Open a fresh can and you hear the same hiss every time. That sound is the most quoted fact in the sport: tennis balls are pressurized, and the pressure is what makes them bounce. The figure usually attached to that hiss is "about 12 psi above atmospheric" or "around 27 psi inside." You will find both numbers, stated with equal confidence, on dozens of sites. Only one can be right, and the truth is that the manufacturers who fill the cans rarely publish either.
We went looking for which tennis ball specifications are actually fixed by rule, which are physics, and which are folklore that hardened into fact through repetition. The honest answer sorts cleanly into three piles.
What is actually regulated about a tennis ball
The short version: the International Tennis Federation (ITF) sets the diameter, the mass, the rebound height, and the deformation under load. Those four numbers are the settled part, and they are public.
A standard (Type 2) ball must measure 6.54 to 6.86 cm in diameter and weigh 56.0 to 59.4 grams. Dropped from 254 cm (100 inches) onto concrete, it must rebound to between 135 and 147 cm — roughly 53 to 58 percent of the drop height. It must deform a specific amount under an 8.165 kg load and recover within a tolerance. These are written into the ITF's Rules of Tennis and the associated ball-approval testing, and they are the reason a ball from one approved brand behaves much like another.
Notice what is not on that list. The ITF specifies how high the ball bounces. It does not specify the internal pressure that produces the bounce. That is left to the manufacturer, and it is where the confident numbers start to wobble.
How a pressurized ball loses its bounce, step by step
To see why the pressure number matters and why it's slippery, follow what happens to a ball after the can opens.
A standard ball is a hollow rubber core with a gas inside it at higher pressure than the surrounding air. The can is sealed at that same elevated pressure, so while the ball sits on the shelf, nothing leaks — the pressure inside the ball and inside the can are balanced.
Open the can and the balance breaks. The air around the ball drops to normal atmospheric pressure. Now the gas inside the ball is pushing outward against a lower outside pressure, and rubber, despite feeling solid, is slightly permeable. Gas molecules migrate through the rubber wall, slowly, from high pressure to low. As the internal pressure falls, the ball deforms more on each bounce and returns less energy. The rebound height drifts below the ITF window. The ball goes "dead."
This is why competitive play burns through cans so fast. The decay starts the moment the seal breaks, not the moment you start hitting. Hitting accelerates it — repeated impact fatigues the rubber and felt — but the clock is the gas, and the gas began leaking on opening.
Where the pressure number came from
Here is the small history. The pressurized tennis ball as we know it traces to vulcanized rubber work in the 19th century, but the number people cite is harder to source than the citation suggests.
Walk the trail backward and most "internal pressure" claims lead to secondary sources — equipment encyclopedias, coaching sites, physics-homework explainers — quoting one another. The figure of roughly 12 psi (about 0.8 atmospheres) above ambient is the most common, and it is plausible: it sits in the right range to produce an ITF-legal rebound. But the major manufacturers (Wilson, Penn, Dunlop, Slazenger) do not publish a target internal pressure as a spec the way they publish nothing about it at all, because the rule they must meet is the bounce, not the pressure. A maker can hit the rebound window with somewhat different internal pressures depending on rubber thickness and compound.
So the belief — "tennis balls are pressurized to X psi" — has a source, but the source is mostly other people stating it. The physics is sound. The specific digit is folklore wearing a lab coat. The honestly defensible statement is narrower: the gas inside is at higher pressure than the air outside, by enough to keep rebound in the ITF range, and the exact figure varies by model and is not a published standard.
The fuzz: what the felt is actually for
The felt covering is the other thing readers ask about, and here the explanations are usually right but overstated. The wool-and-synthetic nap does three measurable jobs and one assumed one.
It increases aerodynamic drag, which slows the ball through the air and makes the game playable rather than a blur. It increases the contact patch and friction against the strings and court, which is what lets players impart spin. And it protects the rubber from abrasion, extending the ball's life. The fourth claim — that fuzz meaningfully changes bounce height — is the weakest. The rebound is governed mostly by the pressurized core; felt condition affects flight and grip far more than it affects how high a fresh ball bounces.
As felt wears flat, drag drops and the ball flies faster and longer. That, not the pressure loss alone, is why a balding ball feels "quick" before it feels dead.
A working rule of thumb
If you want one practical handle on all of this, use the seal, not the look of the felt.
- Treat an opened can as a clock: a pressurized ball loses usable bounce within a few hours of hard hitting or a few days of sitting open, whichever comes first.
- Judge deadness by bounce, not fuzz. Drop it from shoulder height onto a hard floor; a live ball returns to roughly your knee.
- For practice where bounce consistency matters less, pressureless balls have no gas to leak — they bounce via the rubber wall itself and stay consistent for months, at the cost of a heavier, less lively feel.
The numbers worth keeping
| Property | Standard (Type 2) ball |
|---|---|
| Diameter | 6.54–6.86 cm |
| Mass | 56.0–59.4 g |
| Rebound (from 254 cm) | 135–147 cm |
| Internal pressure | Higher than ambient; not a published standard |
| Felt | Wool/synthetic nap; drag, spin, durability |
The first three rows are rules you can verify. The fourth is the one to hold loosely.
The question that's still open
Here is what we could not settle: how long an unopened pressurized can actually holds its bounce. The seal slows the leak, it does not stop it — cans aren't perfectly airtight, and rubber stays permeable on both sides. Anecdotes range from "fine after a year" to "flat after eighteen months," but we found no published, controlled shelf-life study from a manufacturer or the ITF putting a real number on it.
So the most basic question about the most common object in the sport remains unanswered in print: how long does a sealed can of tennis balls really last before the bounce drifts out of spec.