40 mm
A regulation table tennis ball has a specified diameter of forty millimetres
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Photo: Universe Greg · CC BY-SA 3.0
The short answer
ITTF Law 2.3.1 specifies a spherical ball with a diameter of 40 millimetres. The neighbouring laws specify a mass of 2.7 grams and a matt white or orange surface.
Why it's true
The size of a table tennis ball is stated as a diameter, meaning the distance across the sphere through its centre. The ITTF Laws of Table Tennis gives that value as 40 millimetres. It is not the ball’s circumference, and it is not a radius that must be doubled before comparing it with equipment.
The law pairs size with other properties rather than treating a small round object as sufficient. A regulation ball must be spherical, weigh 2.7 grams, and have the specified material and surface characteristics. These simple requirements establish a common basic object for a game in which spin, bounce, and rapid changes of direction are central.
By the numbers
40 mm
Specified ball diameter
2.7 g
Specified ball mass
20 mm
Radius calculated from the stated diameter
How we know
The primary text is the ITTF Handbook’s Laws of Table Tennis, section 2.3. In the 2022 handbook, Law 2.3.1 gives the diameter, Law 2.3.2 gives the mass, and Law 2.3.3 says the ball is made of celluloid or similar plastics material and is white or orange and matt. This article identifies that edition rather than pretending the source is undated.
The handbook distinguishes the laws from the additional regulations used for international competitions. The basic size statement is therefore not a complete manufacturing approval manual. A ball intended for a sanctioned event must satisfy the relevant approval and competition requirements as well as matching the headline dimensions in the laws.
Worth remembering
- Forty millimetres describes diameter, not radius or circumference.
- The laws also specify mass, colour, material, and finish.
- Basic dimensions do not replace competition equipment approval.
In context
A tape measure wrapped around the ball measures a different quantity from a straight diameter measurement. For everyday understanding, the key is to identify the dimension before comparing numbers. For formal equipment assessment, use the relevant technical testing procedure rather than an improvised household measurement.
The simple headline is useful because it is easy to remember, but the surrounding clauses keep it honest. They describe the same ball from several angles: geometry, mass, material, and appearance. That combination is more informative than treating forty millimetres as the entire definition of regulation equipment.
A closer look
Diameter, radius, and circumference
A 40 millimetre diameter corresponds to a 20 millimetre radius. Using the ideal-sphere relationship, the circumference is about 126 millimetres. These calculated quantities explain the geometry, but neither replaces the law’s actual diameter wording. Saying “a forty millimetre ball” is convenient shorthand only when the reader knows which dimension is being described.
The distinction also matters when comparing sports equipment. A tennis ball specification may be expressed as a diameter range, while another sport’s rules may use circumference or a differently framed size standard. Comparing the raw numbers without checking the measurement type can create a false impression of relative size. Here the relevant measurement runs straight across the table tennis ball.
Colour and finish are part of the specification
The permitted basic colours in the cited law are white and orange, and the finish is matt. Those are explicit requirements, not a claim that every promotional or recreational ball sold in other colours is intended for official competition. A novelty ball can be fun to use while still being different from the equipment described by the laws.
The ball works with a defined table and net, but those components have their own measurements. The net is much higher than one ball diameter, and the table provides a larger rectangular bounce surface. Compliance is therefore a system-level question: correct ball size cannot make an uneven table or incorrectly installed net conforming. Each component has to meet its own applicable requirements.
Sphericity is a separate part of the statement and should not disappear behind the diameter number. An object can have one measured width of forty millimetres without being a sphere. The law describes a round ball, not an arbitrary shape that happens to match one dimension when held in a particular direction. This is another reason casual size checks are limited: they may help someone understand the scale of the equipment, but they do not replace the technical assessment needed to establish consistent shape and suitability for approved competitive use.
Related facts
Examine the construction requirements for a different flying object. A regulation feathered badminton shuttle has sixteen feathers.
Compare the specified target height in a different sport. A regulation basketball ring sits 3.05 metres above the floor.
Compare another rule that fixes the size of a playing surface. A regulation tennis court is 23.77 metres long.
Compare a numerical restriction on the equipment a player may use. Golf limits a player to fourteen clubs during a round.
Key terms
- Table-tennis ball diameter
- The ITTF-specified diameter of a spherical ball: 40 millimetres.
- Ball mass
- The specified mass of a table-tennis ball: 2.7 grams.
People also ask
Is forty millimetres the radius?
No. It is the diameter, the full distance across a sphere through its centre. The corresponding radius is twenty millimetres. Confusing those terms would describe a ball twice as wide as intended, so the distinction is more than a minor vocabulary issue.
Does the ball have to weigh a particular amount?
Yes. Law 2.3.2 specifies 2.7 grams. Size and mass are separate requirements: a hollow object can have the right outside dimension but a different weight. Checking one property does not establish the other or certify the ball for a particular competition.
Can a regulation ball be orange?
Yes. The cited law allows white or orange and requires a matt finish. The choice between permitted colours is separate from the diameter rule. A brightly patterned souvenir ball should not be assumed to meet official competition requirements merely because it is approximately the right size.
Does the law require only celluloid?
No. The wording in the cited handbook is celluloid or similar plastics material. It would be inaccurate to quote the material rule as an exclusive celluloid requirement. Modern approval questions should be checked against the applicable ITTF equipment lists and technical documents, not answered from a misleading abbreviation.
Does a printed size guarantee tournament approval?
No. A printed size claim addresses only part of what matters. International competition regulations and equipment approval requirements are additional considerations. Organisers should use the applicable approved equipment rather than infer approval from a familiar diameter, colour, brand, or decorative star marking alone.
