Tennis ball bounce test: heights, steps and results

What is the official tennis ball bounce test?

The International Tennis Federation (ITF) has tested bounce the same way since 1925. In its ball approval test, a ball is dropped from 254 cm (100 in) onto a smooth granite block, and the height of the rebound is measured by video.

Everything is controlled. Balls are kept for 24 hours at 20 ± 2 °C (68 ± 4 °F) and 60 ± 5% humidity, and they are squeezed first so every ball starts the same way. That matters, because heat and humidity both change the result.

To be approved, a ball's rebound must fall inside the range for its type. There are four adult types, covered in our guide to tennis ball types.

How high should a tennis ball bounce?

Here are the official ranges, plus what they work out to from the two heights you can find on any court: the centre of the net, 0.914 m (3 ft), and the top of the net posts, 1.07 m (3 1/2 ft). The net-height figures are our own arithmetic, scaling the official rebound by the drop height. They ignore air resistance and the fact that balls bounce slightly better at lower impact speeds, so a real ball may come up a little higher from net height.

Kids' balls have their own lower ranges from 254 cm: green 120 to 135 cm, orange 105 to 120 cm and red 85 to 105 cm.

How to run the bounce test at home

You can get close to the lab test with a tape measure, some painter's tape and a phone camera. The hardest part is reading the top of the bounce, which is why the slow motion video helps.

1. Pick the floor. Use smooth concrete, a hard court or a stone floor. Avoid wood, carpet, clay and grass, which soak up energy and give a lower reading.
2. Mark the wall. Stick tape at 254 cm (100 in) from the floor, and add marks every 5 cm or 2 in from 120 to 160 cm (47 to 63 in).
3. Let the balls settle. Leave them indoors for an hour or more, so they are at room temperature. Around 20 °C (68 °F) is ideal.
4. Squeeze each ball a few times in your hand. The ITF pre-compresses balls before testing, and this evens out a ball that has sat still.
5. Set up the phone. Put it on the floor or a chair a couple of metres away, filming in slow motion, with the scale marks in view.
6. Drop, do not throw. Hold the ball with its bottom level with the 254 cm mark and open your fingers. No push, no spin.
7. Read the bottom of the ball at the top of its first bounce from the video. That is your rebound.
8. Do three drops per ball and take the middle value. Then drop a new ball of the same type the same way and compare.

What your result means

The official range is for new balls in a lab. Your floor, your release and the temperature all shift the number, so the most useful comparison is your ball against a new ball on the same floor on the same day. The bands below are our rule of thumb, anchored on the ITF's 4.0 cm durability limit, not an official scale.

Why the floor changes the result

The lab uses granite because it barely moves when the ball hits it, so almost all the lost energy comes from the ball. Real courts are different. Physicist Rod Cross gives a vertical coefficient of restitution of about 0.75 for a tennis ball on concrete or a hard court, and 0.6 to 0.75 on grass, depending on the grass and soil.

Clay is a special case. The ball digs in and gets pushed upward, so its vertical bounce stays lively while it slows a lot horizontally. Either way, a court is not a granite block. That is why you should never judge a ball only against the lab number on a soft surface. Compare it with a new ball on the same spot.

Temperature and altitude

The ITF notes that high temperatures affect the rubber and increase bounce height, which is why its test runs at a fixed 20 °C. Cross and Lindsey found balls bounce a lot higher on a hot day than a cold one, and treated their own 73 °F (23 °C) tests as not valid for pass or fail for that reason. We have found no reliable figure for how many centimetres a degree is worth, so test both balls at the same temperature.

Altitude has a similar effect. A Wilson engineer told the WTA that a regular ball at about 4,000 ft (1,219 m) or higher bounces about three inches higher. That is why the ITF approves a separate high altitude ball with a lower rebound range.

The bounce test in one number

Physicists describe a bounce with the coefficient of restitution, or COR: the ball's speed just after the bounce divided by its speed just before. For a straight drop it equals the square root of rebound height divided by drop height.

Working back from the official ranges gives a COR of about 0.73 to 0.76 for a new Type 2 ball. That is our calculation, ignoring air resistance. A ball with a lower COR keeps less of its speed every bounce, and the gaps between its bounces shrink faster. You can hear that, which is the idea behind measuring bounce by sound.

Common mistakes

• Throwing the ball down or letting it roll off your fingers with spin.
• Mixing up top and bottom. Measure the drop and the rebound to the same point on the ball. The bottom is easiest.
• Testing a cold ball from the car against a warm ball from the house.
• Using a wooden floor or a mat. Both give lower readings than the lab.
• Judging by eye from far away. Slow motion video is far more reliable.

Doing it with your phone

If marking a wall sounds like too much effort, BallCheck does the bounce test by sound. Put your phone on the floor, drop the ball from net height and it times each bounce to get the COR, then compares it with a new ball of the same type. Premium also shows an estimated official test rebound for the ball, placed on the ITF range. The reference values assume lab conditions, so drop a new ball on your court first to calibrate. The free web version runs in your browser now.