How to test if your tennis balls are dead

Know when the ball is dead. Play the one in your hand.

Key facts

1
By ITF rule, a tennis ball dropped from 254 cm must rebound to between 134.6 and 147.3 cm.
ITF ball approval specification
2
A tennis ball's coefficient of restitution is about 0.75, so it returns only about 56 % of its energy after being squashed.
Tennis Warehouse University, "Tennis Ball Performance Testing"
3
The strings return about 95 % of their stored energy, so the strings, not the ball, do the springing.
Brody, Cross & Lindsey (2002)
4
A served ball arrives at the returner at about 75 % of the speed it left the racquet.
Brody, Cross & Lindsey (2002)
5
The ball sits on the strings for about 4 milliseconds at contact.

Why it works

A tennis ball is a pressurised rubber shell wrapped in an air brake. Hit it and it flattens. Springing back, it keeps only about half of what went in. That's why the strings, not the ball, do the springing.

The felt drags in the air and grips the strings. On landing, the court takes horizontal speed through friction and gives back vertical speed through restitution. Clay bites and lifts, grass skids and stays low, and hard courts are tuned by the sand in their coating.

A ball that has lost pressure returns less of both. It starts sliding toward the bottom of the legal band the moment you open the can. It bounces lower and flies shorter, and your swing quietly adapts to it.

The line to say on court
“The strings spring. The ball leaks.”

Drill: The can log

5 min · alone

Open a can. Mark 254 cm on a door frame with a ruler beside it. Drop one ball and note the rebound height.
Play your session. Afterwards, drop the same ball from 254 cm and note the height again.
Repeat after every session, and note the session where the rebound drops out of the 134.6 to 147.3 cm band.

Number to beatA rebound of 135 cm or more from a 254 cm drop

Faults and fixes

Practising on dead balls and blaming your swing

The ball lands shorter and lower, and your swing adapts to it.

FixTest the can and retire it on the number.

Thinking the ball is the trampoline

The ball loses about half its energy, while the strings return about 95 %.

FixTune the string bed for power, not the ball.

Reading the bounce without the surface

Friction and restitution differ by court.

FixExpect lift on clay and skid on grass.

Questions players ask

How high should a tennis ball bounce?

Dropped from 254 cm, a legal ball should rebound between 134.6 and 147.3 cm under ITF rules.

How do I know when my tennis balls are dead?

Drop one from 254 cm next to a ruler. If it rebounds below 134.6 cm, it's outside the rule and should be retired.

Does the ball or the strings give the power?

The strings. The ball keeps only about half its energy when it's squashed, while the strings return about 95 %.

How fast do tennis balls lose pressure?

There's no reliable per-day figure. The honest way to know is to drop-test your own balls after each session.

What this doesn't cover

There's no sourced figure for how fast a ball loses pressure per day, so the only reliable number is your own drop test. How much felt wear changes play, versus how much players just perceive it, is unverified. Surfaces also change with weather and wear. The ITF rebound figures should be checked against the current specification.

Sources

ITF ball approval specification
ITF, Classified Surfaces and Court Pace Classification
Tennis Warehouse University, "Tennis Ball Performance Testing"
Cross, "Modeling the bounce of a gas-filled ball"
Brody, Cross & Lindsey, The Physics and Technology of Tennis (2002)

Play the pointHow to tell a new tennis ball from a dead oneYou tell a dead tennis ball by its bounce: it comes up lower and deader than a new one. A ball dies twice. First the felt fluffs up, drag rises and it flies slower and shorter. Later the internal pressure leaks and the felt mats, so the bounce drops. When the balls change, your depth and targets change with them.StrikeHow string tension and type affect spin in tennisSpin comes mostly from string snapback, not tension. During contact the main strings slide sideways and snap back as the ball leaves, adding rotation. Slick polyester slides further than nylon and, in one test, gave roughly 20% more spin. Tension barely changes ball speed, so a steeper swing path is the bigger fix. Strings also lose tension from the first minute.StrikeWhy topspin kicks up and slice stays low after the bounceTopspin kicks up and forward after the bounce because the court's friction grabs the bottom of the ball and throws it on, while slice skids through low. At touchdown, friction takes horizontal speed and restitution returns part of the vertical speed. So read the spin in the air and set your racquet for the height the ball will reach, not the height it arrives at.Play the pointHow clay, grass and hard courts change your gameClay, grass and hard courts play differently because of two things at the bounce: friction, which takes forward speed, and restitution, which sets bounce height. Clay grips the ball and kicks it up, so points run longer. Grass is slick, so the ball skids low and rewards the serve and first strike. Hard courts sit between, tuned by the sand in the coating.