Do tennis balls expire? Unopened cans explained

Do unopened tennis balls go flat?

Slowly, yes. A pressurised tennis ball holds air at about 10 to 13 psi (70 to 90 kPa) above the air outside, according to the ITF. That is what the sealed can is for: it is pressurised too, so there is little pressure difference to push air out of the balls while they wait to be sold.

The seal is not perfect forever. As Mental Floss puts it, a sealed can will not keep balls fresh for years, because air will still find its way out. We found no ball maker or ITF figure for how long a sealed can stays good, so anyone quoting an exact shelf life is guessing.

In practice, a can bought recently and kept indoors is very likely to play like new. A can found at the back of a cupboard after several years is worth testing before a match.

Why tennis balls come in pressurised cans

Pressurised packing goes back to the 1920s, when Pennsylvania (now Penn) and Wilson started selling balls in sealed tubes. The point was to keep balls at full pressure until the moment they were played. The pressurised can is the reason you hear a hiss when you open it: that is the pressurised air inside the can escaping.

The hiss is also a useful clue. If an old can opens with no hiss at all, the seal has probably failed and the balls have been at normal air pressure for a while.

Opened, unopened and pressureless: what to expect

What happens once you open the can

Opening the can lets the air around the balls drop to normal pressure. Now there is a real difference between inside and outside, and air starts working its way out through the rubber core. Play speeds it up and adds wear to the felt and the rubber.

How fast this happens is not well documented. We found no ITF or manufacturer figure for psi lost per day. A pressurizer maker's patent claims balls can lose up to 40% of their internal pressure after one day of several hours of play, but that comes from a company selling the cure. Mental Floss says an average player gets about two weeks of prime use. Both are rough guides at best.

Our guide on how long tennis balls last goes through what changes the rate, from your level to your court.

Does storage matter?

Yes, mostly through temperature. University of Illinois physicists note that warming a ball makes it easier for air to work its way through the rubber. That is helpful when you are trying to push air back in, and unhelpful when the ball is already losing it. So keep opened balls somewhere cool rather than in a hot car or in the sun.

Temperature also changes how a ball plays on the day. The ITF says high temperatures make the rubber livelier and increase bounce height, which is why lab tests run at 20 °C (68 °F). A ball from a freezing garage will feel dead until it warms up, even if it is fine. Let old balls sit at room temperature before you judge them.

For sealed cans, a cool, dry indoor shelf is the best you can do. For opened balls, a pressurizer or a sealed pressure tube is the only storage that tries to stop the leak. No independent published test has measured how much they help.

How to check an old can of tennis balls

1. Bring the can to room temperature, ideally around 20 °C (68 °F), for a few hours.
2. Open it and listen. A clear hiss means the can still held pressure. No hiss means the seal had probably failed.
3. Squeeze each ball. A ball that squashes far more easily than a new one has likely lost a lot of pressure. Treat this as a hint, not a verdict.
4. Find a hard, flat floor: concrete, a hard court or stone.
5. Hold one old ball and one ball from a fresh can side by side at net height, 91.4 cm (3 ft), and let go together.
6. Watch the first bounce. A new standard ball comes back to roughly 49 to 53 cm (19 to 21 in) from that height. If the old ball is clearly lower, it has lost pressure.
7. Repeat three times, swapping hands, so an uneven release does not fool you.
8. Look at the felt. Even a ball that bounces well is past it if the felt is patchy or the seam is splitting.

Do pressureless balls expire?

Pressureless balls are at normal air pressure, with a thicker rubber core doing the work, according to the ITF. They do not go flat in the can or out of it. Cross and Lindsey's lab study found the softening caused by air loss does not happen in pressureless balls, although two of them got slightly stiffer over five months as the rubber aged.

That makes them a good choice for a ball you want to keep in a bag for months, or for a ball machine. See our guide to pressureless and pressurised balls for how they compare in play.

What about high altitude?

Altitude changes the pressure difference too. At higher altitude, the outside air pressure is lower, so a ball pressurised for sea level is effectively more pressurised. The ITF approves a separate high altitude ball with a lower bounce for play above 1,219 m (4,000 ft), and says pressureless balls used there must be acclimatised for 60 days or more at that altitude. If you move a can between altitudes, test the balls where you will play.

Test old cans in seconds

If you have a cupboard full of half used cans, BallCheck can sort them quickly. Put your phone on the floor, drop each ball from net height and it compares the bounce with a new ball of the same type, then gives a keep, pressurize or retire call. Reference values assume lab conditions, so drop one fresh ball on your court first to calibrate. The free web version works in your browser today.