3 cubic inches | about 13.4 cubic inches |
The weight range is 5 percent wide, and the volume range is about 9 percent wide. In practice manufacturers aim for the middle of both. A ball at the light end of the range and one at the heavy end differ by a quarter of an ounce, or about seven grams, a difference you would barely notice by hand and one that can matter slightly in flight.
Notice what the rule does not say. It does not fix the core's exact composition, the number of layers or the bounce. Those are the manufacturer's choices, along with a separate set of tests that Major League Baseball runs on the finished product, covered below.
Layer by layer, from the center out
A major-league ball is usually described as a series of concentric layers. The exact specifications are the manufacturer's, so the table gives the general structure rather than precise dimensions.
| Layer | Material | What it does |
|---|---|---|
| Core | Cork, often described as a small sphere or "pill" | Provides the center and some of the bounce |
| Inner coating | Rubber | Surrounds the cork and adds resilience |
| Outer coating | Rubber, typically in a second layer of a different color | Completes the core and supports the winding |
| Winding | Wool yarn in layers, then finer yarn | Builds the ball up to size and gives it its stiffness |
| Cover | Cowhide, in two pieces | Protects the winding and provides the surface the pitcher grips |
| Stitching | Red waxed thread | Holds the cover on and forms the seams |
The core sets the ball's character. Cork is light and resilient, and the rubber layers around it add the spring. The yarn then does most of the work of creating the ball's mass and firmness: it is wound under tension, so the finished ball is a dense, tightly packed sphere. The way the yarn is wound is one of the reasons two balls can feel different even when they meet the same weight limits.
The cover is cowhide. The rule book allows either horsehide or cowhide, and modern major-league balls are cowhide. The two pieces are cut into a shape sometimes described as a figure eight or a dog bone, which, when wrapped around a sphere, fit together with a single seam that winds around the ball.
How the pieces become a ball
The manufacturing process, in the order a ball experiences it, goes through six stages.
- The core. A small cork center is encased in rubber, in two layers, to form a hard core.
- The winding. Machines wind wool yarn around the core under controlled tension, building up layers until the ball is close to the final size. A finer yarn goes on last, to give a smooth surface. The wound ball is weighed and checked.
- The cement. The wound ball is typically coated with an adhesive, so the cover will bond to it.
- The cover. The two pieces of cowhide are cut to a pattern so that they match exactly, and the leather is prepared for stitching.
- The stitching. Workers sew the cover on by hand with needle and red waxed thread. A ball has 108 double stitches. The work is done by hand because the leather must be pulled tight and each stitch set to a consistent depth.
- The finish. The ball is pressed so the seams settle and the shape is true. It is stamped and inspected for weight, size and appearance.
The major-league ball has been made by Rawlings since 1977, and its balls for Major League Baseball are manufactured in Costa Rica. The company also makes balls for other levels of play. It is worth knowing that these facts reflect the supplier arrangements as of the 2025 season, and arrangements can change.
Why 108 stitches matter
The red stitches are the most recognizable feature of a baseball, and they are also part of how the ball flies. Raised seams create drag as the ball moves through the air, and they give a pitcher something to grip and the air something to push against.
The relationship between the seams and the spin is the subject of the physics of a curveball. The short version is that a ball with seams behaves differently from a smooth sphere. A ball spinning through the air has one side with seams moving into the airflow and one side moving with it, and that asymmetry produces forces that change the path. A pitcher who grips the seams one way creates a different spin from a pitcher who grips them another way.
The height of the seams is a design variable, and small changes in it can alter drag. That is why seam height appears whenever the league debates whether the ball has changed. In the second half of the 2010s, home-run totals rose, and a scientific committee convened by Major League Baseball reported that a decrease in the ball's air resistance accounted for part of the increase, with other factors also at work. This article does not try to say more than that, since the cause of the change in drag was not settled in the public record and the details of any change belong to the manufacturer and the league.
The mud
A new baseball is slick. The cover is smooth and shiny, which makes it hard for a pitcher to grip and for a catcher to hold. Before each major-league game, clubhouse attendants rub the balls with a special mud to take off the gloss, a practice that dates to the late 1930s. The mud is gathered from a spot in New Jersey and has been sold to teams in this form for decades. It is fine and smooth, with no grit, and it leaves the ball with a dull, tacky finish that pitchers can control.
