How to measure the pressure in a tennis ball at home - ITF's deformation test explained

Why the bounce test alone doesn't give you the full picture

The bounce test is the classic home test for tennis balls. Drop the ball from 254 cm, measure how high it bounces, and compare it to the ITF requirement of 135-147 cm. It tells you if the ball still has enough internal overpressure to bounce within the normal range. But it doesn't tell you if the ball has the correct compression - if the rubber core still provides the right resistance when the string hits it.

Two balls can have the same bounce height and feel completely different on the string. One can be lively with full compression, the other can be softer and more sluggish upon contact. The difference lies in the stiffness of the rubber core - how much force is needed to compress the ball. This is the measurement ITF calls forward deformation.

This is what the ITF's official deformation test looks like

The ITF Technical Booklet describes the official test as follows: the ball is placed between two flat parallel plates. A pre-load of 1.587 kg is applied. The pressure is then increased by an additional 8.165 kg, and the amount the ball compresses is measured - the so-called forward deformation. The requirement is that the ball must compress between 0.50 and 0.60 cm under the extra pressure for a type 2 ball according to the ITF.

The ball is then further compressed to a total deformation of 1 inch (2.54 cm), the pressure is reduced back to 8.165 kg, and what the ball has ended up with is measured - the so-called return deformation. The requirement is 0.67-0.91 cm. The difference between forward and return tells something about how well the material returns energy to the string upon contact.

It is a precise test that requires electronic equipment with calibrated plates and force measurement. The equipment is expensive, heavy, and not designed for on-court use, meaning there is no official courtside equipment to test used balls. But the principle can be approximated at home.

What you can realistically measure at home - and what you cannot

You cannot precisely replicate the ITF's official test at home. You don't have the calibrated plates, you don't have the electronic force apparatus, and you cannot measure 0.50-0.60 cm compression with a ruler by eye. What you can do is perform a comparative test that tells you if your used balls still behave like your fresh ones - or if they have become noticeably softer.

The principle is simple: if you press two balls together with the same force, the softer ball will compress more. And if you have a new reference ball and a ball you want to test, you can compare their response under a fixed load. This gives you an indication - not a number - but it's better than guessing from feel.

Step-by-step: your own comparative deformation test

This test gives you a relative measurement - not an absolute one. You measure how your test ball performs compared to a new reference. It is precise enough to tell you if the ball is clearly degraded or still holds up.

1. Allow both balls to acclimatize. Let them sit at room temperature for 30 minutes. Temperature affects both pressure and the stiffness of the rubber - a cold ball tests differently from a warm one.
2. Place the kitchen scale on a firm surface. A table or a floor tile. Tare the unit.
3. Place the new ball on the scale. It weighs approximately 58 g. Note the precise weight.
4. Measure the ball's diameter with the ruler. A new ITF-approved tennis ball is 6.54-6.86 cm in diameter. Note the measurement.
5. Place the hard book/cutting board on top of the ball. Make sure it lies flat.
6. Press down evenly on the plate with your hands until the scale shows 8 kg extra. This is approximately the ITF's test force. Maintain constant pressure.
7. Measure the ball's new height between the table and the book while pressure is maintained. Note this number - it is your reference deformation.
8. Release the pressure and let the ball rest for 30 seconds.
9. Repeat exactly the same procedure with the test ball. Same weight on the scale, same measurement of compressed height.
10. Compare the two measurements. If the test ball's compressed height is noticeably less than the reference ball under the same pressure, the rubber has become softer. The ball is losing its playing characteristics.

The test is comparative, not absolute. You must not try to translate your result to the ITF's 0.50-0.60 cm figures - you do not have sufficiently calibrated equipment. But you can say with reasonable certainty whether your test ball behaves like the new one, or if it has become noticeably softer.

What the numbers you find typically mean

If the test ball compresses 5-15 percent more than the new reference under the same pressure, that is the natural difference between a new and a used ball. It's not dead yet. If the test ball compresses 20-30 percent more, then the rubber and pressure are on their way out, and you should consider replacing it. If the test ball compresses over 30 percent more, then the ball is clearly outside the range where it provides serious play.

If you want the full picture, you can combine the deformation test with the bounce test. The bounce test is explained in detail in the article on tennis ball bounce. A ball that passes both tests is very close to fresh. A ball that passes one but not the other is often in the middle of its lifespan - it works, but it's not new.

The myth that you can just look at the ball

A common perception is that you can tell by looking at a tennis ball if it's still good. This is only partly true. The visual wear on the felt - fraying, missing fibers, irregular surface - comes from mechanical wear on the court. The pressure drop and the stiffness of the rubber core cannot be seen from the outside. A ball with beautiful felt can have lost half of its internal overpressure without showing it visually.

This is precisely why a home-measured deformation test is worth doing. It tells you something that visual inspection cannot. If you consistently play with three balls in rotation, you can test once a month and catch the transition early. This is the most practical usage scenario.

What to do if your balls fail the test

If your balls are clearly softer than a new reference, then either the pressure or the rubber structure has changed. Both are irreversible for the ball itself - you cannot make a soft ball stiff again. You can only decide whether you want to live with a slower contact or change the ball.

The more interesting question is how to avoid reaching that point too quickly. An important factor is how well the balls are stored between sessions. Storage and humidity affect both felt and pressure over time, and winter storage is a separate challenge covered in its own guide.

Our Pressurebox Pro maintains constant internal pressure between your sessions. It does not eliminate the gradual softening of the rubber's structure - that is a separate process driven by repeated deformation cycles during play - but it means that pressure drop is removed as a factor. This extends the period during which your deformation test shows an acceptable result because pressure is no longer the primary culprit.

If you would like a complete overview of tennis ball economics, it can be found in the article on what tennis balls cost annually. And if you want a complete catalog of specific signs that a ball needs to be changed, it can be found in the guide on when tennis balls should be changed.