Volleyball

×

How to play

Space (or a tap on the court) serves and plays every touch. When a prompt appears — DIG, SET, SPIKE, BLOCK — a bar sweeps across it; press Space or tap near the middle for a clean touch. On a phone, the serve sliders sit below the court. A poor touch drops the ball. Arrow keys aim (left/right places the ball, up/down changes depth), [ and ] change serve speed, F switches between a float serve and a topspin jump serve, N starts a new match, H opens this panel.

What is faithful

Every dimension and rule below is taken from the FIVB Official Volleyball Rules 2025–2028 (English, v05), approved by the 39th FIVB World Congress 2024.

• Court 18 × 9 m (Rule 1.1); all lines 5 cm wide and drawn inside the court (Rules 1.3.1, 1.3.2); the centre line's whole width belongs to both courts (Rule 1.3.3); the attack line's rear edge is 3 m from the centre-line axis (Rule 1.3.4).
• Net top 2.43 m for men and 2.24 m for women (Rule 2.1.1), 1 m wide with a 7 cm top band and 10 cm mesh (Rule 2.2). Side bands 5 cm × 1 m above each side line (Rule 2.3). Antennae 1.80 m long, 10 mm across, the top 80 cm above the net in 10 cm stripes, and part of the net (Rule 2.4). Posts 2.55 m high, 1 m outside the side lines (Rule 2.5.1).
• Ball circumference 65–67 cm, mass 260–280 g, inside pressure 0.30–0.325 kg/cm² (Rule 3.1). The pressure is not decoration: it sets the ball's contact stiffness and therefore how much it flattens on landing, which decides line calls, because a ball is in if any part of it touching the floor touches the court, lines included (Rule 8.3).
• Rally scoring to 25 with a two-point lead, the deciding fifth set to 15, match to three sets (Rules 6.2, 6.3). A serving team that wins the rally does not rotate; a receiving team that wins it rotates before serving (Rules 12.2.2.1, 12.2.2.2, 7.6.2).
• Three hits, and a block contact is not one of them (Rules 9.1, 14.4.1). Four hits, double contact and catch are faults (Rules 9.3.1, 9.3.4, 9.3.3).
• The Libero may replace only a back-row player (Rule 19.3.1.1) and may not serve, block or attempt to block (Rule 19.3.1.3).
• A ball is out if any part of it crosses the net plane outside the crossing space, which the antennae bound at the sides (Rules 8.4.4, 10.1.1).

The float serve

Every other side force in a ball game comes from spin. A float serve is the opposite: it is struck flat with no rotation, and a non-rotating sphere near its critical Reynolds number has an unstable, oscillating wake, so the lateral force wanders and the ball breaks late. Wei, Lin and Liu measured this in a wind tunnel in 1988 and found the fluctuation of the lateral force to be the same order of magnitude as the mean drag — whether or not the seam was symmetric to the flow, which is what rules out a seam explanation. The effect is confined to a narrow speed band: Asai and colleagues put the critical Reynolds number for a volleyball near 2.2–2.7 × 10⁵, and Hong and colleagues measured 2.6–3.4 × 10⁵ depending on the ball and the panel orientation. Here the drag crisis is centred on Hong's figure for the Mikasa V200W, the ball used at the 2020 Tokyo Olympics.

The ball leaves your hand above that band and decelerates into it, which is why the break comes late and why the deviation peaks at a particular flight distance rather than at a particular launch speed. In this model that peak sits at 17 m of flight, which is where Laios and colleagues measured it in 2026 over 1224 real float serves, with a mean deviation of about 0.30 m. Only one amplitude constant was fitted; the location of the peak is a prediction.

What is different, and what is guessed

See CREDITS.txt for the full list. In short: this is an independent reimplementation, not FIVB software and not affiliated with FIVB. Substitutions, time-outs, intervals, sanctions and the refereeing team are not modelled. Everything the rules leave to the referee's judgement — a catch, a double contact, interference, screening — is either decided by a stated numeric proxy or never called at all, and each one is listed. The ball's coefficient of restitution is not specified anywhere in the FIVB rules, so it is derived from the specified pressure and labelled as reconstructed.