Not all balls are rubbed the same way. The technique is to work a small amount of mud into the surface with the hands, wipe off the excess and leave the ball ready for play. Clubs follow league rules on how this is done and on how balls are stored afterward.
The practice matters for fairness. A ball with the sheen left on is harder to throw accurately, and a pitcher who puts other substances on the ball breaks the rules. The rules on foreign substances are a topic of their own and have changed in recent years, so for any current detail check the league's published rules.
What makes a ball bounce
The bounce of a baseball is measured by the coefficient of restitution, a ratio that describes how much of the speed a ball keeps after it hits a surface. A ball with a higher coefficient comes off the bat faster, all else being equal, and travels farther. Major League Baseball's specification for the ball includes a permitted range for this measurement, and balls are tested to make sure they fall within it.
Several things affect it. The core and winding set the base value. The moisture in the ball changes it: a ball that has dried out is typically lighter and bouncier, and a ball that has absorbed moisture is heavier and deader. This is the logic behind the humidor, a controlled-humidity storage room. The Colorado Rockies began storing their balls in a humidor in 2002, because dry Denver air was drying out the balls and adding to the offense. Major League Baseball later required humidors in every park. For how the altitude and the air interact with the ball, see altitude, weather and baseball at Coors Field.
Temperature matters too. Warm air is less dense than cold air, so a batted ball meets less resistance on a hot day. These effects are small for any one pitch and add up across a season.
Measuring the ball in flight
The same technology that tracks pitches now tracks the ball after contact, as described in the article on pitch-tracking technology. Systems record the speed and angle of the ball off the bat, and from those numbers analysts can estimate how far a ball would have gone in neutral conditions. This is how fans can see the exit speed and launch angle of a batted ball, and the figures depend on the tracking system and the season.
The tracking data is also what let league scientists study whether the ball itself had changed. If the same swing speed produces a different exit speed from season to season, something about the ball or the bat has changed. That kind of analysis is why the committee in the previous section could point to drag at all.
Balls for other levels
The ball used in the majors is not the only kind, and it is not the one you want for every player.
Youth balls. Beginning players in many leagues use softer balls, often sold as reduced-injury-factor balls, that have a softer core than a standard ball. They are designed to hit with less force. Little League and other organizations publish which balls to use at each level, so check your league's rule. See the site's youth pages and the governing body at Little League.
Practice balls. Many practice balls have a synthetic cover rather than leather, which makes them cheaper and longer lasting. They do not fly or feel exactly like a leather ball.
High-school and college balls. These are generally leather balls built to a specification set by the governing body, which can differ from the pro version in seam height and construction. In 2015 the NCAA adopted a ball with lower, flatter seams, meant to increase offense, a change that also shows how much a small change in the seams can matter.
Pitching-machine balls. Machines use balls designed to survive the stress of being thrown at speed, often with dimpled or synthetic covers.
If you are buying a ball for a player, the first step is to check what the league requires. The second is to match the ball to the age and level of play: a softer ball for a beginner, a leather ball for league play and a durable synthetic ball for backyard practice. No ball can make a young player safe, so use proper equipment and follow your coach's instructions.
Common questions
How many stitches are on a baseball? A major-league ball has 108 double stitches in red waxed thread.
What is inside a baseball? A cork center, two layers of rubber, layers of wool and finer yarn, and a cowhide cover.
Why are baseballs rubbed with mud? To remove the factory shine so pitchers can grip the ball. The mud used by major-league clubs comes from New Jersey and has been used since the late 1930s.
Who makes the major-league ball? Rawlings, since 1977, with balls made in Costa Rica as of the 2025 season.
Why do some balls go farther than others? Small differences in the core, the winding, the moisture and the seams change the bounce and the drag, which is why leagues test the balls they use.
Where to go next
If you want to know how the ball behaves in the air, read the physics of a curveball and pitch-tracking technology. For the equipment that handles it, see how a wooden bat is made. For ballpark conditions, the site's Coors Field page shows one park where the air changes the ball's flight.