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Ball math

A regulation table-tennis ball is exactly 40 mm across

40 mm in diameter

Current ITTF rules specify a spherical ball with a diameter of 40 mm and a mass of 2.7 g.

Standard-defined

Why this is interesting

Forty millimetres looks like a simple measurement. But changing the diameter of a sphere changes much more than its width.

Because sphere volume grows with the cube of the diameter, a small change in diameter produces a much larger percentage change in volume.

That became especially relevant when table tennis moved from a 38 mm ball to a 40 mm ball.

How this measurement works

The diameter changed from 38 mm to 40 mm — an increase of only about 5.3%.

But sphere volume is proportional to diameter³, so 40³ ÷ 38³ ≈ 1.166. A 40 mm sphere therefore has approximately 16.6% more geometric volume than a 38 mm sphere.

Two extra millimetres can mean nearly 17% more volume

That's why comparing only one linear dimension can be deceptive. A small increase in diameter affects all three spatial dimensions simultaneously.

Why did it change?

On 23 February 2000, the ITTF Congress agreed to increase the ball diameter by 2 mm.

The change came during a period when advances in equipment and technique had made the sport faster and more spin-heavy. ITTF historical material describes the organization as trying to balance high-level competition with longer, more engaging rallies and spectatorship.

The move from 38 mm to 40 mm was small on a ruler but significant for the sport.

How certain is this?

Standardized. The current 40 mm diameter and 2.7 g mass are defined by ITTF rules. The 16.6% volume difference is a mathematical consequence of comparing 38 mm and 40 mm spheres.

Values fixed by the governing rules of the sport.

Sources & further reading

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